UCONN IPM: Integrated Pest Management:Greenhouse:Weed Controls

Maintaining weed-free growing conditions is necessary to produce high quality greenhouse crops. Insects and diseases can be kept to a minimum only if proper weed control practices are carried out regularly, along with appropriate control measures.

Weeds may compete with desirable crop plants for light, water and nutrients. The presence of weeds also reduces the aesthetic value of the crops grown and creates a poor impression to customers. Weeds are also a primary reservoir of insects such as aphids, whiteflies, thrips, (See photo) and other pests such as mites, slugs and pathogens. Many common greenhouse weeds such as chickweed, oxalis, bittercress, dandelion and ground ivy can become infected with tospoviruses (impatiens necrotic spot virus (INSV) and tomato spotted wilt virus (TSWV) while showing few, if any visible symptoms. (See photo). Thrips can then vector the virus to susceptible greenhouse crops. Weeds can also harbor other plant damaging viruses that are vectored by aphids.

An integrated weed management program will help to effectively manage weed populations. This approach includes preventative measures, sanitation, physical barriers, handweeding and the selective use of postemergance herbicides. Prevention and Sanitation

Weeds and their seeds can be inadvertently brought into the greenhouse on infected plant material, tools, and equipment. Seeds can be moved in soil, by the wind, by irrigation water, animals and people. Creeping wood sorrel, (Oxalis corniculata), hairy bittercress (Cardamine hirsuta), prostrate spurge (Euphorbia humistrata), common chickweed (Stellaria media) and other weeds are persistent problems in greenhouses. These annual weeds reproduce primarily by seed, with several generations occurring per year. (See photo).

Prevention and sanitation are the grower's first line of defense. Weed seeds are easily blown into the greenhouse though vents and other openings. Keep weed seeds and rhizomes out of the greenhouse by using sterile media," clean " plant materials, and controlling weeds outside the greenhouse. Screening vents and other openings will help to limit the entry of wind blown seed, as well as insects.

When scouting, identify the type of weed (broadleaf or grass), its life cycle (annual, biennial or perennial) and its location. It is critical to remove weeds from greenhouse pots, benches and floors before they flower and produce seed. Physical Barriers

The use of a physical barrier such as a weed block fabric will help to limit weed establishment on greenhouse floors. Leave the weed fabric bare so it can be easily swept. Covering the weed fabric with gravel will make it difficult to remove any spilled potting media providing an ideal environment for weed growth. Regularly hand pull any escaped weeds before they go to seed. Repair any tears in the weed block fabric.

Few herbicides are labeled for use in a greenhouse due to the potential for severe injury or death of desirable plants. This injury may occur in a number of ways including:

Herbicide vapors are easily trapped within an enclosed greenhouse and can then injure crop plant foliage.

Always be sure the herbicide selected is labeled for use in the greenhouse. Carefully follow all label instructions and precautions. It is the applicator's responsibility to read and follow all label directions. Use a dedicated sprayer that is clearly labeled for herbicide use only.

Some of the symptoms of herbicide injury include discolored, thickened and stunted leaves. Sometimes, the growing point of young seedlings is injured, severely stunting their growth. (See photo). Symptoms may be similar to those caused by nutritional imbalances, viruses or air pollution injury. Proper diagnosis is needed to determine the causal agent. In many cases, symptoms are so severe that the injured plants cannot be sold.

Herbicides are generally classified according to their mechanism of action and how they are used (premergence and postemergance).

Preemergence herbicides are applied before weeds emerge. They provide residual control of weed seedlings and can persist for many months, and in some cases, over a year. Preemergence herbicides continue to vaporize, causing crop damage. Currently, there are no preemergence herbicides labeled for greenhouse use.

(Editor's note: Surflan (oryzalin) is no longer registered for use in enclosed greenhouse structures).

Selective use of Postemergence Herbicides
Postemergence herbicides are applied after the weeds have emerged. In the greenhouse, several postemergance herbicides can be used under greenhouse benches and on the floors (See Table 1).

There are of two different types of postemergance herbicides: contact and systemic. Contact herbicides kill only the portion of the plant that the herbicide contacts, so good spray coverage is generally needed. Contact herbicides such as Scythe^Ž are best applied to small succulent seedlings under greenhouse benches and on the floor. Reward Ž is also a contact herbicide but has high mammalian toxicity.

Systemic herbicides are absorbed and move through the plant. The target weeds must be actively growing for the herbicide to be effective. Systemic herbicides such as Roundup^Ž and Finale^Ž are best applied to actively growing weeds when temperatures are above 50^oF. However, Roundup^Žcan only be used in an empty greenhouse between crops. Finale^Ž is similar to Roundup^Ž as it is a non-selective, systemic herbicide. However, Finale^Ž produces symptoms more rapidly (often within 2 to 4 days) than Roundup^Ž (for 7 days or more on most perennial weeds). Systemic movement to deeper roots and rhizomes is not as complete as with RoundupŽ so it is possible that some deeply rooted perennials may regrow after application with FinaleŽ.

Currently, often five herbicides can be used when crops are in the greenhouse: ScytheŽ, EnvoyŽ, RewardŽ, FinaleŽ, FusiladeŽ II, (See Table 1). Scythe Ž, FinaleŽ and RewardŽ are non-selective herbicides. They should not come in contact with desirable crop foliage.

Irrigating crops too soon after applying a herbicide can wash it off the target weeds and reduce its effectiveness.

WEED MANAGEMENT OUTSIDE THE GREENHOUSE

Maintain a 10 to 20-foot weed-free zone around the greenhouse. A geotextile fabric can be used outside the greenhouse to prevent weed growth. Or maintain a boundary of turfgrass, such as a mix of chewing, hard and creeping red fescues to help prevent weed growth. Thrips do not reproduce well on these grasses. Also, fescues are not yet known to be hosts of tospoviruses.

Herbicides may also be used to control weeds outside the greenhouse. The label should state if use near greenhouses is permitted. Close the greenhouse vents and openings during herbicide applications to prevent drift inside to sensitive crops. Do not use any auxin type herbicides, such as those labeled for broadleaf weed control in turf, near greenhouses. Their volatility and the extreme sensitivity of greenhouse crops to these herbicides can result in severe injury.

TABLE 1. HERBICIDES LABELED FOR USE WITHIN A GREENHOUSE
Trade Name	Chemical Name	EPA Reg. No.	Marketing/ Manufacturing Company	Mechanism of Action	REI
For use when crops are in the greenhouse
Envoy^Ž	Clethodim	59639-78	Valent USA	Selective, postemergence herbicide. Works by contact, and is a lipid synthesis inhibitor. No residual activity.	24
Finale^Ž	Glufosinate-ammonium	432-1229	Bayer	Non-selective, postemergence herbicide. Systemic. Works by disrupting cell membranes.	12
FusiladeŽ II	Fluazifop-P-butyl	100-1084	Syngenta	Selective, postemergance herbicide. Systemic. Inhibits cell division by blocking an enzyme involved in lipid biosynthesis.	12
Reward Ž	Diquat dibromide	100-1091	Syngenta	Non-selective. Works by contact. Cell membrane disruptor.	24
ScytheŽ	Pelargonic acid and related fatty acids	62719-529	Dow Agro Sciences	Non-selective, postemergance herbicide. Works by contact.	12
For use in an empty greenhouse between crops
Glyphosate Pro II	Glyphosate	524-517-72112	Prokoz, Inc.	Non-selective, postemergence herbicide. Systemic. Inhibits enzyme found in plants essential to form specific amino acids. No residual soil activity.	4
Roundup Pro^Ž	Glyphosate	524-475	Monsanto Company	Non-selective, postemergance herbicide. Systemic. Inhibits enzyme found in plants essential to form specific amino acids. No residual activity.	4
Roundup Pro ConcentrateŽ	Glyphosate	524-529	Monsanto Company	Non-selective, postemergance herbicide. Systemic. Inhibits enzyme found in plants essential to form specific amino acids. No residual soil activity.	4
Roundup ProDry	Glyphosate	524-505	Monsanto Company	Non-selective, postemergance herbicide. Systemic. Inhibits enzyme found in plants essential to form specific amino acids. No residual soil activity.	4
Touchdown ProŽ	Glyphosate	100-1121	Syngenta	Non-selective, postemergance herbicide. Systemic. Inhibits enzyme found in plants essential to form specific amino acids. No residual soil activity.	12
If any information in this table is inconsistent with the label, follow the label. By law, in order to use a pesticide product, it must be registered in the state. The products registered by a company change from year to year. The information in the following tables is accurate as of publication but is subject to change. back

TABLE 2. HERBICIDES AND THEIR CHARACTERISTICS
Trade Name	Common Name	Target Weeds	Rate/1 gal.	Comments
For use when crops are in the greenhouse
Envoy^Ž	Clethodim	Annual and perennial grasses	0.65 to 1.3 fl oz	Apply to actively growing grasses only under greenhouse benches. Does not control sedges or broadleaf weeds.
Finale^Ž	Glufosinate-ammonium	Annual and perennial grasses, and broadleaf weeds	2.0 to 4.0 fl oz	Apply to actively growing weeds under greenhouse benches. Air circulation fans must be turned off during application. Apply as a directed spray, using a large droplet, low-pressure type nozzle to reduce drift. Avoid drift and direct contact with desirable vegetation. Do not use in greenhouses containing edible crops. No soil activity.
FusiladeŽ II	Fluazifop-P-butyl	Annual and perennial grass weeds.	0.75 oz	Does not control broadleaf weeds or sedges. Thorough coverage is important for good activity. Optimum weed control is achieved when young, actively growing weeds are treated.
Reward Ž	Diquat dibromide	Annual broadleaf and grass weeds	0.75 oz	For general weed control beneath greenhouse benches. Avoid contact with desirable foliage as injury may occur. Optimum weed control is achieved when young, actively growing weeds are treated. High mammialian toxicity.
ScytheŽ	Pelargonic acid and related fatty acids	Annual and perennial broadleaf and grass weeds.	4.0-6.6 fl oz.	Avoid contact with desirable vegetation. Apply to young, succulent weeds. See label for higher rates to be applied for burndown of perennials. Cool or cloudy weather may slow down activity. Provides no residual weed control, but leaves a strong odor.
For use in an empty greenhouse between crops and outside greenhouses
Roundup ProŽ Glyphosate Pro II	Glyphosate	Annual and perennial weeds.	0.66 2.6 fl oz.	Use higher rates for hard to kill perennials. Desirable vegetation must not be present during application. Air circulation fans must be turned off to reduce drift. No residual soil activity.
Roundup Pro Concentrate Ž	Glyphosate	Annual and perennial weeds.	0.5-1.0 fl oz	Use higher rates for hard to kill perennials. Desirable vegetation must not be present during application. Air circulation fans must be turned off to reduce drift. No residual soil activity.
Roundup ProDry	Glyphosate	Annual and perennial weeds	0.4 to 1.5 oz	Desirable plants must not be present during application. Air circulation fans must be turned off to reduce drift. No residual soil activity.
Touchdown Pro Ž	Glyphosate	Annual and perennial weeds.	06-2.6 fl oz	Use higher rates for hard to kill perennials. Desirable vegetation must not be present during application. Air circulation fans must be turned off to reduce drift. No residual soil activity
If any information in this table is inconsistent with the label, follow the label. By law, in order to use a pesticide product, it must be registered in the state. The products registered by a company change from year to year. The information in the following tables is accurate as of publication but is subject to change.

Leanne Pundt, Extension Educator, Commercial Horticulture, University of Connecticut

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The information in this material is for educational purposes. The recommendations contained are based on the best available knowledge at the time of printing. Any reference to commercial products, trade or brand names is for information only, and no endorsement or approval is intended. The Cooperative Extension system does not guarantee or warrant the standard of any product referenced or imply approval of the product to the exclusion of others which also may be available.All agrochemicals/pesticides listed are registered for suggested uses in accordance with federal and Connecticut state laws and regulations as of the date of printing. If the information does not agree with current labeling, follow the label instructions. The label is the law.Warning! Agrochemicals/pesticides are dangerous. Read and follow all instructions and safety precautions on labels. Carefully handle and store agrochemicals/pesticides in originally labeled containers immediately in a safe manner and place. Contact the Connecticut Department of Environmental Protection for current regulations.The user of this information assumes all risks for personal injury or property damage.Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Kirklyn M. Kerr, Director, Cooperative Extension System, The University of Connecticut, Storrs. The Connecticut Cooperative Extension System offers its programs to persons regardless of race, color, national origin, sex, age or disability and is an equal opportunity employer.

WEED MANAGEMENT OUTSIDE THE GREENHOUSE

For use when crops are in the greenhouse

For use in an empty greenhouse between crops

For use when crops are in the greenhouse

For use in an empty greenhouse between crops and outside greenhouses