99
Contents

1.4.1 Introduction 101

1.4.2 Environmental Controls and

Instrumentation 102

1.4.2.1 Temperature 102

Cooling 102

Heating 104

Temperature sensors and controls 105

Heat conservation systems 106

1.4.2.2 Humidity 106

Humidification 106

Dehumidification 106

Humidity sensors and controls 107

1.4.2.3 Light 107

Shading 107

Artificial lighting 108

Light sensors and controls 108

1.4.2.4 Carbon dioxide 109

Increasing carbon dioxide 109

Carbon dioxide sensors and controls 109

1.4.2.5 Irrigation 109

Irrigation systems 109

Monitoring and regulating irrigation 110

1.4.2.6 Fertilization 110

Fertilization systems 110

Monitoring and regulating fertilization 113

1.4.2.7 Environmental control systems 113

Discrete and dedicated controls 113

Analog and integrated controllers 113

Climate control computers 113

1.4.2.8 Nursery security and emergency equipment 115

1.4.3 Benches and Supports for Containers 118

1.4.3.1 Pallets 120

1.4.3.2 Benches 120

Stationary benches 120

Movable benches 121

1.4.3.3 Construction materials 122

1.4.4 Handling Equipment for Materials and

Seedlings 125

1.4.4.1 Conveyors 126

1.4.4.2 Hand carts and motor vehicles 128

100

1.4.5 Seedling Production Equipment and

Supplies 129

1.4.5.1 Containers 129

Types of containers 129

Cleaning and sterilizing used containers 130

1.4.5.2 Artificial growing media 132

Components 132

Mixing growing media 132

1.4.5.3 Sowing lines 135

1.4.5.4 Container filling equipment 136

1.4.5.5 Sowing equipment 138

Shutterbox seeders 138

Vacuum plate seeders 139

Automated and precision seeders 141

Seed mulching (gritting) 141

1.4.5.6 Transplanting equipment 144

1.4.5.7 Pesticide application equipment 144

Portable sprayers 145

Boom sprayers 145

1.4.6 Seedling Harvesting Equipment 146

1.4.6.1 Grading and packing lines 148

1.4.6.2 Storage 149

1.4.7 Summary 150

1.4.8 Literature Cited 151
1.4. 1 Introduction

In chapter 3 of this volume, we designed a propagation 

environment that is suited to both the climate on the site and the 

biological requirements of the crop. Now, in this chapter, we will 

discuss some of the equipment and materials that are needed to 

maintain that environment and produce a crop of seedlings.

This chapter consists of two general parts. The first part (section 

1.4.2) briefly discusses the types of equipment that can be used to 

modify the propagation environment to optimize the six factors 

limiting to seedling growth: temperature, humidity, light, 

carbon dioxide, water, and mineral nutrients. This section is 

intended as a general introduction for nursery developers who are 

designing new nurseries as well as nursery managers wanting to 

upgrade their existing facilities. (More specific informa tion on the 

biophysical concepts of the limiting factors, how to modify each 

factor, and monitoring and control systems are given in volumes 

three and four of this series.)

The second part of this chapter (sections 1.4.3 to 1.4.6) deals 

with the materials and equipment needed to produce a crop of 

seedlings, from the types of benches to kinds of storage 

systems. A short discussion of the basic concepts introduces 

each step in the seedling production process. (More details are 

provided in volumes two, six, and seven of this series. Volumes 

six and seven are still in preparation at this time.)

101
1.4.2 Environmental Controls and

Instrumentation

Seedlings raised in open growing compounds can be supplied with 

irrigation and fertilization, and light and temperature can be 

controlled to a minor degree. Container nurseries that grow their 

seedlings in some sort of a propagation structure have the 

potential to control all 6 limiting factors. The degree of control 

depends on the type of structure and the environmental control 

equipment that is provided. For planning purposes, most 

equipment costs vary with the size of the propagation area, 

although some are fixed costs (table 1.4.1).

1.4.2.1 Temperature

The options for temperature control in open compounds are very 

limited. Shelterhouses or greenhouses are generally equipped 

with heating and cooling equipment that correspond to the type of 

structure and the nursery climate.

Cooling. The only form of temperature control possible in open 

growing compounds is cooling with irrigation. This technique can 

also be used in propagation struc102
tures. Because any surface, including foliage, from which water 

evaporates is cooled, short bursts of irrigation will lower the 

temperature of the seedling without raising the soil moisture to 

harmful levels.

Temperature control becomes much more complicated in 

propagation structures because of solar heating. During the day, it 

is often more difficult to cool a greenhouse than it is to heat one 

because many wavelengths of sunlight are converted to heat 

energy and the covering stops air exchange. A recent study found 

that half of the solar energy entering a greenhouse on a bright 

sunny day increases the air temperature (Roberts and Giacomelli 

1992).

Modifying the propagation structure is the first cooling strategy. 

Shelterhouses are popular because the sides can be raised to 

allow natural ventilation (see fig. 1.3.5A). With established 

greenhouses, shading should be considered if the crop can grow 

well at lower light intensities. Some forest and conservation 

species can grow reasonably well at lower light intensities but 

many, including most of the commercial conifers, do not. Of course 

this depends on regional climate. In the Intermountain West, 

sunlight intensities are generally high, so shading should be 

considered. In climates with many days of cloudy weather, 

however, further reduction in light intensity may not be acceptable. 

Automatic shade curtains are relatively expensive (fig. 1.4.1 A), but 

they do give the grower good control over light intensity in the 

propagation area. (Seedling shade tolerance is discussed in 

volume three, pages 80-82 and shading techniques, pages 28-29.)

Cooling equipment operates on two biophysical principles: first, 

increasing air flow replaces warm air in the boundary layer 

around the seedling and second, a
substantial amount of latent heat is absorbed when water 

evaporates. There are three types of cooling systems, which may 

be used singly but more typically in combination. Equipment costs 

vary with the type of system (table 1.4.1). (Cooling equipment is 

covered in volume three, pages 22-30).

Convection ventilation. This energy-efficient type of cooling relies 

on vents at the top and sides of the propagation structure. When 

the vents are open, hot air escapes out the top and draws in cooler 

and drier air from the sides. Vents can be opened manually or more 

commonly as the first stage in a sequential control system. 

Unfortunately, convection ventilation occurs best during colder 

weather when there is a strong gradient between inside and outside 

temperatures, and it is also dependent on the direction and speed 

of the wind (Roberts and Giacomelli 1992).

Fan ventilation. Fan ventilation is more reliable and efficient than 

convection cooling, and fully controlled propagation structures 

usually have one wall of exhaust fans to draw the air through them.. 

Exhaust fan systems work best in structures under 45 m (150 ft) in 

length and must be properly sized and installed for optimum 

efficiency (Bartok 1993). Even shelterhouses are often equipped 

with portable fans to increase air exchange (fig. 1.4.1 B). The new 

horizontal air flow (HAF) systems can maintain air temperatures 

within 1.8 °C (2 °F) with as few as 4 circulating fans per structure 

(Bartok 1994).

Evaporative cooling. Fully controlled houses can also be 

equipped with evaporative cooling systems, but their efficiency 

depends on the climate. The drier the ambient air, the greater the 

cooling potential. Fans draw the intake air through a porous 

medium called a wet wall, which is kept wet with water from a 

recirculation tank (fig. 1.4.1 C). Efficient systems can cool the 

intake air to quite close to the wet bulb temperature. Most wet walls 

consists of vertical pads and require a recirculation tank and pump 

to keep them wet, but there are also horizontal pad systems that 

are sprayed and require no recirculation tank. Alternatively, fog 

nozzles can be installed at the air intake, allowing evaporative 

cooling to occur for some distance into the greenhouse.

Heating. The first principle of heating is to control the movement 

of heat, which moves by mass flow, conduction, and radiation. 

Control of mass flow means having a tight greenhouse, so that air 

enters and leaves the greenhouse only when and where it is 

supposed to. Conduction is retarded by insulation, and 

double-layer

104

coverings can greatly retard heat loss. Z he second principle is to 

add or subtract heat to maintain the desired temperature. The sun 

is a major source of heat that can be captured and stored in a 

shelterhouse with the sides lowered or a totally enclosed 

greenhouse. The thermal mass, or heat-storing capacity, will be 

greatest when the propagation structure is full. Because of the 

high latent heat of evaporation, irrigation practices must be 

considered in heating calculations.

Propagation structures can be warmed by two basic types of 

heating systems that differ in location and method of heat 

distribution. Central heating systems use large boilers that are 

located in the headhouse or in a separate structure and pump 

steam or hot water through pipes to a range of houses. Although 

they use a variety of different fuels and heat distribution techniques, 

unit heaters are used to heat individual propagation structures. 

(Heaters, fuels, and heat distribution systems are discussed in 

volume three, pages 30-36.)

Steam or hot water heaters. Generally associated with large 

central heating systems, these heaters distribute hot water or 

steam through plain or finned pipes that are located around the 

baseboard perimeter of the propagation structure or under the 

benches (fig. 1.4.2A). The pipes radiate heat, which then circulates 

through the propagation structure by convection. Baseboard 

heating eliminates cold air flow next to walls and is best suited to 

fully controlled greenhouses where the sidewall does not open. 

Because baseboard systems lose their effectiveness with 

increasing greenhouse width, they are better for single structures 

rather than for gutter-connected ranges. Under-bench heating 

pipes are beneficial for plant growth but impede access and are 

more expensive.

Hot water pipes can also be laid in an insulated concrete floor. 

Because the greenhouse floor acts as a heat reservoir, floor 

heating systems are most beneficial when plants are grown directly

on low pallets. However, floor heating systems can be expensive 

to install and cannot react quickly to heat demand or to a change in 

desired temperature.

Forced air heaters. Because they are relatively inexpensive and 

easy to install, forced air heaters are popular for shelterhouses or 

individual greenhouses (fig. 1.4.2B). They also have a quick 

response time. Direct-fired forced air heaters can use a variety of 

fuels or they can receive hot water or steam from a central boiler. 

They are often connected to a fan-jet circulation system,
which distributes hot air through large perforated tubes. Forced-air 

heaters are often mounted overhead but this is inefficient because 

hot air rises. Although they hinder material handling, heat 

distribution tubes situated under the benches warm the root 

systems and generate better convective heat distribution. Moving 

air up through the seedling foliage also reduces disease problems.

Infrared heaters. These unit heaters are iron pipes that are 

mounted overhead throughout the greenhouse and burn gas or oil 

to generate radiant heat. Reflectors radiate infrared radiation 

downward, heating the plants but not the air. They have a 

moderate response time and work best when air movement is 

minimized. Like any equipment mounted overhead, they create 

shade.

Temperature sensors and controls. Mechanical thermo stats 

have either a bimetal strip or liquid-filled tube that changes length 

or configuration with temperature and in so doing, operates an 

electric switch. They are simple, rugged, and require no power. If 

the heating and cooling equipment are separately controlled, they 

can act as automatic backups for each other. On the other hand, 

separate thermostats can work against each other and cannot be 

connected to computer control systems.

Newer temperature control systems operate from electronic 

sensors, such as thermocouples or thermistor, that change 

resistance or voltage in response to temperature. Electronic 

thermostats have an average temperature differential of less than 

1.1 °C (2 °F), compared to mechanical thermostats, which can 

vary from 2.2 to 5.5 °C (4 to 10 °F) from the true temperature 

(Greenhouse Manager 1994a). These electronic thermostats 

require logic devices to read the resistance or voltage and 

calculate temperature. They may be operated by simple readout 

devices, dedicated temperature controllers, or environmental 

control computers and have several advantages. A single sensor 

can operate all of the heating and cooling equipment and can 

"stage" temperature control, making it impossible for the heating 

and cooling to work against each other. Proportional control is 

possible-instead of a simple ON/OFF response, the system is 

proportional to the deviation from
the setpoint which makes for more precise and efficient 

temperature control. These systems are readily amenable to 

computer control and can incorporate many special features, such 

as separate day and night setpoints, modulating vent or steam 

valve control, automatic CO Z injection, and continuous data 

collection. As a safety measure, many computer systems feature 

multiple sensors that average readings and provide a cross check 

in case one sensor fails.

Heat conservation systems. A variety of energy conservation 

devices are available ranging from a simple temperature set-back 

at night to complex systems of electrically deployed thermal 

insulation blankets. Controls for these devices can be an electric 

time clock switch or a photocell sensing the coming of night and 

activating the system. Often, these same systems can control 

blackout or shade curtains (fig. 1.4.1 A). Computer control systems 

often incorporate several sophisticated tactics for energy 

conservation in addition to simply deploying energy conservation 

hardware.

There are specific strategies for operating efficient heating 

systems. The first is staging the different components of the 

heating and cooling system. The second strategy is maximizing the 

time the greenhouse spends in "neutral," because nothing is being 

spent to heat or cool it. The only limitation is the tolerance of the 

crop for the variation in temperature, and that will vary with the 

species and stage of growth of the seedlings. The third strategy is 

having a separation in switching temperatures, eliminating short 

ON/OFF cycles that wear out equipment faster. A temperature 

differential (hysteresis) is built into mechanical thermostats and 

can be adjusted in many. A common differential is about 0.8 °C (1 

.5 °F) which means that for cooling, OFF is 1.6 °C (3 °F) lower 

than ON. A wider differential should be used if the crop can tolerate 

it. Differentials of any size can be programmed into computer 

systems, which have more complex control strategies that can 

maintain extremely accurate temperatures without proportional 

staging or hysteresis.

The Greenhouse Climate Control Handbook (Acme 1988), 

ASH RAE (1989), and The Ball Redbook (Ball 1991) are good 

sources of additional information on greenhouse cooling and 

heating.

106

1.4.2.2 Humidity

Humidity control is not feasible in open compounds, shadehouses, 

or shelterhouses with their sides raised. Humidification equipment 

is occasionally used in fully controlled greenhouses, especially for 

vegetative propagation. Dehumidification requires no special 

equipment but is practical only in enclosed structures.

Humidification. Humidification is most commonly needed in arid 

climates and particularly during the winter when colder air is 

brought into the propagation structure and warmed. Moisture can 

be added to the atmosphere in a variety of ways. If it is added as 

steam, it will already have absorbed the heat of vaporization and 

will not cool the greenhouse. Steam systems are expensive, 

however, and are cost effective only when a steam heating 

system is already in place. Humidity is also increased by short 

bursts from the irrigation system and from evaporative pad cooling 

systems.

Fog or mist systems are more commonly used for humidification 

(fig. 1.4.3A). The difference between fog and mist is one of particle 

size. Mist droplets are large enough to settle out in a few minutes 

and cause surfaces to become wet, whereas fog particles remain 

suspended. Fog systems humidify better but are also much more 

expensive (table 1.4.1). The choice between fog and mist will 

depend on the type of crop and cultural objectives. Water may need 

to be filtered to remove salts or other suspended particles, which 

can plug the nozzles. Under hot, dry conditions, a typical 

greenhouse ventilation system produces an air exchange every few 

minutes, and so fog and mi st systems must be capable of delivering 

short bursts of moisture to humidify the dry replacement air.

Dehumidification. Although high humidity is a chronic problem in 

wet climates, it is generally only necessary to dehumidify the 

propagation environment after irrigation in most climates. Growers 

typically open the vents for dehumidification if the ambient 

conditions are suitable, whereas ventilation plus heating will work 

regardless of the outside humidity. Underbench heating tubes are 

particularly effective in dehumidifying the seedling canopy, or 

some nurseries use portable fans or blowers (fig. 1.4.313). (See 

volume three, pages 61-64 for more information on humidification 

and dehumidification.)
Humidity sensors and controls. There are a variety of sensors 

for humidity. Mechanical humidistats have hair elements that 

change length with changes in humidity, triggering ON/OFF 

switches. They are not very accurate (+ or -10% RH) but are good 

enough for most greenhouse purposes. Hair elements can get wet 

without being ruined and are easily and cheaply replaced when 

they wear out. However, they are also not compatible with modern 

computer control systems.

Electrical resistance elements are impregnated with hygroscopic 

salts and change resistance with humidity. They are suitable for 

use with computer controls, but cost more than hair elements and 

may lose their calibration when exposed to free water. New bulk 

polymer sensors are superior in this regard. Dew-point sensors 

calculate the humidity from the ambient temperature and the 

temperature at which dew forms on a mirror, which reduces its 

reflectance. These are very accurate when kept clean and 

calibrated but are very expensive and more accurate than is 

needed for container nursery use (More information on monitoring 

humidity is provided in volume three, pages 66-69.)

1.4.2.3 Light

There are three properties of sunlight that can be modified in forest 

and conservation nurseries: intensity, duration, and quality. In most 

open growing compounds, sunlight is not controlled, but in 

propagation structures, the options increase with the sophistication 

of the nursery design.

Shading. The intensity of sunlight can be reduced with shadecloth, 

lath, and shading compounds that are applied to transparent 

coverings. Sophisticated retractable shade systems are now 

available that can sense sunlight intensity to maximize the light 

reaching the crop during the entire day and under different cloud 

conditions (fig. 1.4.1 A). Because sunlight is converted to heat 

when it reaches the crop, artificial shading also is used as the first 

stage of cooling. Specialized shading systems, called blackout 

curtains, are sometimes used in high-latitude nurseries to exclude 

sunlight for several hours during the day and induce dormancy in 

sensitive species. Similar systems are used to extend heat 

retention curtains during winter nights to reduce heating costs 

(table 1 .4.1).

107
Artificial lighting. There are two types of lighting systems used in 

forest and conservation nurseries. Photosynthetic lighting is 

used to supplement sunlight intensity during the winter in nurseries 

at high latitudes. High-intensity lighting systems are expensive to 

install and operate, however, and so are only considered 

economical under special circumstances (table 1.4.1). 

Photoperiodic lighting modifies daylength to prevent seedlings 

from becoming dormant by reducing the duration of the dark 

period. This is the most common type of artificial lighting used in 

container nurseries, and photoperiod lights have been used in all 

types of propagation environments, from open growing compounds 

to fully controlled greenhouses.

Photoperiodic lighting involves both duration (how long the lights 

are left on) and timing (when the lights are activated). Duration is 

either continuous or intermittent, and photoperiod lights are turned 

on after dark or before dawn to extend daylength or at short 

intervals during the night (fig. 1.4.4A). The best lamp depends on 

the type of application. Incandescent, fluorescent, and high-intensity 

discharge lamps differ significantly in light intensity and quality, and 

each requires its own type of fixtures and controls. Blackout curtains 

are used in high-latitude nurseries to artificially shorten the 

daylength during summer and induce dormancy (fig.1.4.4B).

Light sensors and controls. The type of control system is 

determined by the type of lighting system and the choice of lamp. 

For example, fluorescent and high-pressure sodium lamps cannot 

be switched on and off frequently but are very efficient when 

operated for extended periods. Incandescent lamps, on the other 

hand, are only used for intermittent lighting. Photoperiodic lighting 

is typically installed in either fixed or mobile systems. Fixed lights 

are mounted either overhead or at oblique angles around the 

perimeter of the growing area and are controlled by photocells and 

timing devices. Some nurseries install photoperiod lights on a 

movable irrigation boom to produce intermittent lighting patterns. 

Container nursery developers should contact horticultural lighting 

experts and talk to other nurseries before designing lighting 

systems. The book Lighting for Plant Growth (Bickford and Dunn 

1972) is the best single source of information on horticultural 

lighting systems. (Shading and artificial lighting systems are also 

discussed in volume three, pages 83-116.)

108
can be generated by combustion of propane or natural gas in 

burners located throughout the house. Carbon dioxide can be 

added anytime during daylight hours whenever the vents are 

closed. The strategy is to begin raising the CO Z level several 

hours before dawn and then shutting the generators down when 

ventilation begins.

Carbon dioxide sensors and controls. Carbon dioxide 

enrichment is controlled by solenoid valves that regulate the burner 

or the tank, and is synchronized with the vents or fans. The rate of 

CO2 addition can be set using rules of thumb regarding the size 

and leakiness of the greenhouse, but it is better to measure the 

rate. There is some relatively inexpensive testing equipment on the 

market that is adequate for determining the rate of CO2 enrichment. 

Once the equipment has been calibrated, periodic spot checks are 

useful but continuous monitoring is not necessary. For 

sophisticated environmental control systems, infrared gas 

analyzers can monitor and control CO2 levels very precisely. The 

book CO2 Enrichment in the Greenhouse: Principles and Practices 

(Hicklenton 1988) is the best reference on horticultural control of 

carbon dioxide. (Also, see volume three, pages 132-138, for more 

detailed information.)

4 2.5 Irrigation

An efficient and reliable irrigation system is required for all 

container nurseries and, in contrast to other types of 

environmental control equipment, the basic systems are similar 

from open compounds to fully controlled greenhouses.

Irrigation systems. Sprinkler irrigation is the norm in forest and 

conservation nurseries. Drip irrigation is not practical with small 

containers, and other irrigation techniques, such as capillary mats, 

do not permit air pruning of the root system. A typical sprinkler 

irrigation system consists of a pump, pressure tank, pipes, and 

sprinkler heads. Nurseries in developed areas sometimes can use 

water from municipal water systems and pipe it directly through a 

pressure regulator to the sprinklers. If a well is necessary, the 

water is pumped to a pressure tank that acts as temporary storage 

and a pressure buffer. Although galvanized piping was used in 

older nurseries, plastic polyvinyl chloride (PVC) pipe is now the 

most common because of its low cost and desirable physical 

properties. The building codes in some States stipulate which 

types of pipes must be used so local officials must be contacted 

(Bartok 1991).

109

1.4.2.4 Carbon dioxide

Carbon dioxide enrichment is not widely practiced in forest and 

conservation nurseries but is relatively economical in fully controlled 

propagation structures (table 1.4.1). The practicality depends on the 

type and condition of the structure because it is difficult to maintain 

proper carbon dioxide levels in leaky houses.

Increasing carbon dioxide. There are two options for supplying 

carbon dioxide (CO2) to propagation structures: pressurized gas and 

combustion of fossil fuels (fig. 1.4.5). Pure CO2 can be injected from a 

pressurized tank through perforated pipes (the safest technique), or 

CO2
Within the propagation area, two irrigation systems are common: 

fixed sprinklers or a mobile boom (fig. 1.4.6A&B). Fixed sprinklers 

are laid out on a grid and are the most economical option (table 

1.4.1). Mobile irrigation booms are more expensive but apply water 

more evenly and only to the propagation areas, thus reducing 

runoff. A wide variety of different boom irrigation systems are now 

available. New computer-equipped booms offer precise control of 

irrigation, allowing sections to be skipped or irrigated repeatedly to 

meet the requirements of specific crops (Greenhouse Manager 

1993A). Details on designing an irrigation system are presented in 

Aldrich and Bartok (1989), Pair and others (1983), and Melby 

(1988).

Monitoring and regulating irrigation. The irrigation methods used 

in container nurseries vary considerably, based on crop 

requirements and the type of irrigation system. Small propagation 

areas may be watered by hand with a hose, and this is still the best 

technique for some sensitive species. The next step in complexity is 

a fixed array of sprinklers that are manually operated. This 

increases the size of the propagation area that can be controlled, 

but someone has to be there to turn it on and off. Manual operation 

has some definite quality control advantages because the irrigator is 

always around to make sure that the system is operating properly 

and that the seedlings are receiving the proper amount of water. 

This can be inconvenient on weekends and holidays, however, and 

so most nurseries use some sort of automated irrigation system.

The simplest form of irrigation controller is a time clock to control 

a series of solenoid valves that turn the water on and off in each 

irrigation zone of the propagation area. These controllers can be 

programmed to time the duration of irrigation in each zone, thus 

allowing the irrigator to adjust the amount of water applied to the 

demands of each crop. This makes irrigation at night and on 

weekends possible but has the disadvantage that the same 

amount of water is applied every time regardless of climate or crop 

condition. Therefore, growers using time controllers try to arrive at 

some estimated plant water use. For example, some nurseries 

monitor evapotranspirational losses by weighing a typical block of 

containers. Because water is a major part of block weight, growers 

can monitor water use by weighing the blocks and determining 

when to irrigate (fig. 1.4.60.

110

Environmental computers use "on-demand" control systems to 

regulate irrigation by monitoring accumulated light, vapor pressure 

deficit, or evaporative demand. Research with tensiometers, which 

measure the matric potential of the growing medium, has also 

shown promise (Whitesides 1993). Although these new computer 

systems offer new possibilities, personal monitoring of irrigation is 

still the norm in forest and conservation nurseries. Irrigation is such 

a critical part of nursery culture and seedling water use can change 

so rapidly in propagation environments that reliance on a fully 

automated control system is not recommended without regular 

supervision. (See volume four, pages 95-109, for more information 

on irrigation systems and monitoring techniques.)

1.4.2.6 Fertilization

Because the artificial growing media used in most container 

nurseries are essentially infertile, the 13 mineral nutrients that are 

required for normal plant growth and development are supplied 

through fertilization.

Fertilization systems. The primary fertilization methods in forest 

and conservation nurseries are (1) adding soluble fertilizers to the 

irrigation water (fertigation), and (2) incorporating solid fertilizer 

into the growing medium. Most container nurseries inject soluble 

fertilizer through the irrigation system because it is the easier and 

more accurate way to apply and monitor mineral nutrition. There 

are two ways of doing this. The fertilizer can be added to a large 

tank of water, dissolved, and then pumped onto the crop. 

Alternatively, the ferti lizer can be made up as a concentrated 

solution that is then injected into the water line during irrigation (fig. 

1.4.7A). The latter is the most common method because, at 

concentration ratios of 1 :100 or 1:200, the tanks and injectors take 

up much less room than a tank large enough to hold the diluted 

fertilizer solution. Many different fertilizer injectors are available, 

from simple hose siphons to mechanical pumps using water 

pressure or electricity; compared to other nursery equipment, 

injectors are relatively inexpensive (table 1.4.1). Fertilizer injectors 

are normally located with the irrigation controls in the headhouse. It 

is a good idea to install backflow preventers on the water lines to be 

sure that potable water does not get contaminated with fertilizer; in 

fact, they are legally required in many areas.
111
112
The second fertilization technique is incorporation of slow-release 

fertilizers into the growing medium when it is being mixed (fig. 

1.4.7B). This method is less popular because it is difficult to obtain 

even mixing of the fertilizer pellets in the small volume of growing 

media in the small containers typically used in forest and 

conservation nurseries. The other drawback is that, once the 

slow-release fertilizers are incorporated, there is no way to control 

the nutrient release rate. Some container nurseries use a 

combination of slow-release fertilizers and fertigation.

Monitoring and regulating fertilization. Liquid fertilizer injection is 

controlled with the same equipment as the irrigation system. 

Automated controls that monitor inline salinity and are linked to the 

overall environmental computer system are now available. Salinity 

measurements only give a rough idea of total fertilizer levels, 

however, not the concentration of individual mineral nutrients. 

Although sensors for specific nutrient ions are available, they are 

not practical for operational nurseries. Specialized fertigation 

controls (fig. 1 .4.70 use computer programs to regulate the 

concentrations of individual nutrients as a proportional response of 

total salinity, and can maintain accuracies to within 10% (Labbate 

1994). Many growers monitor fertility by testing the concentration of 

nutrients in the applied irrigation water or the leachate that drains 

from the container. There is no automated way to monitor the 

amount of dry fertilizer that is incorporated into growing media. (See 

volume four, pages 26-58, for more information on fertilization 

methods and monitoring.)

1.4.2.7 Environmental control systems

Discrete and dedicated controls. These single-function 

controllers (e.g. thermostats) regulate one piece of equipment with 

a simple ON/OFF switch. Currently, around 90% of existing 

greenhouses still rely on mechanical thermostats (Greenhouse 

Manager 1994a). One common thermostat contains a bimetallic 

strip that is sensitive to changes in temperature and activates a 

switch when it senses a change. Thermostats are used to regulate 

temperature control equipment such as heaters, fans, and vents 

(table 1.4.2). Other types of discrete and dedicated controls do not 

sense the propagation environment at all but use time clocks to 

regulate irrigation solenoids, photoperiod lights, or carbon dioxide 

generators. They can operate independently or be linked in 

sequence. This redundancy of function means that if one device 

fails, the rest of the system keeps working. Discrete and dedicated 

controls are inexpensive but

must be calibrated routinely. Thermostats are particularly 

unreliable, and response varies from instrument to instrument 

and can change over time (Nelson 1991).

Analog and integrated controllers. A wide variety of controllers 

are available that can regulate many different environmental 

variables (table 1.4.2). They use proportioning thermostats and 

other electric sensors to gather information from the propagation 

environment, and then rely on electronic logic circuitry to process 

this information, make decisions, and operate a single piece of 

environmental control equipment. Analog controllers are limited to 

a single sensor and can control only one propagation environment 

(Ball 1991). Many are "hardwired systems," which means that they 

are not directly programmable, whereas others have limited 

programmability. Analog controllers can also be wired to activate 

alarm systems, such as when the temperature decreases past a 

set point.

Environmental controllers are commonly linked to provide multiple 

functions and operate in stages to maintain the desired 

temperature by switching an increasing sequence of heating or 

cooling (fig. 1.4.8). For example, consider a situation in a 

greenhouse early in the morning. Neither the heating nor cooling is 

operating; the house is in "neutral." As the greenhouse heats up, 

the temperature reaches the first cooling setpoint and one of 

several fans comes on. If that gives sufficient cooling, nothing 

further happens, but if the temperature continues to rise and 

reaches the second cooling setpoint, a second fan comes on. If all 

of the fans are on, and cooling is still not adequate, then th e pump 

comes on and begins circulating water through the wetwall and 

evaporative cooling-the third stage of cooling. As the greenhouse 

cools down, the sequence reverses. Heating stages consist of a 

sequence of burners and heat distribution fans.

Climate control computers. The computer revolution has 

radically changed the way in which the environment in propagation 

structures is controlled. Climate control computers use 

microprocessors to combine information from a variety of sensors 

and provide an integrated view of all factors in the propagation 

environment (fig. 1.4.9). Computers can sense and record climatic 

information from an outside weather station as well as atmospheric 

or growing medium conditions within the propagation structure. 

Climatic indexes, such as vapor pressure deficit, used to be 

difficult to monitor in operational nurseries but this task is now 

possible with computers.

113

Computers are essential in high-tech greenhouses to integrate all 

the various environmental control equipment. Instead of only 

switching a single piece of equipment ON or OFF, climate control 

computers can modulate, which produces an infinite number of 

adjustments. They also compile and analyze the full complement of 

environmental data to make "intelligent" decisions (Argus Control 

Systems 1990). For example, during the winter, light sensors tell 

the climate computer that the sun is setting, and so the demand for 

heat can be anticipated before the temperature actually begins to 

drop (Ball 1991). Increasing energy costs and concern about 

excess fertilizer runoff make computer control

systems even more attractive. Documented energy savings can 

range from 15 to 30% for typical nurseries to as high as 40 to 60% 

for high-tech greenhouses with all the latest equipment (Whitesides 

1991).

A typical system consists of a central computer terminal and 

individual controllers and alarms at different locations throughout 

the various propagation environments (fig. 1.4.9). One terminal 

should be located in the headhouse so that personnel can 

instantaneously monitor all environmental factors in each 

propagation environment, as well as analyze stored data to 

compute trends and detect problems. Many growers also install 

computer terminals in their homes so that they can respond to 

potential problems without having to go to the nursery. One of the 

greatest advantages of climate control computers is that they are 

able to accurately record what really happens in the nursery so 

that this information can be used for troubleshooting and finetuning environmental control equipment (Bartok 1993). Computers 

also can be linked to a more sophisticated alarm system that can 

be redundantly programmed. With this technology, computers can 

often identify the location and nature of a problem, so that the 

grower is not needlessly bothered.

Nurseries with propagation structures larger than 2,000 m2 (21,500 

ft2) can usually justify computer control systems, which pay for 

themselves in 3 to 5 years (Mackenzie 1993). Computer systems 

come in many different models, offering a wide variety of different 

features (Greenhouse Manager 1994b). Developers should consult 

with other nurseries and suppliers to make sure that the system is 

well matched to their requirements. Maintenance is rarely a 

problem because replacement parts can usually be obtained by 

overnight courier, and companies offer specialist support by 

telephone.

1.4.2.8 Nursery security and emergency equipment

Forest and conservation seedlings are a valuable crop and so it 

makes good sense to protect this investment with a security 

system. Seedlings grown in greenhouses are particularly 

succulent and can be damaged or killed by only a few hours of 

excessively high or low temperatures. Less sophisticated security 

systems are needed for simpler growing structures or open 

compounds, which may only require a low-temperature alarm for 

unseasonal frosts.

115
A variety of different security systems are available and can alert 

the nursery manager in case of electrical power failure, mechanical 

problems with environmental control equipment, fire, or burglary. 

Alarm systems are relatively inexpensive, ranging from around $50 

for a simple bell temperature alarm to automatic telephone dialing 

units that can be purchased and operated for a couple of hundred 

dollars per year. The cost varies with the type of sensor and alarm 

system. Many types of sensors are available that can detect almost 

any type of emergency situation, from simple thermostats or 

thermisters that detect temperatures that are too high or too low to 

ultrasonic sensors that can signal an unwanted intruder (Bartok 

1987). Alarm systems range from bells and sirens to sophisticated 

alarms that can automatically dial a sequence of phone numbers in 

case the first is busy or does not answer. Computer alarm systems 

can detect

116

and indicate specific equipment failures. These "smart systems" 

allow the grower to diagnose many problems from home so that 

traveling to the nursery may not be necessary.

Standby electrical generators are essential for most nurseries 

because power failure is an unfortunate reality, especially in 

remote areas where many container tree nurseries are located. 

Loss of electrical power can be disastrous during cold periods 

because most growing structures rely on electricity for fuel ignition 

and heat distribution. Conversely, in hot climates, electricity is 

needed to run ventilating fans and cooling system water pumps. 

Used generators are available from surplus equipment stores and 

should be sized to operate all critical equipment. A minimum of 1 

kilowatt (kW) of generator capacity is generally adequate for every
184 m2 (2,000 ft2) of growing area (Nelson 1991). Small to 

medium-sized generators can utilize existing natural gas or 

propane but larger machines are best suited to diesel fuel. Because 

gasoline can become stale, it should only be considered for small 

portable generators that are used on a regular basis. With proper

ventilation, generators can be located inside structures, or outside 

on a concrete pad (fig. 1.4.10A). To limit noise, a residential-grade 

exhaust system should be specified. Fully automatic control 

systems are available that constantly monitor line voltage, start and 

stop, and recharge their batteries without an operator. All 

generators must be serviced routinely and should be run through a 

full load test at least once a year (Charlton 1992).

Propagation structures that rely on fossil fuels for heat rarely have 

alternative heating systems to ensure that heat is available in case 

of loss of the primary fuel supply. Problems with natural gas 

supplies are rare, but supplies of fuel oil or propane should be 

monitored and refilled regularly to maintain an adequate reserve. 

Portable radiant heaters can be used as an inexpensive backup 

heat source as long as proper ventilation can be provided (fig. 1 

.4.1 OB). Salamander heaters burn kerosene at a rate of 1.9 to 3.8 

liters/h (0.5 to 1.0 gal/h) and can protect up to 140 m2 (1,500 ft2) of 

production area in a propagation structure (Nelson 1991).

Crops grown with photoperiod lights or blackout curtains are 

extremely sensitive to equipment failure. For example, if 

photoperiod lights fail for only one night, shoot growth can cease 

and the seedlings will set a dormant terminal bud—a situation 

that is difficult or sometimes impossible to reverse within the 

growing season. The most direct way to monitor the lighting 

system is to link a photocell to the lighting controls, which is a 

standard feature of most computer alarm systems.

117
1.4.3 Benches and Supports for Containers

The process of growing seedlings consists of a sequence of 

operations that begins when the seeds or cuttings are delivered to 

the nursery and ends when the seedlings are shipped to the 

outplanting site (fig. 1.4.11). Before this can happen, however, the 

propagation area must be outfitted with some sort of support 

system for the containers.

Container crops can be grown directly on the floor, on pallets, 

racks, or raised benches, and the choice is critical from both a

biological and operational standpoint. The way in which forest and 

conservation plants are positioned affects their growth and 

development. Whereas other crops may be grown directly on the 

floor or on traditional benches, container tree seedlings have
aggressive root systems that quickly grow out the bottom of the 

container (fig. 1.4.12A). The roots from seedlings in containers on 

the ground will grow into the soil (fig. 1.4.12B). These external 

roots must be pruned before the seedlings can be shipped, which 

not only requires extra labor but also reduces seedling quality. To 

cause roots to desiccate and air prune, an air space must be

provided under the container. Some containers are designed so 

that they promote air pruning (fig. 1.4.12C), but others must be 

grown on benches with wire mesh or other supports to create the 

necessary air layer (fig. 1.4.12D).
Operationally, containers must be positioned so that they make the 

most efficient use of the propagation area and can be easily 

handled and moved. The container support system must be 

compatible with the handling system; a nursery that is designed to 

move seedlings with forklifts will have a different system than one 

using conveyors. Aisles provide access for workers and equipment, 

but they also reduce production space. Main aisles of 0.9 to 1.5 m 

(3 to 5 ft) and side aisles of 0.5 to 0.8 m (1 .7 to 2.5 ft) are typical 

(Aldrich and Bartok 1989).

1.4.3.1 Pallets

Container nurseries that are designed to maximize materials 

handling use some sort of wood, synthetic, or metal pallets that 

can be handled by pallet jacks or forklifts (fig. 1.4.13A). The 

dimensions of the pallets must be designed so that they can be 

easily moved by the handling equipment and also must fit through 

the door of the propagation structure (fig. 1 .4.13B). Access must 

also be considered for buildings such as the headhouse, if that is 

where the containers will be sown or the seedlings packed. 

Pallets should be constructed so that they will stack easily to save 

space when not in use (fig. 1.4.13C). Calculations for designing 

pallets so that they accommodate the maximum number of 

containers as well as fit within the propagation structure are given 

in section 1.3.4.3 of this volume.

1.4.3.2 Benches

Benches put seedlings at a safe and comfortable working height for 

thinning, weeding, and other cultural tasks, including inspections for 

diseases and other problems. In propagation structures with 

heating and cooling systems, benches allow good air circulation 

under the crop. Under-bench heating is not only more efficient, but 

warm air rising through the seedlings warms their roots, stimulating 

better growth, and also dries their foliage, reducing the incidence of 

foliar disease. For maximum space efficiency, benches can be 

designed specifically for a given type of container (fig. 1.4.14A) 

although most nurseries use a standard bench that will 

accommodate several container types.

Stationary benches. The surface dimensions vary considerably, 

but bench height is fairly standard at about 70 to 80 cm (28 to 32 

inches), which is best for safe and comfortable working. Widths 

from 1.2 to 1.5 m (4 to
cost of stationary benches can vary considerably based on type 

of materials and design features (table 1.4.3).

Movable benches. Movable or rolling benches are a rather 

recent innovation that, while allowing good access, increase the 

production space efficiency as much as 10 to 25%. They are 

constructed of metal or wood and are of two general designs (fig. 

1.4.15). The lateral design features permanent bench supports, 

but the bench tops move laterally so that an aisle can be created 

at intervals by moving the bench tops by hand

5 ft) allow workers to reach containers in the middle (Aldrich and 

Bartok 1989). Many stationary benches are homemade, consisting 

of wooden or concrete block legs and wooden or metal frames (fig. 

1.4.14A&B). If they are not designed for one particular type of 

container, bench tops are constructed of heavy-gauge wire 

screening (fig. 1.4.12D) or galvanized expanded metal (fig. 1 

.4.14C) so that any type of container can be used. The
or with special handles (fig. 1.4.15B&C). Benches as long as 61 m 

(200 ft) can be moved in this manner and, when the access aisle 

is not needed, then the sections can be rolled together to produce 

a continuous bench. Aisles are typically about 46 cm (18 in) wide 

although they can be expanded to 69 cm (27 in) if more space is 

required (Aldrich and Bartok 1989). The end-to-end movable 

benches feature a permanent middle aisle and pallets that move 

end-to-end on benches with rollers. With this design, a side aisle 

can be created where needed and the pallets can even be moved 

out of the propagation structure through the endwall (fig. 

1.4.15D&E). Movable benches can be fabricated at the nursery or 

purchased commercially (Greenhouse Manager 1993); their 

higher cost must be balanced against the increased production 

space (table 1.4.3).

122

1.4.3.3 Construction materials

In the humid nursery environment, the frames of pallets or benches 

should be constructed of aluminum or galvanized steel to resist 

rusting, or pressure-treated wood must be used to prevent decay 

(see section 1.3.3.4 for further discussion). The bench supports 

can be made of several different materials, including treated wood, 

metal pipe, or concrete blocks. Raised benches should be able to 

support at least 122 kg/m2 (25 pounds per square foot). Bench 

tops are made of wire mesh, expanded metal, or wooden slats to 

promote good drainage and air pruning. Wire mesh tends to sag if 

not properly supported, whereas expanded galvanized metal, 

although more expensive, is much sturdier (Langhans 1980).

Molded plastic pallets and benches are just recently being 

marketed, and some are constructed of recycled plastic or 

plastic/wood composites. The surface of these new materials will 

not splinter and is smooth so that containers will slide easily. 

Plastics also clean easily, which is a real consideration for 

preventing development of algae and moss and sanitizing between 

crops. Lumber made of recycled plastic/wood composites is 20 to 

30% heavier than natural wood, and some types flex slightly under 

warm temperatures. Costs are competitive with those of 

pressure-treated lumber (Sorvig 1993).

124
1.4.4 Handling Equipment for Materials and

 Seedlings

After the container support system has been selected, then the best 

way to move seedlings and materials throughout the nursery must 

be decided. Every nursery must have a system for moving 

materials from storage areas to work areas, and containers in and 

out of the growing area (fig. 1.4.11). Some nurseries use the same 

method of transport throughout all processes, such as those 

designed around a pallet system, and some can be quite 

sophisticated (fig. 1.4.16A&B). Other nurseries use a combination 

of equipment.

The handling system must be decided early in the nursery 

development process because it helps determine both structure 

design and location. In designing a handling system, emphasis 

should be placed on minimizing wasted time and motion, and flow 

diagrams should be sketched for all key operations such as 

container sowing and packing. The novice developer should visit 

other container nurseries during these

operations to get an idea of the processes involved. The handling 

system should be designed so that key equipment and workers are 

supplied with a constant flow of materials. For example, the critical 

operation in a hand sowing line is done by the person doing the 

seeding, and so keeping this person supplied with filled containers 

will reduce labor expenses and make the entire operation go more 

smoothly. Many nurseries use conveyors to move sown containers 

into the propagation area and mature seedlings to the packing 

shed. Bartok (1986) provides a good discussion of other 

considerations for designing an efficient, but safe, system for 

handling materials.

Typical handling systems use conveyors, carts, and pallets to 

move seedlings and materials through the nursery. Their costs 

will vary with the sophistication of the system.

125
1.4.4.1 Conveyors

Conveyors are often used to speed container movement during 

sowing or packing and to move filled containers to the propagation 

areas. There are 4 common types: roller conveyors, belt conveyors, 

chain conveyors, and trolley conveyors. Roller conveyors are 

unpowered and are available in 1.5- to 3.0-m (5- to 10-ft) sections 

on which the materials are physically pushed by hand (fig. 1.4.17A). 

They are somewhat limited because materials must have smooth, 

flat bottoms or be placed on a piece of plywood. Belt conveyors (fig. 

1.4.17B) are powered by electric or hydraulic motors and are 

available in a variety of widths and lengths; heavy-duty models can 

handle 136 to 182 kg (300 to 400 lbs). Folding and extension belt 

conveyors come in sections and can reach as far as 30 m (100 ft). 

Chain conveyors are similar but use moving chains instead of belts; 

they also require containers with smooth bottoms (fig. 1.4.17C). The 

last category is trolley conveyors, which are not powered and run 

on an overhead track connected to the frame of the growing 

structure (fig. 1.4.17D). This use must be considered during 

construction so that the structures can handle the added load 

(Bartok 1991 b).
Conveyors are particularly useful during sowing and packing to 

move the containers along the line and also to supply materials to 

the workers (fig. 1.4.17E). With large automated container filling 

and sowing equipment, belt elevator conveyors are used to keep 

hoppers supplied with growing media. With the variety of different 

conveyors that are available, nursery developers must carefully 

consider the full range of uses before making a selection.
1.4.4.2 Hand carts and motor vehicles

A wide variety of wheeled carts, trailers, and motor equipment has 

been used to move seedlings and materials in container nurseries. 

Smaller nurseries rely on hand carts and pallet jacks for materials 

handling (fig. 1.4.18A). Forklifts and tractors with forklift 

attachments have many uses in container nurseries but can only 

be used on solid floors (asphalt or concrete) that can support such 

equipment (fig. 1.4.18B). Small motor carts with trailers are 

commonly used, but operating internal combustion engines in 

closed spaces where people are working is dangerous. So, 

electrically powered or propane-powered equipment is 

recommended (fig. 1.4.18C). Nurseries growing their seedlings in 

open compounds can use fuel-powered tractors and trailers, some 

of which have been custom-made to handle containers (fig. 

1.4.18D).
1.4.5 Seedling Production Equipment and

Supplies

The objective of this section is to give the nursery developer an 

idea of what will be needed to produce that first crop. The 

container tree nursery system can be broken down into separate 

processes (e.g., sowing seed) that consist of operations (e.g., 

placing the desired number of seeds per container). Each of these 

processes have specific requirements including equipment (e.g., 

seeding machine) and supplies (e.g., seed and electricity). (See 

section 1.1.4 for more discussion on this concept and 

terminology.)

To be most efficient and cost effective, seedling production 

equipment should be chosen so that it fits into an overall sequence 

(fig. 1.4.11). All the various steps in the process can be done by 

hand and so the need for equipment is dependent on the size and 

complexity of the nursery and the resources available. Worker 

safety is also a consideration because many operations can lead to 

fatigue and injury, especially tasks requiring repetitive motion, and 

so should be mechanized if possible. The propagation method for a 

particular species is also a consideration. With some crops, 

equipment cannot be justified, but for others, it would be strongly 

advised. For example, a nursery growing 2 million pine seedlings 

could significantly cut production costs by purchasing a seeder. 

Another nursery, specializing in small orders of native plants with a 

variety of seed types and sizes, would do better to sow everything 

by hand. The time interval in which the process must be 

accomplished is also significant. In evaluating the need for 

equipment, keep in mind how often and how fast the process must 

occur. If it will occur often, or there is a lot of it to do in a short time, 

it will pay to mechanize. However, if the process occurs only once 

or twice a year or it can be spread over considerable time, it may 

be more cost effective to keep the mechanization simple and hire 

more workers. This is particularly true in developing countries, 

where labor is relatively inexpensive and nurseries provide 

much-needed local employment.

1.4.5.1 Containers

Types of containers. Many different sizes and types of 

containers have been used in forest and conservation nurseries, 

and the choice will depend on the species of plant being grown, 

the type of nursery system, and the conditions on the outplanting 

site. Nurseries that grow their seedlings on contract will often 

have the container type controlled by the specifications of the 

contract, and some contract customers will even supply the 

containers. The choice of container type is one of the most critical 

in nursery development because it affects the design of the 

propagation area, type of benches, and the choice of handling 

and production equipment.

Although tree improvement seedlings and other specialty stock are 

often grown in large single containers (fig. 1.4.14C), the majority of 

forest and conservation plants are grown in aggregate containers 

known as blocks, trays, or racks (see fig. 1.3.21). The individual 

containers within the block are called cells or cavities. Besides 

being much smaller in volume than the typical container used to 

grow other ornamental or horticultural crops, many woody plants 

have aggressive root systems that will spiral in normal containers, 

especially at the bottom. Root spiraling greatly reduces seedling 

quality because it can cause instability after outplanting (fig. 

1.4.19A). To solve this problem, special containers that are unique 

to the forest nursery industry have been designed with 

anti-spiraling ribs or are coated with root pruning chemicals (fig. 

1.4.19B). (An estimate of typical container costs is provided in 

table 1.5.1 in chapter 5 of this volume. A comprehensive 

discussion on the biological and operational consideration in 

selecting a container is provided in volume two, pages 3-35.)

129
Cleaning and sterilizing used containers. Although some 

nurseries use disposable paper or plastic bag containers, the 

majority are more durable and are designed to be reused (fig. 

1.4.20A). Before doing so, however, the containers must be 

cleaned to remove cull seedlings and residual growing media, and 

they should also be sterilized to kill fungal pathogens, moss, algae, 

and liverworts. For small nurseries, the blocks are hand cleaned by 

shaking them upside down to remove loose material and then 

immersing them in a tub of disinfectant solution or hot water (fig. 

1.4.20B). Larger nurseries have automated equipment called 

block washers that

130

use high-pressure water nozzles to clean the containers and 

steam or dip tanks to sterilize them (fig. 1.4.20C). Although 

commercial models are available, many nurseries have developed 

their own container cleaning systems, which typically use a section 

of conveyors to handle the containers as they move through the 

process. Considering cost and worker safety, steam and hot water 

can be just as effective as chemical sterilants (Peterson 1991). 

(Chemicals used to sterilize containers are discussed in volume 

five, pages 82-83.)

1.4.5.2 Artificial growing media

Although native soil is still used in some developing countries, the 

"soil" used in modern container nurseries is actually an artificial 

medium that is a blend of organic and inorganic components. (An 

example of typical growing medium costs is provided in table 1.1.5. 

A detailed discussion of growing media can be found in volume 

two, pages 43-81.)

Components. Most forest and conservation nurseries in the United 

States and Canada use an approximate 1 :1 mixture of vermiculite 

and sphagnum peat moss. Perlite can be added to increase 

porosity, and nurseries where peat moss is prohibitively expensive 

are using pine bark and other organic composts. A wide variety of 

premixed growing media can be purchased commercially in bulk or 

in loosely filled bags (fig. 1.4.21A). Several suppliers offer custom 

mixing to the grower's specifications and this option should be 

carefully evaluated before any mixing equipment is purchased. 

Slow-release fertilizer and other amendments are frequently added 

to many brands of premixed growing media. Some of these 

amendments, such as dolomitic limestone, can cause growth 

problems with some species. Therefore, growers should always 

specify that their media be prepared without amendments unless 

they are specifically requested.

Both vermiculite and sphagnum peat moss can be purchased in 

bulk, which is less expensive, and stored until they are needed. 

Vermiculite comes loose in plastic bags, but peat comes in 

compacted plastic-wrapped bales (fig. 1.4.21 B). Because plastic 

breaks down rapidly in sunlight, the bags should be stored inside or 

under plastic or canvas tarpaulins outside. Although commercial 

media are considered to be sterile, there have been problems with 

some sources of peat moss in recent years, and so many growers 

are requesting steam pasteurization of their media (fig. 1.4.21 C). 

Properly held, growing media or their components can be stored for 

many years without any loss in quality. Many nurseries, however, 

prefer mixing their own growing media not only because it can be 

less expensive but also because it allows precise control over 

composition and quality.

132

Mixing growing media. The object of this procedure is to 

thoroughly mix the components without destroying the physical 

structure of the peat and vermiculite. Both peat moss and 

vermiculite are fragile. Overmixing reduces porosity, which can lead 

to seedling growth problems and potential root disease. Small 

nurseries mix the components by hand on the floor or in a tank. The 

mixed material is usually shoveled onto a conveyor or auger that 

moves it to a hopper or bin that supplies a potting table or container 

filling equipment. Growers who handle their media properly will 

achieve adequate mixing, and consider the labor to be cheaper 

than the purchase of mixing equipment. Hand mixing introduces 

more variation into the operation, however. Variation in the 

uniformity of the growing medium and in the compaction within the 

container are the cause of many problems later in the growing 

season, because they affect both water and mineral nutrient 

availability.

Several types of mixing equipment are commercially available and, 

when properly used, can produce more-uniform growing media. 

Cement mixers can be used to mix small batches of media (fig. 1 

.4.22A), but they must be closely monitored so that mixing is 

uniform but the structure of the particles is not broken down. Larger 

nurseries can justify the expense of a batch mixer that use 

paddles and augers to mix 0.75 to 3.00 m3 (1 to 4 yd3) of growing 

medium at one time (fig. 1.4.22B). More specialized and expensive 

continuous mixing equipment is designed to deliver the growing 

medium to a container filling and sowing line at rates as high as 38 

m3/h (50 yd3/h). Continuous equipment uses ribbon mixers that 

meter growing media components from separate bins and 

incorporate them in the desired proportions (fig. 1.4.22C). These 

specialized mixers are also designed to blend incorporated 

fertilizers and other amendments at precise rates (Gleason 1986). 

Nurseries normally use this equipment to bulk-mix large volumes of 

growing medium and store it until needed. None of these mixers is 

foolproof, however, and so proper operation and monitoring are 

important in achieving a uniform, high-quality growing medium.


1.4.5.3 Sowing lines

Sowing is one of the most critical operations in container nursery 

culture, and also one of the most labor intensive. Therefore, many 

nurseries use a variety of equipment to improve the quality of the 

various tasks in the sowing process, while minimizing slow and 

expensive hand labor. Some nurseries assemble a sequence of 

different types of equipment into a sowing line. The process 

typically consists of four sequential operations:

1. Filling the containers (fig. 1.4.23A)

2. Tamping or dibbling the medium (fig. 1.4.23B)

3. Sowing the seed (fig. 1.4.23C)

4. Covering the seed with mulch (fig. 1.4.23D)
The equipment used in each of these operations is discussed in the 

following sections. Several manufacturers have developed 

mechanically sophisticated automated sowing equipment that 

combines the entire sowing line into one sequential process. The 

components of the different automated sowing lines vary between 

manufacturers, but all consist of a series of machines assembled in 

proper sequence and are typically connected with conveyors (fig. 

1.4.23E). The main advantage of automated sowing equipment is 

convenience-the entire process is contained in one package that 

can be serviced by the same company. Most offer exchangeable 

components like the vacuum sowing drum to adjust for different 

containers and seeds. Automated sowing lines are relatively 

expensive, however, and have the disadvantage that one 

component may not perform as well as the others. Nurseries that 

build their own sowing lines can use different types of equipment 

and change them if a new type of equipment becomes available.

1.4.5.4 Container filling equipment

In small to medium-sized nurseries, containers can be loaded 

with growing medium with very simple equip

136

ment, but in larger nurseries, mechanization becomes 

necessary to save labor and time. A large variety of potting 

machines and flat fillers are commercially available (Roskens 

1993).

Container filling consists of two simultaneous operations: filling the 

containers with growing medium, and compacting it uniformly. 

Containers can be filled by hand, but this introduces undesirable 

variation into the operation and is relatively slow. Many nurseries 

use a homemade filler that consists of a hopper to store the 

growing medium and a filling surface where the medium is 

compacted into the containers (fig. 1.4.24A). The growing medium 

flows out of a gate in the hopper and is then spread manually 

across the top of the container block and into the cells. Some filling 

machines have an eccentric camshaft underneath that is driven by 

an electrical motor. This machine alternatively raises and drops the 

container block, increasing the uniformity of compaction and greatly 

speeding-up the filling process. Several commercial fillers are 

available that can accommodate forestry containers (fig. 1.4.24B). 

These consist of a hopper to hold the mix and an integral conveyor 

to move the mix to where it is sprinkled over the top of the container 

block, which moves by on another conveyor

Figure 1.4.23 

(continued)—

Although some 

nurseries 

assemble a 

sowing line out

of different 

equipment, 

several brands of 

automated 

sowing lines can 

be purchased to 

do the entire 

process (E).
while being shaken with a motorized vibrator. Surplus growing 

medium drops into a bin so that it can be recycled to the hopper.

After the containers have been filled, the surface of the growing 

medium must be shaped or compacted to make space for the 

seed—a process known as tamping or dibbling. Handmade 

dibbles can be made of wood or plastic and consist of a plate with 

plugs conforming to the diameter of the cavities with a length equal 

to the depth of the depression desired. When the plates are 

pressed onto the top of the containers, the plugs compact and 

shape the medium in each cavity (fig. 1.4.24C). One automated 

dibbler uses a pneumatic press with dibbles that have a convex or 

conical tip to create a conical depression, which forces the seed to 

roll to the center of the cavity (fig. 1.4.24D). Some automatic 

sowing lines feature a rotary drum with dibbles positioned to 

conform to the spacing of the cavities in the container (fig. 

1.4.23B).
1.-4.5.5 Sowing equipment

Hand seeding, although possible with all species of forest and 

conservation plants, is generally considered to be too slow to be 

cost effective where labor costs are high. Most conifer and many 

broadleaf seeds can be sown mechanically with good precision as 

long as seed quality is high, whereas very small, very large, or 

irregularly shaped seeds are normally hand sown. Container 

nursery operations require clean, high-quality tree seed of known 

germination capability. Clean seed simplifies sowing and increases 

its speed and accuracy because all seeders work better with clean 

seed. High-germination seed (greater than 85%) permits sowing 

fewer seeds per cavity, which reduces sowing and thinning costs. 

(See chapter 2 in volume six for specific information on seed quality 

and sowing.)

Shutterbox seeders. The simplest and least expensive seeder is 

the shutterbox, a rectangular box with a seed reservoir at one end 

and a seeding frame at the other (fig. 1.4.25A). The seeding frame 

corresponds to the outside dimensions of the container block or tray 

and contains a bottom plate with a grid of holes that correspond to 

the pattern of cavities in the container. This means that a shutterbox 

is custom built for a specific type of container and if different 

container types need to be sown, then a shutterbox will have to be 

constructed for each. The top layer of the seeding frame is a shutter 

with holes of the same pattern as the bottom plate, but offset to the 

side (fig. 1.4.25B). The holes in the shutter are drilled just large 

enough to hold the number of seeds to be sown, usually 2 to 6. 

Different shutters are needed to sow a different number of seeds 

per cavity or for another type of seed.

To operate the shutterbox, the operator hand-sweeps seed from 

the reservoir onto the shutter, making certain that each hole is filled 

(fig. 1.4.25C). Excess seed is swept back into the reservoir. When 

the shutter is moved laterally so that its holes are aligned with 

those in the bottom plate, the seeds drop into the container (fig. 

1.4.25D). Operation of a shutterbox requires practice, but 

experienced workers can achieve acceptable seeding rates with 

this simple device. Shutterboxes can be custom-made of wood, 

metal, or plastic, and are also available commercially. They work 

best with roughly spherical seeds like those of pine species, and 

will jam if seeds are dirty.

138
Vacuum plate seeders. The vacuum plate seeder consists of a 

hollow frame that is connected to a shop vacuum cleaner (fig. 

1.4.26A). The frame is the same size as the container block and 

has a grid pattern of holes over the bottom surface corresponding 

to the positions of cavities in the container blocks. The number of 

holes at each position determines the number of seeds per cavity. 

When the hollow frame is held over the seed tray and a vacuum is 

applied, seeds are held on the bottom plate (fig. 1.4.26B). The 

frame is then

aligned over the container block and the vacuum is broken, 

allowing the seeds to fall into the container cavities (fig. 

1.4.26C). Vacuum plate seeders can be homemade, and 

manufactured ones are relatively inexpensive.

Shutterbox and vacuum plate seeders are ideal for small to 

medium-sized nurseries because they require low initial capital 

investment and are surprisingly productive. Both are reasonably 

precise in sowing the desired

139
number of seeds per cavity, but most nurseries use hand sowing 

to correct deficiencies (fig. 1.4.26D). Actual production figures 

vary considerably based on differ140

ences in sowing lines, labor, seed quality, and supervision. 

However, the potential production is 300,000 to 500,000 cavities 

per machine per day.
Automated and precision seeders. The next level of container 

seeders are more mechanically sophisticated and expensive but 

are very efficient at delivering exact numbers of seed to each 

cavity (Reid 1994). Several models of automated seeders can be 

purchased that use a variety of seed delivery techniques, and 

sowing precisions of greater than 90% are possible. Vacuum drum 

seeders feature a hollow drum that picks up seeds from a hopper 

and drops them, one row at a time, when a container passed 

underneath on a conveyor belt (fig. 1.4.23C). Some automated 

seeders hold one or two seeds at a time on a vacuum nozzle and 

then distribute them through drop tubes to the containers (fig. 

1.4.27A). Most can accommodate different types of containers with 

slight modifications, but vibratory feeders can handle different 

sizes or shapes of seed better than the vacuum models. Vacuum 

seeders are generally faster, however.

The most sophisticated type of seeding equipment are precision 

seeders that are specifically designed to place one seed per cavity, 

thus eliminating the need for thinning or transplanting (Reid 1994). 

Sowing efficiencies can be as high as 98% (fig. 1.4.27B). Some 

models pick up single seeds with needle-point vacuum heads (fig. 

1.4.27C), others use vibrating feeders to isolate single seed 

(singulation) and electric eyes to control the seeding rate, whereas 

yet others use vibratory singulation and pocket drop distribution 

systems (fig. 1.4.27D). Precision seeders should only be used with 

the highest quality seed: well cleaned, uniform in size, and of very 

high germination rate. The production capabilities of the various 

models vary widely, but all are relatively expensive. Precision 

seeders can usually be justified in larger nurseries with efficient 

sowing lines. Precision seeding technology is still developing and 

so, before deciding to purchase a precision seeder, nursery 

developers should talk to other nurseries to learn how they function 

under operational conditions.

The ultimate in precision seeding is to place a pregerminated 

seed in each container, assuring nearly 100% cell occupancy and 

eliminating costly blanks or thinning costs. Research into 

pregermination protocols and experiments in liquid or gel sowing

are continuing, and equipment has already been perfected for 

some horticultural crops. The problem with most forest and 

conservation species is that seed quality is extremely variable due 

to the difficulty of cleaning and processing the seed as well as 

complex seed dormancy. Nurseries growing large seedlots of 

species with uniformly clean, high-germination seed may find use 

for this new equipment, however.

Seed mulching (gritting). After the containers are seeded, the 

seed is covered with a thin layer of sand, grit, vermiculite, perlite, 

or some other mulching material. Seed mulches physically hold the 

seed in place and prohibit excessive growth of algae, moss, or 

liverworts, which can delay or completely inhibit seed germination 

(fig. 1.4.28A). White materials like perlite or poultry grit are 

recommended because they reflect sunlight to keep the seed cool 

and moist (fig. 1.4.28B). The objective of seed mulching is to apply 

the mulch evenly over the top of the container block to the depth 

desired. Mulches that are too thin will allow the seed to dry out, 

whereas those that are too thick may inhibit seed germination. 

Irregularly applied mulch may be worse than none at all because it 

will cause uneven germination rates and complicate later cultural 

practices (fig. 1.4.28C). Mulching is sometimes done by hand, but 

more often by simple machinery that is often homemade (fig. 

1.4.28D). Automated seeding lines accurately meter out the mulch 

into each container as it passes underneath on a conveyor (fig. 

1.4.23D).

141


1.4.5.6 Transplanting equipment

At the present time, transplanting seedlings from one container to 

another is not common in forest and conservation nurseries. 

However, some nurseries hand transplant extra seedlings to fill 

empty cells during the germination stage, especially when seed is in 

short supply. In the last several years, several types of mechanical 

transplanters have been developed to transplant miniature plugs of 

flowers and vegetables into other larger containers (Onofrey 1993). 

There is also a need for transplanting equipment for tree seedlings 

because production space in greenhouses is expensive and the

cost of hand transplanting is prohibitive. One automatic 

transplanter in the development stage transplants miniature 

seedling plugs grown under intensive culture into larger containers 

that can be raised under extensive culture (Hodgson 1993). The 

seedlings are grown for a few weeks in Miniblock® containers with 

a special growing medium that holds the root plug together. The 

transplanting process consists of predibbling the larger capacity 

containers (fig. 1.4.29A) and then mechanically pushing the 

miniplugs down into the cavities (fig. 1.4.29B). Operational trials of 

this new propagation technology are currently underway and initial 

results look promising.

1.4.5.7 Pesticide application equipment

In spite of a grower's best efforts at prevention, it will occasionally 

be necessary to use pesticides to protect the crop. In container tree 

nurseries, pesticides are typically applied as seed treatments, liquid 

sprays and drenches, baits, and aerosols. The best application 

method will depend on several factors: the type of pesticide, legal 

restrictions, characteristics of the target pest, sensitivity of the crop, 

environmental conditions, and type of equipment.

144
The majority of pesticides are applied as sprays, however. Foliar 

sprays are used for exposed insects and pathogens, and drenches 

for root problems. Both techniques require mixing the pesticide with 

water and applying it to the problem area with portable sprayers or 

injecting it through the irrigation system. Foliar sprays require 

moderate- to high-pressure nozzles to ensure even coverage of the 

seedling shoot, and hollow- or solid-cone nozzles are 

recommended. Drenches require lower pressure flooding nozzles 

that allow the pesticide to infiltrate the growing medium completely. 

The pesticide formulation will also affect the type of spray 

equipment because some materials, such as wettable powders, 

require agitation. Bohmont (1983) is an excellent source of 

information on all aspects of pesticide use. (See volume five, pages 

76-93 for specific information on pest management strategies, and 

pesticide application, handling, and storage.)

Portable sprayers. Growers should constantly monitor their 

crops so that pest and disease problems can be detected when 

the problem is still confined to small scattered areas. Then, these 

areas can be spot-treated with portable sprayers (fig. 1.4.30A). 

Portable sprayers come in all sizes, and the choice will depend on 

the amount of pesticide that needs to be applied. Growers should 

never mix more chemical than can be used in one application, 

making hand sprayers ideal for small jobs, but higher capacity 

equipment is needed for more extensive spraying (fig. 1.4.30B).

New technology is revolutionizing pesticide application. Although 

they are more expensive, low-volume sprayers are rapidly 

replacing traditional hydraulic sprayers for nursery applications 

(Bartok 1992). Whereas traditional sprayers use large volumes of 

diluted pesticides, low-volume sprayers use relatively small 

quantities of concentrate. They save time and money because 

they cover the crop more evenly and quickly, and because they 

use a concentrate, the labor and potential hazards of mixing are 

eliminated.

Electrostatic sprayers are particularly attractive for container 

nursery use because the small portable models that are currently 

available can be used safely at close distances (fig. 1.4.30C). 

These sprayers produce very small positively charged droplets that 

disperse well because they repel each other. Coverage is excellent 

because the droplets are attracted to the negatively charged plant, 

and so can reach even the bottoms of leaves. Air-assisted 

electrostatic sprayers are more effective than gravity or mechanical 

types (Lindquist and Powell 1991).

Boom sprayers. When larger areas require treatment, boom 

sprayers are the most efficient way to apply pesticides. Nurseries 

with fertilizer injection equipment can also inject pesticides and 

other chemicals directly into the irrigation system. Both foliar 

sprays and drenches can be applied, depending on the nozzle 

used and the length of time of application. This technique is also 

safer because the operator does not have to be in the greenhouse 

while the application is in progress. Although fixed sprinkler 

systems can also be used, their poorer coverage means more 

pesticide will be wasted and there will be more runoff (Dumroese 

and others 1992). Pesticides can be applied to open growing 

compounds with mobile spray booms (fig. 1.4.30D).

Possible water pollution from agricultural pesticides is an 

increasing concern and so anyone purchasing pesticide 

application equipment should consider pollution potential in 

addition to cost and efficiency.

145

1.4.6 Seedling Harvesting Equipment

The next step in the seedling production cycle involves moving 

seedlings to an area where they can be graded and packed for 

storage (fig. 1.4.11). Some nurseries do this in the aisles of the 

propagation area, whereas others move the seedlings to the 

headhouse or other work area. The way in which the seedlings will 

be delivered to the outplanting site determines what type of 

harvesting and storage system will be needed. There are two 

options: shipping the seedlings in their growth containers, or 

removing them and shipping them in boxes. Nurseries that choose 

the former store their seedlings in sheltered storage and use 

specially equipped trucks to deliver them to the outplanting site. 

Sophisticated equipment has been developed to handle the 

seedlings at the nursery and load them onto the delivery trucks (fig. 

1.4.16A&B). When seedlings are shipped in their containers, 

storage racks must be designed to protect the seedlings and 

prevent them from being crushed (fig. 1 431 Al_

Not every container cavity yields a usable seedling, however, and 

the process of eliminating plants that are too small or that have 

other obvious defects is called grading or culling. The main 

advantage of culling is that the volume and weight of storing and 

shipping the seedlings are lessened, and planting will be more 

efficient. Nurseries that can produce a high percentage of 

shippable seedlings can avoid the extra cost of grading. Most 

seedling users expect that only plantable seedlings will be shipped, 

and so nurseries assemble grading and packing lines to process 

seedlings for storage and shipping.
1.4.6.1 Grading and packing lines

The process of extracting seedlings from their containers, grading, 

and packaging them- commonly called pulling and wrapping—has 

become increasingly popular in recent years. This is a 

labor-intensive process, and so nurseries combine the sequence of 

tasks into grading and packing lines that are connected by 

conveyors (fig. 1 .4.31 B). These can be rather simple set-ups in 

which all the steps are completed by hand, from pulling the 

seedlings and grading them, to wrapping them and placing them 

into storage boxes. Larger nurseries use a variety of equipment to 

speed up the process and make it more efficient.

In addition to reducing labor costs, the mechanization of grading 

and packing has become necessary because of

the high incidence of injuries to workers. Many forest and 

conservation seedlings have aggressive root systems and develop 

a firm plug by the end of the growing cycle. The roots of some 

species even grow into small holes in the walls of the container 

cavities, especially with Styrofoam blocks. This makes the 

seedlings difficult to remove from the containers, and nursery 

workers on the packing line often develop tendonitis and other 

chronic wrist and lower arm injuries. To make the seedlings easier 

to extract, many nurseries have developed custom equipment. 

Mechanical shakers use a jolting motion to loosen the seedling 

plugs from their containers (fig. 1.4.32A) as the first step in the 

packing line. This is followed by seedling extractors that physically 

push one row of seedlings at a time from the container and onto 

the grading belt (fig. 1.4.32B).
Once the workers have extracted the seedlings from the containers, 

they visually inspect them and place them on the grading belt in 

bunches of 5 to 25 seedlings. The next step is to bag or wrap the 

bunches of seedlings and place them into the storage boxes. 

Bagging machines (fig. 1.4.32C) hold a supply of plastic bags that 

are automatically inflated by a flow of air, making it easier to insert 

the seedlings. The bags are just deep enough to enclose the root 

plugs to retard desiccation, but keep the seedling shoot uncovered 

to reduce the chance for storage molds. Other nurseries use plastic 

film to wrap the root plugs of the seedlings into bundles before 

boxing (fig. 1.4.32D). The final step in the grading and packing 

process involves sealing the storage boxes, marking them with the 

species, seedlot, number of seedlings, and other important 

information. Then they are transported to storage until they can be 

shipped (fig. 1.4.11).

1.4.6.2 Storage

After the seedlings have been transported to the storage areas, 

there is no need for any other equipment until they are shipped to 

the outplanting site. In sheltered storage, seedlings should be 

stored on the ground to protect the roots systems, which are much 

more susceptible to freezing that the tops. They should be bunched 

together to minimize the perimeter and their root systems insulated 

with sawdust or other material (fig. 1.4.33A). Seedlings in 

refrigerated storage must be stored on racks or tiered pallets to 

support their weight and avoid crushing (fig. 1.4.33B). The storage 

boxes should be lined with a thin plastic bag to retard moisture loss 

while still permitting oxygen exchange. Properly packaged 

seedlings do not require humidity control in the storage building. 

(Sheltered storage and refrigerated storage are discussed in 

section 1.3.5.4, and will be covered in detail in volume seven of this 

series.)
To be most efficient and cost effective, seedling production 

equipment should be chosen so that it fits into an overall 

sequence. All the various steps in the process can be done by 

hand, and so the need for equipment is dependent on the size 

and complexity of the nursery as well as the available technology, 

finances, and labor. With some crops, equipment cannot be 

justified but, for others, would be strongly advised. In evaluating 

the need for equipment, nursery managers must keep in mind 

how often and how fast the operation must occur. If the process 

will occur frequently or must be done in a short time, it may pay to 

mechanize. Worker safety is also a consideration because many 

operations can lead to fatigue and injury and so the process 

should be mechanized if possible.

1.4.7 Summary

Two general categories of equipment are used in container tree 

nurseries. A wide variety of equipment is used to modify the 

propagation environment to optimize the six factors limiting to 

seedling growth: temperature, humidity, light, carbon dioxide, 

water, and mineral nutrients. The degree of modification depends 

on the type of structure and the environment control equipment 

that is provided. Seedlings raised in open growing compounds can 

be supplied with irrigation and fertilization, and light and 

temperature can be controlled to a minor degree. Container 

nurseries that grow their seedlings in some sort of a propagation 

structure have the potential to control all the limiting factors. These 

factors can be controlled individually or, in fully controlled 

environments, sophisticated controls (including computers) balance 

all factors to produce conditions that are ideal for seedling 

propagation. Forest and conservation seedlings are a valuable 

crop and so nurseries use security systems to detect electrical 

power failure, mechanical problems with environmental control 

equipment, fire, or burglary.

The second category of container nursery equipment includes the 

materials and equipment needed to produce a crop of seedlings. 

Growing tree seedlings consists of a sequence of processes that 

begin when the seeds or cuttings are delivered to the nursery and 

ends when the seedlings are shipped to the outplanting site. 

Container crops can be grown directly on the floor, on pallets, 

racks, or benches, and the choice is critical from both a biological 

and operational standpoint. Biologically, tree seedlings must be 

supported so that their roots will air -prune, and operationally, 

containers must be positioned so that they make the most efficient 

use of the propagation area and can be easily handled and moved. 

The container support system must be compatible with the 

handling system, and nurseries use forklifts and conveyors to 

move seedlings and supplies.

150
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