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Agriculture Handbook No. 149

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CONTENTS

Page

Location and construction of greenhouses 1

Lettuce varieties for forcing 3

Production and care of seed 5

Rotation and intercropping 5

Greenhouse soil and its management 7

Soil sterilization 8

Soil preparation 10

Starting the plants 11

Soil for seedlings 11

Planting the seed 11

Seed treatments to improve

germination 12

Transplanting 12

Establishing the crop plants- 13

General care of the crop 15

Watering 15

TemperatTire and ventilation 15

Cultivation 16

Page

Harvesting and marketing 16

Diseases and their control 17

Drop 17

Bottom rot 18

Gray mold 18

Downy mildew 18

Anthracnose 18

Rosette 19

Tipburn 19

Root knot 19

Insect pests and their control 19

Preventipn of infestations 20

Precautions in the use of insecticides 20

Aphids 20

Caterpillars 21

Greenhouse whitefly 21

Two-spotted spider mite 21

Slugs, snails, and sowbugs 21

Garden symphyHd 22

List of scientific names 22

Thompson, Ross Calvin, 1896-

Growing lettuce in greenhouses [by Ross C. Thompson, S.

P. Doolittle and T. J. Henneberry, Washington] U. S. Dept.

of Agriculture [1958]

22 p. lllus. 24 cm. (cU. S. Dept. of Agriculture] iVgriculture

handbook no. 149)

"Replaces Farmers' bulletin 1418, Lettuce growing in greenhouses."

1. Forcing (Plants) 2. Greenhouse management. c2. Greenhouse

cultivation] 3. Lettuce. i. Title. (Series)

SB351.L6T47 1958 685.52 Agr 58-355

U. S. Dept. of Agr. Libr. lAg84Ah no. 149

for Library of Congress c*nt

This publication replaces Farmers' Bulletin 1418, Lettuce Growing

in Greenhouses.

Washington, D. C. Issued November 1958

For sale by the Superintendent of Documents, U. S. Government Printing Office

Washington 25, D. C. - Price 15 cents
By Ross C. THOMPSON/ horticulturist, and S. P. DOOLITTLE, pathologist, Crops

Research Division, and T. J. HENNEBERRY, entomologist, Entomology Research

Division, Agricultural Research Service

Growing lettuce under glass is one of the oldest vegetable-forcing

industries. Lettuce is well adapted for forcing during the colder

months of the year, when light and temperature are less favorable for

other vegetable-forcing crops. Much lettuce was grown in hotbeds

and coldframes before greenhouses were generally used for growing

vegetables. Some lettuce is still grown in frames in early spring and

late fall, when only limited protection from cold is required. However, most of the forced crop is now grown in greenhouses especially

designed for growing vegetables, and therefore the market can be

supplied with fresh lettuce throughout the winter, when the weather

is too cold for growing crops in frames.

With the expansion of the field-grown lettuce industry into regions

with mild winters, the demand for greenhouse-grown lettuce has

gradually declined. Although field-grown head lettuce can now be

obtained in most markets the year round, there is still a demand for

high-quality greenhouse-grown lettuce during the winter. Growing

lettuce under glass still constitutes a sizable industry in certain districts, where its production has become highly specialized and the

winter market has been well organized and maintained. The industry

has now become fairly well stabilized. It centers around Grand

Rapids, Mich., Rochester, N. Y., Ashtabula, Cleveland, and Toledo,

Ohio, and Boston, Mass.

Growing lettuce in greenhouses is an expensive and highly specialized business, and it should not be undertaken commercially without

practical knowledge in the growing and marketing of the crop.

LOCATION AND CONSTRUCTION OF GREENHOUSES

Since many growers of greenhouse lettuce also plant other crops,

they should consider the requirements of these crops in planning the

construction of new greenhouses. Lettuce, tomatoes, and cucumbers

are the important vegetable-forcing crops, and many growers plant

all three at different seasons. Since lettuce is grown chiefly during

the winter, when the days are short and there are many hours of dull,

cloudy weather, it is important that greenhouses be constructed so

as to afford the maximum of light. It is important that the tempera-

' Retired July 31, 1958.
2 AGR. HANDBOOK 149, U. S. DEPT. OF AGRICULTURE

ture be properly controlled and that drafts be reduced to the minimum

consistent with good ventilation. Efficient heating equipment is

essential.

Some important points to be considered in selecting a location for a

vegetable-forcing greenhouse include (1) nearness to market, (2)

availability and cost of labor, (3) supply and cost of fuel, (4) soil

suitable for the crops to be grown, (5) water supply, and (6) freedom

from smoke and fumes from industrial plants.

The type of lettuce grown in greenhouses is not so well adapted for

long-distance shipment as the hard-headed type grown in the field.

It tends to wilt quickly unless it is kept moist and cool. The delicacy

of the product makes it imperative thatthe crops be grown fairly near

the market.

Labor is a major item in the cost of almost all commodities, agricultural as well as industrial. Capable, well-trained help is often difficult to obtain. The available labor supply should be kept in mind

in selecting a location for a vegetable greenhouse.

The cost of fuel has always been a large item of expense in greenhouse maintenance, and it has risen sharply in recent years. Soft coal

has been the chief source of fuel for greenhouse heating, but oil and

gas have replaced itto some extent.

Although the soir used in greenhouses can be made to suit the crop

to be grown by adding fertilizer, organic matter, lime, and other soil

amendments, it is highly desirable that the range be located where the

soil is naturally suitable for the crops to be grown. The lighter types

of soil are preferred for greenhouse-vegetable production.

There should be an adequate and constant supply of water free of

materials that may be toxic to plants or that may corrode the heating

system. Highly chlorinated water is not desirable. Avoid a high

concentration of salts.

Smoke from industrial plants reduces light intensity through its

presence in the air and forms a film on the glass, which greatly reduces the intensity of the light that reaches the plants. Avoid reducing the light of dull winter days by smoke or fumes. Do not locate

greenhouses near industrial plants because of the smoke and also the

fumes, which may be toxic to certain pi ants.

Most of the newer greenhouses built for growing lettuce are evenspan structures of steel, aluminum, or steel and aluminum. Wood is

used much less than in the past. Although the original cost of wood

construction may be low, the upkeep in painting, glazing, and other

repairs over a period of years adds greatly to the total cost.

Lettuce can be grown in almost any type of greenhouse in which the

lighting is good, except the lean-to type, which is not suitable for commercial lettuce growing.

The ridge-and-furrow type of construction (fig. 1) is widely used to

enclose a large area under one roof. This kind of structure permits

an almost unlimited area to be enclosed with a minimum of obstruction

to interfere with cultural operations and the entrance of light.

Large even-span metal-frame greenhouses (fig. 2) are being widely

used for vegetable forcing. The use of steel and aluminum in the

framework adds greatly to the strength and life of these greenhouses,

and they afford better lighting than the old-style wood-frame

buildiníífs.
GROWING LETTUCE IN GREENHOUSES

FiGUEE 1.—Kidge-and-furrow type of greenhouse, which permits enclosure of very

large areas under one roof.

FIGURE 2.—Steel-frame greenhouse, ideal for vegetable forcing.

Most greenhouse lettuce is grown on ground beds. In large ranges

tlie entire floor of the greenhouse is fertilized, plowed, and tilled in

much the same manner as a field.

LETTUCE VARIETIES FOR FORCING

Of the many lettuce varieties, only a few are suitable for forcing. In

the past numerous varieties of heacl, Cos, and leaf lettuce were grown

under glass, but now two or three constitute almost the entire commercial crop.

Only in the Boston district is head lettuce forced in quantity. In the

past Belmont, May King, Big Boston, and Salamander were grown

under glass with varying success.

Bel-May is a small butterhead variety of lettuce (fig. 3, A) developed by the Massachusetts Agricultural Experiment Station. It is

still the important greenhouse variety in the Boston market-garden

district, but it is not grown in other districts.

None of the crisp head varieties of lettuce are well adapted for growing in greenhouses. Paris White, Trianon, Bath, Express, and other
AGR. HANDBOOK 149, U. S. DEPT. OF AGRICULTURE

BX-6794X, BX-e795X

FIGURE 3.—The two most important varieties of lettuce grown under glass : A,

Bel-May, a small butterhead variety ; B, Grand Rapids, a leaf, or nonheading,

variety. •
GROWING LETTUCE IN GREENHOUSES 5

varieties of Cos lettuce have at times been grown as forcing crops but

with limited success.

Of all the types of lettuce, the leaf variety Grand Eapids (fig. 3, B)

is the most popular, and it is also the most important greenhouse

forcing variety. There are several strains of Grand Rapids, each

adapted to particular locations and growing conditions. The Washington strain, sometimes called Grand Rapids Forcing, was developed

by the United States Department of Agriculture. It is very popular

with the Grand Rapids, Mich., growers. A forcing strain of Grand

Rapids that is resistant to tipburn was developed by the Ohio Agricultural Experiment Station. Sometimes known as Resistant Grand

Rapids, it appears a few days earlier than the other strains. A third

one, Grand Rapids U. S. No. 1, also developed by the Depar'tment,

is an important strain in New York and other districts where lettuce

is forced. It is slower to bolt than the other Grand Rapids strains.

PRODUCTION AND CARE OF SEED

Only those lettuce varieties that are known to be adapted for forcing

should be grown under glass. High-quality seed of a good strain is

even more important for the forcing industry than for field production.

The supply of such seed is often limited, and therefore many large

growers have found it profitable to produce their own seed.

By carefully selecting the most desirable plants for seed production,

a strain can be developed that is especially adapted to local conditions

and resistant to certain diseases. In working for disease resistance,

selection should be made under conditions favorable for disease and

where disease is present. A pound of seed will be produced from 50

to 75 well-grown lettuce plants. Lettuce averages about 350,000 seeds

per pound. The seed of Grand Rapids is small and well below the

average in size. A pound of high-quality lettuce seed, if properly

handled, will produce enough plants to set an acre.

Plants selected for seed production should be transplanted from the

beds to some part of the greenhouse where they can be given special

attention (fig. 4). Do not save seed plants that are even slightly offtype lest the strain deteriorate. Unless the greenhouse is screened and

all insects are kept out, cover each plant with a light-weight muslin

bag just before the first flowers open. Keep the coverings on until all

flowers have set seed. If the bags are left on until the seed is mature,

the plants can be cut and left in the bags until seed-cleaning time.

After a desirable strain has been isolated, it need not be increased

every year. Enough seed for 2 or 3 years can be grown and kept until

used if it is stored in a cool, thoroughly dry place. Lettuce seed loses

its viability rapidly under humid conditions.

ROTATION AND INTERCROPPING

Growing lettuce in greenhouses should be considered in relation to

growing other forcing crops. Very few growers produce a single crop

throughout the year. At least part of the year most of them grow

other vegetables, such as tomatoes or cucumbers, or some kind of

flowering plant.
AGR. HANDBOOK 14 9, U. S. DEPT. OF AGRICULTURE

FIGURE 4.—A crop of Grand Rapids lettuce seed nearing maturity in a greenhouse.

Since lettuce is the only one of the three important vegetable-forcing

crops that thrives during the short, dull days of winter, growers

usually plan to have it occupy the greenhouse during this period.

A crop of tomatoes is often followed by a crop of lettuce, which is followed by a spring crop of tomatoes or cucumbers. Two or three crops

of lettuce sometimes follow a fall crop of tomatoes. If cucumbers are

used in the rotation, they are usually grown as a spring crop, when

light and temperature are more favorable than during the fall and

winter months. Rotation plans in common use in large ranges are

given in table 1. If it is desired that lettuce be available for cutting

continuously for an indefinite period, plantings should be made on

several successive dates, so that only part of the crop will reach cutting

size at one time.

TABLE 1.—Greenhouse crop rotations, with planting and harvesting

dates

Plan and crop Seed sown Plants set Crop harvested

Plan 1 :

Tomatoes _ _ July 1-15

Nov. 15-30

Feb. 1-15

July 1-15

Nov. 15-30

Feb. 1-28

Aug. 1-15

Oct. 15-30

Jan. 1-15

Aug. 15-30

Dec. 15-31

Apr. 1-15

Aug. 15-31

Dec. 15-31

Mar. 15-31

Sept. 1-15

Nov. 15-30

Mar. 1-15

Dec. 15-31

Lettuce - __ ^ Mar. 15-31

Cucumbers

Plan 2:

Tomatoes

July 1-31

Dec. 15-31

Lettuce Mar. 15-31

Do June 1—15

Plan 3:

Lettuce

Do

Tomatoes -__

Nov. 15-30

Feb. 1-28

Before Aug. 1
GROWING LETTUCE IN GREENHOUSES

FIGURE 5.- -Leaf lettuce intercropped with cvicunibers. The lettuce is to be

harvested before the cucumber vines begin to run.

In plan 1, the time between the removal of the cucumbers and the

planting of tomatoes may be used for soil sterilization. In plan 2, it

is possible to grow a third crop of lettuce in part of the space. Growers

who follow this cropping plan usually count on 2i/^ crops of lettuce.

Since leaf lettuce can be grown to salable size in about 8 weeks from

the transplanting date, it is often planted between rows of tomatoes

or cucumbers (fig. 5) and is harvested before the tomatoes or cucumbers reach such size as to interfere too much with its growth.

Intercropping of lettuce presents difficulties, and the product is

usually not of the highest quality. Since cucumbers and tomatoes

both require temperatures too high for lettuce, either lettuce or the

crop it is grown with is likely to suffer from improper temperature.

The shading effect of tall-growñng tomatoes and cucumbers where

the vines are trained on trellises makes growing conditions unfavorable for lettuce. In general, intercropping tomatoes or cucumbers

witli lettuce is not a good practice.

GREENHOUSE SOIL AND ITS MANAGEMENT

Growing lettuce and other vegetable-forcing crops in greenhouses

is an intensive and expensive operation. Maximum production must

be reached quickly and maintained continuously. Most ordinary

field soils are not suitable and often require large amounts of organic

matter, fertilizer, and lime. Sometimes sand or ashes are added to

change the texture and physical properties and thus to make the soil

better suited for intensive crop production. Few^ field soils are ideal

for greenhouse-vegetable growing. If it is possible to select the soil

upon which the greenhouses are to be constructed, choose land that

will require as little modification as possible.

For greenhouse-vegetable production the soil should be lighter than

is generally considered suitable for growing the same crops in the

471105°—.58 2
8 AGR. HANDBOOK 149, U. S. DEPT. OF AGRICULTURE

field. The lighter soils are favored because (1) they are more easily

handled in transplanting, tilling, and sterilizing than heavy soils;

(2) their surface dries more quickly, and thus disease is less apt to

become established and spread; and (3) they are more easily kept

in good physical condition, and since they are less likely to become

packed, they afford better aeration.

Where the original soil is too heavy, make it more friable by incorporating sand and well-decomposed organic matter. Muck is ideal

for supplying organic matter and making soil more friable. However, muck alone is not suitable for vegetable forcing.

Good drainage is essential. Unless the land selected for the greenhouse site is already well drained, provide proper drainage when the

greenhouse is constructed. Tile installed for drainage may also be

used for soil sterilization. If underlain by a heavy soil, the tile can

be used for subirrigation.

Soil Sterilization

The control of plant pests in the greenhouse is more necessary and

sometimes more difficult than in the field. Almost universally greenhouse growers practice soil sterilization to control diseases, insects,

nematodes, and other plant pests. Sterilize greenhouse soil at least

once a year whenever it can be done with the least interference with

crop production. This is usually during the summer, when high temperatures are unfavorable for greenhouse crops.

Most large greenhouse ranges for forcing vegetables are equipped

to sterilize with steam, which is the most effective and widely used

method of controlling soil-borne plant pests. New greenhouses should

be provided with such equipment. The most generally used devices for

applying steam in soil sterilization are (1) the inverted pan, (2)

perforated iron pipes, and (3) drain tile. The method to be used must

be determined largely by the conditions.

The inverted pan' is best adapted for porous soils, which offer the

least resistance to penetration by steam. The equipment consists of a

wooden or metal pan of almost any convenient size. Galvanized sheet

iron is more durable and lighter'than wood. Convenient-sized pans

are 4 to 6 feet by 10 to 12 feet and 6 to 8 inches deep. However, the

size and shape of the pans should be determined by the dimensions of

the planting beds, the capacity of the boiler, and the means of moving

the equipment. Each pan is provided with a pipe connection for

attaching the steam hose. The pan is inverted, and the edges are

forced into the soil a few inches to prevent the escape of steam. If

the steam pressure is high, it may be necessary to add some additional

weight to hold the pan in the soil. Some large growers have the pan

equipped with wheels and a mechanical device by which the apparatus

is lifted and rolled when it is necessary to move the pan (fig. 6). ^ The perforated-pipe arrangement consists of 4 to 6 perforated iron

pipes, 114 or 11/2 inches in diameter and 20 to 70 feet or more in length,

which are fastened to a header pipe, 2 inches in diameter, at sj)acings

of 12 to 18 inches. The header pipe is provided with a connection for

attachment to the steam hose. The perforations are usually i/s or i/4

inch in diameter and 12 inches apart. The size of the equipment is

determined by the size of the greenhouse, boiler capacity, and help
GROWING LETTUCE IN GREENHOUSES

21369

FIGURE 6.—Mechanically movable steam pan being used for sterilizing greenliouse soil. The equipment is lifted by a special hoist and moved to a new

location on wheels, which roll on the concrete walks.

available for moving. This equipment is buried to a depth of 6 to 10

inches in the soil, and the entire surface is covered with plastic or

a heavy canvas to confine the steam. If two or more sets of equipment are available, one set can be buried and made ready while sterilization is going on in another section. The chief objection to this

method of sterilization is tlie great amount of labor required to bury

the pipes.

A variation of this method is known as the rake, or steam-harrow,

system. The equipment consists of a framework of metal pipes with

tees at intervals of 6 to 8 inches, to which about 6-inch lengths of small

iron pipe, usually ^ inch in diameter, are connected. The lower ends

of these small pipes are closed by flattening to form wedges. Holes,

/s or %6 inch in diameter, which are drilled through each pipe just

above the wedge, provide openings for the escape of steam. The

header pipe is equipped with a connection for attachment to the steam

hose. The perforated pipes, or pegs, of this equipment are forced

into the soil, and the surface is covered with plastic or a heavy

canvas to confine the heat. Where the boiler capacity is sufficient

to supply the steam, several of these rakes can be operated continuously by a group of workmen. The rake is especially well suited

for small greenhouses and ranges having raised benches.

A drain tile 4 inches in diameter and set about II/2 feet deep in rows

11/^ to 2 feet apart can be used for both steam sterilization and subirrigation. However, tile lines may be a source of trouble, because

they provide a shelter and breeding place for crickets, roaches, and

other pests.
10 AGR. HANDBOOK 149, U. S. DEPT. OF AGRICULTURE

In order to destroy the most resistant plant pests, heat the soil to a

depth of several inches to 212° F. for a half hour or more. The time

required for the soil to reach this temperature will depend on the

amount of steam, soil texture, and type of equipment used. Excessive

heating breaks down organic materials and leaves the soil in a poor

condition for the growing of plants. Plant growth is more satisfactory when steam-sterilized soil is permitted to lie idle for 3 to 4

weeks before it is used.

.Soil Preparation

The best soil for forcing crops is one naturally deep, well drained,

and friable. In large ranges where the crop is grown on ground beds,

the soil remains in place from year to year. Its productivity is maintained by the addition of manure, or other organic matter and fertilizer, and by sterilization. Animal manure is the best source of organic

matter. Apply it at the rate of 1 ton to each 1,500 square feet of surface, or about 30 tons per acre. Because of the increasing scarcity and

cost of animal manure, greenhouse growers have had to find substitutes

for part of the organic matter and to supplement manure with

chemical fertilizers. Muck is a good source of organic matter and can

be used as a substitute for manure if supplemented with ^ chemical

fertilizer. Experiments show that good results can be obtained with

15 tons per acre of manure plus half a ton per acre of a 3-12-4 fertilizer or other commercial fertilizer having a similar formula.

Except on highly fertile soils already well supplied with phosphorus, lettuce yield generally increases with an application of

phosphorus in an available form. Superphosphate is preferred to

the less soluble forms of bonemeal and ground rock phosphate.

Foliage crops like lettuce use large amounts of nitrogen. However,

' avoid overfeeding with available forms of nitrogen such as nitrate of

soda and sulfate of ammonia. Excessive stimulation with nitrogen

causes a rank, succulent growth that is subject to disease. Lettuce does

not have a high potassium requirement. However, best results with

lettuce are generally obtained when a complete fertilizer supplying

nitrogen, phosphorus, and potassium in the proportion of about 1 part

of nitrogen, 2 or 3 parts of phosphorus, and 1 part of potassium is used.

When making a heavy application of chemical fertilizer, spread it

uniformly and work it thoroughly into the soil before planting. Side

dressing with nitrate of soda or sulfate of ammonia after the crop

has become established should not exceed about 200 pounds per acre.

Continued use of large amounts of nitrate of soda may in time result in

toxic residues.

Lettuce is more sensitive to soil reaction than are tomatoes or

cucumbers. When tomatoes or cucumbers are grown in rotation with

lettuce on the same soil, note especially the requirement of lettuce,

Avhich does best on soils that are only slightly acid. If the soil is

moderately or very acid, correct the acidity by applying lime. It is

usually unsafe to exceed the amount indicated by lime-requirement

tests.

Lime may be applied as ground limestone, ground burned limestone,

or hydrated lime. Ground burned limestone gives the quickest results; ground limestone is slow in its reaction. About one-half as
GROWING LETTUCE IN GREENHOUSES 11

much ground burned limestone or about three-fourths as much

hydrated lime is required to give the same change in soil reaction as

a given quantity of ground limestone. Once the soil in a greenhouse

has been brought to the proper condition of acidity, a moderate application once a year as indicated by lime-requirement tests will generally

maintain a satisfactory soil reaction. More lime will be required

when acid-forming materials like sulfate of ammonia are used in the

fertilizer than when base-forming materials are used.

STARTING THE PLANTS

Greenhouse lettuce is usually grown from transplants rather than

by seeding in the beds where the crop is to be grown. Enough seedlings for an entire range can be grown in one greenhouse, where

uniform growing conditions can be maintained. A large part of the

range is thus available for other crops while the lettuce plants are

being started.

The seedlings are grown on solid beds or raised benches with or

without the use of flats. Better plants can generally be obtained if

flats are used, and the plants are more easily handled.

Soil for Seedlings

Use a good grade of composted soil for starting the plants. Prepare the compost pile several months or preferably a year in advance.

This prepared soil should be free of large clods, stones, or trashy

organic matter. Two parts of field sod to one part of stable manure,

preferably horse or cattle manure, makes a good compost. If the soil

is acid, add ground limestone to the compost pile to correct the condition. It is generally desirable to add a few pounds of a complete

fertilizer high in phosphorus to each ton of compost. A 5-10-5

fertilizer or one having a similar analysis is suitable. Cut down the

compost pile and turn once and preferably twice to incorporate the

manure thoroughly. The lime and fertilizer can best be added at

the firstturning.

Large ranges are equipped with soil shredders, through which the

compost is passed before it is used for planting. If the prepared

compost contains coarse material, pass it through a screen. It is

advisable to sterilize all compost used for starting the plants, although

this is not always done. An entire range may be infected with a

disease that originates in the plant bed or seed flat.

It is important that the soil used for starting the plants be friable

enough to fall apart readily and permit the separation of the seedlings

with a minimum of root injury. Small plants start off more rapidly

after transplanting and are much less subject to injury by soil-borne

organisms if the root system is kept intact than if the roots are broken

or otherwise injured.

Planting the Seed

Since greenhouse lettuce does not sell well until local field-grown

lettuce is off the market, set the fall crop so as to be ready to harvest

it soon after the field-grown crop is no longer available. About a

month is required to produce plants for setting. Growth is slower
12 AGR. HANDBOOK 14 9, U. S. DEPT. OF AGRICULTURE

and more time is required to produce the crop during the short days

of winter than when the days are brighter and longer. When lettuce

is not grown with other crops, plant the seed from August 1 to 15

for a crop to mature the last part of October or early November. In

large ranges where several crops are grown during the fall, winter,

and spring, planting and harvesting go on continuously and several

plantings are necessary.

Drill planting of lettuce seed is preferable ; however, the seed may

be planted broadcast. Drop the seed at the rate of 8 to 12 per inch

in very shallow furrows spaced about 2 inches apart. A suitable

planting furrow can be made with the edge of a thin piece of wood

such as a lath. Cover the seed to a depth of not more than one-eighth

inch with muck or leaf mold mixed with sand. Muck is not a good

covering when used alone, as it is too light and tends to be pushed

up in a sheet as the seed germinates, and therefore a weak, elongated

seedling may result. Muck mixed with sand to add weight makes

an ideal cover for lettuce seed. Sometimes burlap or other cloth is

substituted for the soil covering, and it is thoroughly wet after being

laid to help hold the moisture until the seed germinates. When cloth

is used, it is essential to remove it as soon as the seed starts to germinate.

Seed Treatments To Improve Germination

The seed of many lettuce varieties germinates poorly if planted soon

after harvest. Such dormant seed requires a month or two of afterripening in storage under dry conditions before it will germinate well.

Some lettuce seed germinates poorly at temperatures of 75° F. or

above, especially the Grand Rapids variety. When the greenhouse

temperature can be controlled, keep it at 65° to 70° until the lettuce

seedlings have emerged aboveground. Good, viable, nondormant seed

will germinate in less than 24 hours at this temperature. At higher

temperatures most lettuce seed will require at least 3 to 5 days for

emergence, depending somewhat on the depth of covering.

If dormant lettuce seed is to be planted when the greenhouse temperature cannot be held below 75°, germination can be greatly improved by soaking the seed in a i^-percent solution of thiourea for 8

to 10 hours at 65° to 70°. After removal from the solution, the seed

may be planted while damp, or it may be scattered thinly on a dry

surface until dry and then stored until planting time. The increase

in germination due to the thiourea treatment will remain effective for

several months if the seed is stored in a cool, dry place. A similar

treatment with water alone will aid in the germination of most lettuce

seed.

Dormant lettuce seed can be made to germinate more readily if

given a cold treatment just before planting. Germination has improved greatly when dormant lettuce seed is soaked in water for 2

or 3 hours and then held at 39° with good aeration for 4 to 6 days.

With the cold treatment the seed cannot be dried and stored. It

must be planted while moist to benefit from the treatment.

Transplanting

Greenhouse lettuce is usually transplanted, or pricked off, twice,

(1) from the planting bed or seed flats to other flats and (2) from
GROWING LETTUCE IN GREENHOUSES 13

FIGURE 7.—Plants for a 10-aere range transplanted into flats and placed on a

greenhouse bed.

these flats to the beds where tlie crop will be grown. Transplanting

is expensive, but it produces better plants. Do the first transplanting

about a week after the seedlings emerge. The interval between emergence and transplanting depends on the growth rate and the thickness

of seeding. If the seedlings are close together or the temperature

in the greenhouse is high, transplant sooner than if the greenhouse

is cool or the plants are not crowded. If vigorous, stocky plants are

to be obtained, do not delay transplanting until the seedlings become

crowded. The less the roots of the seedlings are broken or injured

in lifting from the plant beds or flats, the sooner the plants will recover

after transplanting.

The seedlings are sometimes transplanted into small pots or paper

bands, but the common practice is to transplant them into standard

planting flats (fig. 7). Allow^ about 2 inches between the plants in

the flats. The flats in general use hold 60 to 75 plants at the 2-inch

spacing. Experienced workers can transplant as many as 5,000 plants

a day. As soon as the plants are set in the flats, water the soil. It is

important that the first watering be thorough. Apply enough water

to wet the soil to the bottom of the flat. Apply the water as a fine

spray so as not to injure the plants by covering the leaves with mud.

Be especially careful if the plants are wilted.

ESTABLISHING THE CROP PLANTS

After transplanting the seedlings, leave them in the growing flats

or pots until good, sturdy plants have developed but not until they

are crowded. Holding too long before the final transplanting results

in elongated, bleached plants that are stunted and start off slowly.

Planting distances of 7 by 7 inches to 9 by 9 inches are generally used.

Some growers plant closer in one direction than in the other to make

cultivation easier. Planting in squares allows the plants to develop

uniformly. The greater number of plants required for close planting
14 AGR. HANDBOOK 149, U. S. DEPT. OF AGRICULTURE

(table 2) adds to the cost of plants, increases the chances for disease,

and may not increase the total yield per unit area enough to justify the

practice.

In setting the plants in the greenhouse beds, some kind of marker

is essential for spacing the plants accurately. One such device (fig. 8)

consists of a broad board with wooden pegs, which are li/^ inches in

diameter and about 2 inches long and are attached at the desired planting distances. It speeds the work of setting and assures accurate

spacing of the plants. The workers kneel on the board while they

are transplanting. As the plants are being set in holes already made

by the marker, other holes are being made.

Retain as much soil as possible on the roots in removing the plants

from the flats, so thatthere will be a minimum check in growth. Running a knife through the soil bet^veen the plants in both directions

TABLE 2.—Planting distance and number of lettuce 'plants required fer

1.000 square feet and per acre

Distance between

plants (inches)

6 by 7

6 by 8

6 by 9

7 by 7

Plants for—

1,000

square

feet

Number

3,429

3,000

2,667

2, 939

1 acre

Number

149,368

130, 680

116, 176

128, 023

Distance between

plants (inches)

7 by 8

7 by 9

8 by 8.

8 by 9.

Plants for1,000

square

feet

Number

2, 571

2,286

2,250

2,000

1 acre

Number

111,993

99, 579

98, 010

87, 120

21861

FiGUKE 8.—Setting lettuce plants in the greenhouse. The board serves as a

support for the workers as well as a marker for spacing the plants.
GROWING LETTUCE IN GREENHOUSES 15

helps to retain the soil around the roots. Too deep setting of lettuce

plants is ruinous. It is better to set them a little shallow than too

deep. The soil should come almost to the bottom pair of leaves. Leaf

petioles and blades are much more subject to decay by soil organisms

than stems.

GENERAL CARE OF THE CROP

Watering

Lettuce, which requires a great deal of water, should never be permitted to suffer from want of moisture. Thorough irrigation is much

better than frequent light wettings. Frequent wetting of lettuce

leaves tends to spread diseases. Moisture favors drop, bottom rot,

gray mold, and mildew. The sooner moisture can be dried from the

leaves after wetting the better. There is less danger of foliage diseases getting started in lettuce during the early stages before the

plants cover the soil and when ventilation and air movement are good.

Subirrigation is ideal for lettuce, because moisture can be kept off the

leaves.

Overhead irrigation insures more uniform distribution of water,

reduces labor costs, and provides less chance for damage to plants

than hand-operated nozzles.

Pipelines equipped with sprinkling nozzles spaced about a foot

apart are fastened to the supporting parts of the greenhouse. A

single line of pipe will handle a 40- to 50-foot width of greenhouse

space. By inserting a swinging joint near the valve end, the entire

line can be rotated so as to cover all the space between the lines.

Temperature and Ventilation

Lettuce is a cool-season crop and thrives best at fairly low temperatures. High temperatures favor rapid growth, but they increase the

chances for disease and may cause weak, spindling plants. Temperatures may be high with less danger to the plants during the early

stages of growth than when the plants are larger and air movement

between them is reduced. It is important that temperatures be held

low as the crop nears maturity.

There should be a difference of 10° to 15° F., and perhaps more, between day and night temperatures; however, sudden changes in temperature should be avoided. The exact temperatures that should be

mamtained within a greenhouse for growing lettuce must be determined to some extent by the conditions outside, as they affect the

amount of ventilation that can be used. After irrigation, when the

humidity within the greenhouse is high, it is best to maintain the temperature a few degrees lower than if the foilage were dry and the

humidity low. Mght temperatures of 45° to 50° and day temperatures 15° higher are satisfactory for lettuce during the winter. However, in early fall and late spring it may be impossible to maintain

these optimum temperatures.

Ventilate the lettuce as much as outside conditions will permit. The

top vents should never be closed tightly, except during very cold or

stormy weather. Stagnant, humid air contributes to the development

of some diseases of greenhouse lettuce.
16 AGR. HANDBOOK 149, U. S. DEPT. OF AGRICULTURE

Cultivation

The removal of weeds, which is one of the chief purposes of cultivating crops grown in the open, is generally not a problem in greenhouses.

Once the soil in a greenhouse has been steam sterilized, weeds should

not be troublesome unless their seeds are introduced in manure and

other materials used in preparing the soil. Shallow cultivation to

break up the surface crust may be beneficial while the plants are small,

especially if the soil contains considerable clay. However, after the

plants have begun to cover the soil, cultivation may do more harm than

good. Cultivation is usually done with ordinary hand tools. Longhandle tools can be used on narrow beds, but some kind of trestlework

on which workmen can stand is desirable for wide beds. If the spacing is wide, wheel hoes may be used for cultivating small plants.

HARVESTING AND MARKETING

The time required to produce a crop of lettuce, from transplanting

in the greenhouse beds until harvesting, ranges from 6 to 12 weeks,

depending on the market demand as regards plant size and the season

of the year. When prices are good, it may be profitable to harvest

before the plants reach maximum size even though this means some

loss in total yield. When lettuce is sold by weight, growers like

to allow the plants to become as large as possible without reduction in

quality. A longer time is required to produce plants of marketable

size during the short days of winter than during the smmier, longer

days. If another lettuce crop is to follow the one about ready to

harvest, the condition of the seedlings for the next crop may make

it necessary to harvest earlier than otherAvise to avoid damage from

holding the seedlings in a crowded condition.

In harvesting, the plants are cut just above the soil surface with a

short-blade knife. Sometimes they are trimmed and packed in the

greenhouse as they are harvested. However, most large establishments

are equipped with washing and packing sheds, and then only the

spoiled, dirty outer leaves are removed in the greenhouse. The plants

are placed in containers and taken to the shed where they are trimmed,

washed, and packed for market (fig. 9).

The washed plants are packed wet, as some moisture is required to

keep them fresh in transit. If the bottom of the packing containers

is tight, holes should be made to permit the escape of the water that

collects atthe bottom.

There is no established standard container for packing and shipping

greenhouse lettuce. Various kinds of packages, baskets, hampers,

boxes, and barrels have been used. Rectangular splint baskets holding

about 10 pounds of lettuce are generally used by Ohio growers. Large

containers are usually lined with heavy paper. Paper linings afford

some protection from freezing when lettuce is shipped in cold weather.

The container covers are of paper to prevent wilting of the lettuce

before it is placed on the market.

Grades and marketing standards are subject to periodic changes.

Information about legal standards can be obtained from the Agricultural Marketing Service, United States Department of Agriculture.
GROWING LETTUCE IN GREENHOUSES 17

FIGURE 9.- -Harvesting lettuce in bushel baskets for removal to the packing shed,

where it is to be trimmed, washed, and packed.

The profit to be expected from a given unit area of greenltonse

lettuce depends on many factors—labor, fuel, fertilizer, and other

production costs, market demand, quality of the product, and yield.

Any estimate of profit that might be expected could only be a rough

approximation. About a pound of lettuce per square foot of greenhouse s])ace is considered a good yield. Yield ])er unit area depends

largely on the stage of development at whicli tlie crop is harvested.

In order to obtain a yield of a pound per square foot of space, the

])lants must reach near-maximum size before cutting. Market demand

and prices may make it more profitable to harvest smaller plants even

though the total yield is reduced.

DISEASES AND THEIR CONTROL

Drop

Dro]) is possibly the most damaging disease of greenliouse lettuce.

It is caused by fungi that persist in the soil. The stem is infected at

the soil line, after which the leaves progressively wilt and the entire

plant eventually collapses. Affected plants show a soft watery rot

of the stems and leaves near the soil. The decayed tissues are soon

covered with a cottony-white fungus growth, which produces black

sclerotia, or seedlike bodies. These are found in or on tlie decayed

leaves and stems, tlieir size depending on the species of fungus present.

Sclerotia are resistant to an unfavorable environment and serve to

maintain the fungus in the soil.

Drop is particularly apt to cause trouble during damp, cloudy

weather. Too high a temperature, improper ventilation, overwatering, and crowding the plants tend to increase losses from the disease.

Proper management of the crop, particularly using water sparingly

during the 3 or 4 weeks before harvest, helps to check the spread of

di'op. When tlie disease appeal's, remove and destroy affected plants,
18 AGR. HANDBOOK 149, U. S. DEPT. OF AGRICULTURE

including taproots and surrounding soil. This helps to prevent infection of adjacent plants and also the formation of sclerotia. When

drop causes severe loss, the best remedy is to sterilize the soil with

steam (seep. 8).

Bottom Rot

Bottom rot is destructive in some sections of the country, especially

to head lettuce, wliose outer leaves rest on the ground. It most frequently occurs as the plants approach maturity. The disease is caused

by a soil-borne fungus. Brown spots develop on the midribs and

blades of the leaves. This begins at the bottom of the plant and is

followed by a brown soft rot that extends into the head. There is a

broAvn fungus growth over the leaves and formation of brown

sclerotia.

Excess moisture favors bottom rot, and losses can often be much

reduced by using no more water than is essential for satisfactory

growth. When this measure is not effective, sterilize the soil with

steam (seep. 8).

Gray Mold

Gray mold is caused by a fungus that is most likely to be damaging

when plants are weakened by unfavorable growing conditions or by

other diseases. It is characterized by a brown soft rot of the leaves,

which become covered with a grayish mass of fungus growth. When

plants are severely damaged, black sclerotia may be produced.

Gray mold often follows tipburn, as the fungus readily attacks the

dead areas caused by tipburn at the leaf margins. Ample ventilation

and soil sterilization with steam (see p. 8) help to prevent or delay

the development of gray mold.

Downy Mildew

Downy mildew is characterized by yellowish or brownish spots on

the outer leaves. The lower surface of the leaves becomes covered

with a white fuzzy growth of the causal fungus.

Insufficient ventilation, overwatering, and fluctuating temperatures

favor the development of downy mildew. Careful management of

the greenhouses, particularly the avoidance of high humidity, is

usually effective in controlling this disease.

Anthracnose

Anthracnose, also known as shot hole or rust, is caused by a fungus

that produces reddish-brown roughly circular spots on the leaves. The

centers of the older spots often drop out and give a shot-hole appearance. On the midribs the spots are narrow and slightly sunken. The

fungus lives on infected plant refuse and its spores are water borne.

Low temperatures and overwatering favor this infection, especially

when water is applied as a coarse spray that spatters soil on the plants.

Plant losses can be reduced by usmg water sparingly. Apply it as a

fine mist, as a slow stream from a hose, or by subirrigation to avoid

spattering soil on the leaves.
GROWING LETTUCE IN GREENHOUSES 19

Rosette

Rosette causes considerable losses of greenhouse lettuce. This disease and bottom rot are caused by the same fungus, which attacks the

plants when they are small. Rosette is characterized by a decay of the

roots that stunts the plants, especially when they have been badly set,

improperly fertilized, or grown on poor soil.

Good growing practices help to reduce loss, but soil sterilization

(see p. 8 ) may be necessary if the disease persists.

Tipburn

Head lettuce and, to a less extent, leaf lettuce are subject to tipburn.

This nonparasitic injury causes the margins of the leaves in the central

part of the plant to turn brown and die; The dead areas rarely extend

more than one-half inch from the edge of the leaf.

Tipburn is most prevalent when temperatures are high and the

plants are making a rapid succulent growth. It often occurs after a

few cloudy days when the greenhouses are kept at rather high temperatures with inadequate ventilation. Slightly lower temperatures than

normal and adequate ventilation will reduce losses from tipburn during the winter. When outside temperatures are high during the fall

and spring, care in the use of nitrogenous fertilizers, watering, and

ventilating will help to maintain a firm growth of leaves and hold the

disease in check.

Root Knot'

Root knot has never been considered particularly destructive to

greenhouse lettuce, but it must be controlled to obtain the best lettuce

yield. During the winter, lettuce greenhouses are usually kept at such

a low temperature that the disease remains dormant and does no damage, but during the warmer weather of fall and spring root knot may

cause serious injury. It is very injurious to both cucumbers and tomatoes, and when one or both of these vegetables are rotated in the

greenhouse, the disease should be controlled.

Steam sterilization of the soil is an effective control measure if done

carefully and thoroughly. The microscopic nematode, or eelworm,

causing this disease works down to a depth of a foot or more and

penetrates under walks and side walls. Satisfactory control cannot

be expected unless the soil is heated to at least 212° F. and to a depth

of at least a foot.

INSECT PESTS AND THEIR CONTROL

Greenhouse lettuce is subject to injury by a variety of insects and

related pests. Those most commonly encountered are aphids, caterpillars, the greenhouse whitefly, the two-spotted spider mite, slugs,

snails, sowbugs, and the garden symphylid. These pests frequently

cause serious damage to the lettuce crop. Try to prevent infestations

and apply control measures when these pests appear.

^Prepared by A. L. Taylor, nematologist, Crops Research Division.
20 AGR. HANDBOOK 14 9, U. S. DEPT. OF AGRICULTURE

Prevention of Infestations

A good sanitary program is important in preventing infestations.

Outside the greenhouse provide a clean strip free from weeds and

crops that may harbor pests of lettuce. In the greenhouse eliminate

trash and plant debris to prevent breeding and hiding places of these

pests. If possible, grow transplants in a separate house free from

pests so as to start with clean plants.

After harvesting a crop, apply a 10-percent parathion aerosol at 1

pound of solution per 25,000 cubic feet of greenhouse space to keep

the greenhouse free of insect pests for a new crop. After applying

the aerosol, keep the greenhouse closed overnight for maximum results.

Fumigation with calcium cyanide at 4 ounces per 1,000 cubic feet of

greenhouse space is also effective.

Precautions in the Use of Insecticides

iTisecticides are poisonous. Handle them with care. Follow the

directions and heed all precautions on the container label.

When handling or mixing concentrated insecticides, avoid spilling

them on your skin and keep them out of your eyes, nose, and mouth.

If you spill any on the skin or clothing, wash it off and change your

clothing immediately. Wear a respirator and goggles when you are

working with concentrated sprays or dusts.

Many insecticides can be absorbed directly through the skin in

hazardous quantities. In applying them, try to keep them off your

skin and away from your eyes, nose, and mouth. When you have

finished using them, wash all exposed surfaces of the body with soap

and water. Change your clothing if you have spilled any insecticide

on it.

Parathion^ TEPP, and calcium cyanide are extremely poisonous.

They should be applied only by a person thoroughly familiar with

their hazards and one who will assume full responsibility for safe use

and comply with all the precautions on the labels.

Store insecticides in closed containers in a dry place where children,

irresponsible persons, and animals cannot reach them.

Aerosols usually contain the poison gas methyl chloride. Regardless of the insecticide in these aerosols, they should be applied only

by a trained operator who will enforce the precautions prescribed by

the manufacturer and assume full responsibility for safety.

Do not use parathion on lettuce within 21 days or TEPP within 3

days before harvest. Do not use malathion within 10 days or nicotine

sulfate within 7 days before harvest. Do not apply DDT to lettuce

after the leaves to be marketed or eaten appear.

Aphids

Three species of aphids, or plant lice—foxglove aphid, green peach

aphid, and potato aphid—are common pests of greenhouse lettuce.

These insects are readily controlled with a parathion or malathion
GROWING LETTUCE IN GREENHOUSES 21

aerosol, except in plots or benches of dense foliage. TEPP is then

the most effective insecticide. TTse a 10-percent parathion or malathion aerosol or a 5-percent TEPP aerosol at the rate of 1 pound of

solution per 50,000 cubic feet of greenhouse space. One application

is usually adequate. Malathion may also be applied to the foliage in

wettable-powder or emulsion sprays. Use 1 pound of malathion per

100 gallons of water. Nicotine sulfate is also effective when applied

to the foliage in a spray containing 1 quart of the 40-percent solution

per 100 gallons of water.

Caterpillars

The caterpillars attacking lettuce include the armyworm, cabbage

looper, and cutworms. These insect pests may be controlled w^ith a

10-percent malathion or parathion aerosol applied at the rate of 1

pound of solution per 50,000 cubic feet of greenhouse space. Malathion may also be applied in a spray as for aphids. A B-percent DDT

dust is effective when applied to the foliage, but it should not be used

after the leaves to be marketed or eaten appear.

Greenhouse Whitefly

The greenhouse whitefly is a common pest of greenhouse lettuce. It

sucks the juices from the leaves. Infested plants become yellowish

green and often stunted.

These insects may be controlled with a 10-percent parathion or

malathion aerosol at the rate of 1 pound of solution per 50,000 cubic

feet of greenhouse space. Eepeat this treatment 2 to 8 times at 2-w^eek

intervals or until satisfactory control is obtained. Malathion may be

used in a spray as for aphid control. A 5-percent TEPP aerosol is

effective when applied at the rate of 1 pound of solution per 50,000

cubic feet of greenhouse space. Eepeat this treatment every 5 days

for 4 to 5 weeks.

Two-Spotted Spider Mite

The two-spotted spider mite is often troublesome. It feeds on the

undersurface of the leaf and produces a chlorotic condition. In extreme cases the plants may be stunted or even killed.

Apply a 10-percent parathion or a 5-percent TEPP aerosol at the

rate of 1 pound of solution per 50,000 cubic feet of greenhouse space.

Tw^o or three applications of the parathion aerosol at 7- to 10-day

intervals are necessary to destroy an infestation. Repeat the TEPP

treatment at 5-day intervals until the infestation is controlled.

Slugs, Snails, and Sowbugs

The presence of slugs and snails is revealed by their trail of slime,

as well as feeding injury. They feed at night and hide by day under

pots, benchboards, flats, or dense foliage.

Effective baits on the market usually contain metaldehyde with

2 j)ercent of calcium arsenate or 5 percent of chlordane. Sprinkle the

bait between the plants, or place about a teaspoonful in a pile at in-
22 AGR. HANDBOOK 149, U. S. DEPT. OF AGRICULTURE

tervals. Two or more applications may be required, as the slugs and

snails may not feed every night but rest for several days between feedings. Use extreme care in applying these baits. Do not apply them

to the foliage or excess residues may be left on the marketed crop.

Sowbugs feed at night or on cloudy days and hide by day under

pots, boards, or trash. Apply a 3-percent DDT dust to the soil for

effective control.

Garden Symphylid

Garden symphylids are present in the loose soil high in organic

matter that is found in most greenhouses. They feed on the roots and

do the most damage to young plants.

Soil sterilization (see p. 8) is the recommended method of control.

First moisten the soil so that the symphylids will congregate near the

surface, heat to 180° F., and hold at this temperature for 4 hours. The

method most used in vegetable greenhouses makes use of buried lines

of drain tiles spaced about 18 inches apart and connected by headers.

Steam at 15 pounds' pressure is introduced into the headers. Suitable covers of plastic or special paper over the soil help retain the

steam and insure sterilization of the soil surface. They also aid in

getting heat penetration to the maximum depth. Sterilization by the

inverted-ipan method will destroy garden symphylids in raised benches.

LIST OF SCIENTIFIC NAMES

Fungi That Cause Diseases of Greenhouse Lettuce and Insects That

Attack It

DISEASES

Common name Causal organism

Anthraciiose Marssonina panattoniuna (Berl.) Magn.

Bottom rot Rhizoctonia solani Knehn

Downy mildew Bremia lactucae Regel

Drop Sclerotinia sclerotiorum (Lib.) DBy. and S. minor

Jagger

Gray mold Botrytis cinérea Pers.

Rosette Rhizoctonia solani Kuehn

INSECTS

Common name Scientific name

Armyworm Pseudaletia unipuncta (Haw.)

Cabbage looper Trichoplusia ni (Hbn.)

Foxglove aphid Myzus solani (Kith.)

Garden symphylid Seutigerella immaculata (Newp.)

Green peach aphid Myzus pérsicae ( Sulz. )

Greenhouse whitefly Trialeurodes vaporariorum (Westw.)

Potato aphid Macrosiphum solanifolii (Ashm.)

Two-spotted spider mite Tetranyelms telarius (L.)

U. S. GOVERNMENT PRINTING OFFICE: 1958