United States 

Department of 

Agriculture Field Guide

Forest 

Service for Managing

Southwestern 

Region Malta Starthistle 

TP-R3-16-1 

January 2010 
 

 

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Malta starthistle (Centaurea melitensis L.)

Sunflower family (Asteraceae) 

Malta starthistle is an invasive plant that has been listed 

as a noxious weed in both Arizona and New Mexico. This 

field guide is intended to serve as the U.S. Forest Service’s 

recommendations for management of Malta starthistle in 

forests, woodlands, and rangelands associated with the 

Forest Service’s Southwestern Region. The region consists 

of 11 national forests in Arizona and New Mexico together 

with 3 national grasslands in New Mexico, Oklahoma and 

Texas. 

Description 

Malta starthistle (synonyms: Napa starthistle, tocalote) 

is an annual invasive weed with foliage and winged 

stems that are grayish to green in color. Its thistle-like 

appearance is similar to yellow starthistle (C. solstitialis) 

but is distinguished by smaller yellow flowers and longer 

seedpods armed with relatively short (less than 1/2 inch) 

spines. Strategies for managing both species are similar. 

Growth Characteristics 

• Winter annual and occasional biennial. 

• Grows erect to 1 to 2 feet tall. 

• Deep, simple taproot. 

• Thick leaves are held in a basal rosette through 

winter and early spring until flower stems bolt. 

Leaves are narrow and smooth edged near the tip 

and lobed at the base and are covered with thick, stiff 

“prickly” hairs and dot-like resinous glands that may 

be overlaid by fine white “cottony” hairs. 

• Insect-pollinated and reproduces by seed. Produces 

1 to over 100 solitary, spiny, yellow flower heads 

(with 1 to over 60 seeds per head) from April through 

September. Flowers are about one-third to one-half 

inch long and have a purple to brown-tinged base 

with a central spine and fine hairs. Seeds are about 

one-tenth of an inch long with gray to tan stripes. 

Ecology 

•	 Impacts/Threats – Malta starthistle is highly 

competitive and often develops dense, impenetrable 

stands that displace desirable vegetation. Malta 

starthistle can be poisonous to horses due to its 

ability to produce a nervous disorder called “chewing 

disease.” However, animals typically avoid the weed 

because of its sharp spines and hairs. The threat of 

injury from spines on the seed heads diminishes 

recreational opportunities, livestock grazing, and other 

values. 

• Location – Occurs on open, disturbed sites such 

as grasslands, rangelands, open woodlands, fields, 

pastures, roadsides, waste places, and cultivated fields. 

Found throughout most of the Western States, some 

Central States, Eastern States, as well as Southern 

States. Ranges up to 7,200 feet. Uncommon in desert 

regions. 

•	 Spread – Seeds adhere to surfaces and thus can be 

carried for long distances on undercarriages of vehicles 

and road maintenance equipment and for shorter 

distances on animals and humans. Birds can also 

transport seeds after eating them. 

•	 Contributing Factors – Excessive grazing favors 

growth of Malta starthistle over grass species, and 

infested land may need to rest for a year and a half 

after treatment before re-introducing grazing. Areas 

with soil disturbance (particularly disturbances along 

roadsides) are highly susceptible to Malta starthistle 

infestations. 

Management 

Malta starthistle grows rapidly as an invasive plant, and 

seeds may remain dormant in the soil for up to 10 years. 

Therefore, starthistle cannot be controlled within a single year 

or by using only one control method. Strategies to contain 

and reduce Malta starthistle populations require long-term 

planning and integrated management: 

• Healthy plant communities should be maintained to 

prevent or limit infestations of starthistle. 

• New populations of starthistle should be detected and 

eradicated as early as possible. 

1 
 •Mechanical, cultural, biological, and chemical 

methods to control starthistle populations should be 

combined whenever possible. 

Choice of control method(s) for Malta starthistle depends 

on the land use and site conditions such as accessibility, 

terrain, climate, density and degree of infestation, nontarget 

flora and fauna present, etc. Other considerations include 

treatment effectiveness, cost, and the number of years 

needed to achieve control. Table 1 summarizes approaches 

for the most common situations involving Malta starthistle. 

More than one control method may be needed for each site. 

Physical Control 

Physical methods to control Malta starthistle should focus 

on removal of seed heads and the root system. These 

methods usually have to be repeated and must be timed 

properly to be most effective. 

Table 1. Control Decisions 

Manual Methods 

Hand pulling and hoeing are effective for small infestations 

of Malta starthistle, but this must be done repeatedly. Plants 

should be removed in early bolt before flowers have opened 

and gone to seed. The taproot should be removed as much as 

possible. 

Mechanical Methods 

When feasible, frequent tillage with a plow or disc will 

control Malta starthistle. Tilling should be done when the 

surface soil is dry since fragmented plant segments can 

regrow in moist soil. Shallow cultivation (five or six times 

a year, 2 weeks apart) should be repeated while leaves are 

present but before plants have flowered. Regular cultivation 

for 2 or more years must be maintained for long-term 

effectiveness. 

Mowing is a commonly used technique that can reduce seed 

production; however, mowing during early plant growth 

can cause greater production of flowers and seed. Some 

Site 

Roadsides 

Physical 

Methods 

Use machinery such as

mowers or graders for

mechanical clearing. 

Cultural 

Methods 

Implement requirements for operation of

vehicles and for reporting infestations along

roads. 

Biological 

Methods 

Little 

researched 

Chemical 

Methods 

Use truck spraying

equipment. Wash underneath 

to prevent spread. 

Rangeland Use tillage or

prescribed fire when

possible. May need

to use hand tools in 

difficult terrain. 

Use certified seed. 

When moving livestock or vehicles through

infested areas, inspect and remove any seeds

from animals, clothing, and vehicles before

entering uninfested areas. 

Do not graze for 11⁄2 years after treatment. 

Little 

researched 

Use ground or aerial

broadcast spraying; however, 

backpack spraying may

be more practical in areas

difficult to access. 

Wilderness 

or Natural 

Areas 

Hand methods may be

needed to protect other 

resources. 

Post signs warning visitors to remove seeds. 

When moving livestock or vehicles through

infested areas, inspect and remove any seeds

from animals, clothing, and vehicles before

entering uninfested areas. 

Do not graze for 11⁄2 years after treatment. 

Little 

researched 

Use backpack sprayers.

Broadcast spraying by aerial

or ground methods may be

used on thicker stands if 

allowed. 

2 
vegetation management experts do not recommend mowing 

at all since mown plants often produce side branches that 

have more flowers, even with repeated mowing and proper 

timing. When appropriate, mowing should take place only 

when plants are in late bud or early bloom stage. Mowing 

should occur regularly (e.g., weekly or biweekly) at a level 

that will remove the lowest branches. Leaves should not be 

left below the level of the cut. 

Prescribed Fire 

Burning conducted from January to April can eliminate 

Malta starthistle during the rosette stage provided there is 

a source of fine fuels sufficient to carry an intense, uniform 

fire. Malta starthistle may also be burned in early to midsummer (late June to early July) during the early flower 

stage. However, prescribed fire operations during this 

period may not be feasible in some areas due to the hazard 

of causing an uncontrolled fire. Burning at other times may 

increase seed production and enhance survival of established 

plants. Research currently underway is investigating the 

combination of fire with followup herbicide treatments for 

improved control, but results are not known at this time. 

Cultural Control 

Early detection and plant removal are critical for preventing 

establishment of Malta starthistle. The local public should 

be educated to help prevent Malta starthistle from becoming 

established. Vehicles, humans, and livestock should be 

discouraged from traveling through infested areas; and 

a program to check and remove seeds from vehicles and 

livestock after going through infested areas should be 

implemented to help stop dispersal. Hay, straw mulch, 

planting seeds, and other related products should be certified 

to be weed-free before use in areas undergoing treatment. 

Table 2. Classical Biological Agents 

Biological Control 

Grazing 

Sheep, goats, and cattle may graze Malta starthistle in 

early spring when plants have developed flowering stems 

but before they have spiny heads. Grazing can reduce 

the presence of starthistle, but owners of horse and other 

livestock should look for signs of toxicity or so-called 

“chewing disease” in starthistle stands that have flowering 

heads. 

Classical Biological Agents 

Biological control agents for Malta starthistle have not been 

researched as well as yellow starthistle, although some 

biological control agents may affect both species. A beetle, 

Lasioderma haemmorrhoidale, that feeds on starthistle seed 

heads was transplanted from the Mediterranean region but 

has little effect in controlling the invasive plant. The limited 

number of agents known to affect Malta starthistle is shown 

in table 2. 

Chemical Control 

The most effective period to spray Malta starthistle is from 

December through April during the seedling to early rosette 

stage since lower rates of herbicide can be applied. When in 

the late rosette or bolting stage, higher rates should be used. 

Herbicides should be applied before flowering when there 

are 4 to 6 inches of growth and good growing conditions. 

Since Malta starthistle is typically an annual, herbicide 

application during or after flowering is ineffective. Label 

instructions and guidelines for mixing and application 

should always be followed. 

Malta starthistle is best controlled with post-emergent 

broadleaf herbicides since these chemicals generally have 

Species 

Bangasternus orientalis 

Type of Agent 

weevil 

Site of Attack 

Eats or uses flowers 

for cocoons 

Impact/Use 

Limited 

Considerations for 

Release 

Little researched 

Puccinia juncea var. solstitialis rust fungus Undetermined Unknown/

California 

Little researched 

3 
 

little or no effect on grass species. The main herbicide 

entry into the plant is through the leaves with only minor 

entry through the roots. All herbicides listed in table 3 will 

effectively control Malta starthistle when properly applied. 

However, they will also impact other emerged, broadleaved species so caution should be taken if non-target 

species need to be protected. This includes woody species 

Table 3. Herbicide Recommendations 

which may also be impacted. Herbicides may be applied by 

backpack sprayers, ATV or UTV sprayers, or conventional 

boom sprayers that are pulled or attached to a tractor or 

truck. Populations of Malta starthistle are rarely extensive 

enough to warrant aerial application of herbicide; however, 

spraying targeted areas by helicopter may be an option when 

large areas are infested by the starthistle. 

Common 

Chemical 

Name 

(active 

ingredient) 

Product 

Example1 

Reclaim

GrazonNext 

Tordon 22K

Grazon P+D

Product 

Example 

Rate per 

Acre1 

(broadcast) 

2/3 to 1 pt 

Backpack 

Sprayer 

Treatment 

Using 

Product 

Example 

1 to 3%2 

Time of 

Application 

Early rosette

stage; use higher

rate at bolting to

bud stage. 

Remarks 

Wet foliage thoroughly. Do not spray when 

plants are defoliated by late freeze, hail,

insects or other unfavorable conditions. 

Effects are shown within 2 to 4 weeks. 

Clopyralid 

Aminopyralid:

2,4-D3 

Picloram4 

Picloram: 2,4-

D3, 4 

1.5 to 2 pt 1 to 3% Same Same 

1 to 3 pt 1 to 3% Same Same 

1 to 2 qt 1 to 3% Same Same 

Dicamba: 

Diflufenzopyr 

Overdrive 4 to 8 oz 1 to 3% Same Same 

Dicamba: 2,4-D3 Weedmaster 1 pt to 1 qt 3 to 5% Same Same 

2,4-D3 Several 

Manufacturers

 1 to 2 qt 5 to 10% Same Same 

Metsulfuron Escort 1 oz NA Same May take 2 to 3 months to show effects. 

Metsulfuron: 

2,4-D: Dicamba3 

Cimarron 

Max 

Rate III: 

1 oz (Part A)

4 pt (Part B) 

NA Same May take 1 to 3 months to show effects. 

Imazapyr Arsenal 1 qt. 1% All stages Spray to have total plant control (e.g.,

along roadsides). May take 2 to 3 months

to show effects. 

1 Trade names for products are provided for example purposes only, and other products with the same active ingredient(s) may be available. Individual 

product labels should be examined for specific information and appropriate use with Malta starthistle.
	

2

 Herbicide/water ratio, e.g., a gallon of spray water with a 3 percent mixture is made by adding a sufficient volume of water to 4 ounces of herbicide until

a volume of 1 gallon is reached (4 oz ÷ 128 oz/gal = 0.03 or 3 percent).
	

3 2,4-D is a restricted use pesticide in New Mexico only. A certified applicator’s license is required for purchase and use.
	

4 Restricted use pesticide. A certified applicator’s license is required for purchase and use.
	

4 
Control Strategies 

In nearly all cases, a long-term commitment (greater than 

3 years) to manage Malta starthistle is necessary to deplete 

the seed bank. Each treatment situation is unique and 

implementing an adaptive management approach using 

different control methods is usually necessary for longterm success. Initial treatment should attempt to eliminate 

as much of the weed population as possible. Secondary 

treatment should include monitoring and additional control 

measures such as spot spraying with backpack sprayers or 

prescribed fire. An option for treatment in the first year is to 

apply herbicide with clopyralid as an active ingredient. The 

herbicide will substantially reduce the starthistle population 

and allow grasses to become established. The herbicide 

treatment can then be followed by prescribed burning in 

the next year (or possibly 2 years). This sequence greatly 

reduces starthistle infestations to insignificant or very low 

levels. The greatest benefit from a treatment sequence 

such as herbicide-prescribed fire is a healthier range plant 

community as indicated by increased species diversity and 

enhancements in forage quality and quantity. 

Similar strategies may be adapted for other treatment 

situations according to local circumstances. For example, 

roadways infested with Malta starthistle may be treated first 

by intensive spraying to control the starthistle. In the second 

step, deep-rooted native perennial grasses can be seeded to 

establish erosion control. In the final state, native broadleaf 

forbs such as lupines may be seeded to restore a more 

balanced mix of plants into the system. 

Further Information 

DiTomaso, J. M., W. T. Lanini, C. D. Thomsen, T. S. 

Prather, C. E. Turner, M. J. Smith, C. L. Elmore, M. 

P. Vayssieres, and W. A. Williams. 2007. Pest notes: 

yellow starthistle. Publ. No. 7402. Univ. Calif. Agric. 

Nat. Resources. Davis, CA. 

DiTomaso, J. Yellow starthistle information. UC Weed 

Research Inform. Center. URL http://wric.ucdavis. 

edu/yst/index.html 

DiTomaso, J., G. B. Kyser, and M. S. Hastings. 1999. 

Prescribed burning for control of yellow starthistle 

(Centaurea solstitialis) and enhanced native plant 

diversity. Weed Sci. 47:233-242. 

Donaldson, S. and D. Rafferty. Identification and 

management of Malta starthistle (Centaurea melitensis 

L.). Univ. of Nev. Coop. Exten. Serv. Fact Sheet-02-

86. Reno, NV 89520. http://www.unce.unr.edu/ 

publications/files/nr/2002/FS0286.pdf 

The use of trade or firm names in this publication is for reader information 

recommendations for their use, nor does it 

imply that the uses discussed here have been

registered. All uses of pesticides must be 

and does not imply endorsement by the U.S. Department of Agriculture of 

any product or service. It does not contain 

registered by appropriate State and/or Federal

agencies before they can be recommended. 

CAUTION: Pesticides can be injurious to

humans, domestic animals, desirable plants,

and fish or other wildlife—if they are not

handled or applied properly. Use all pesticides selectively and carefully. 

Follow recommended practices for the disposal of surplus pesticides and

pesticide containers.