Fact Sheets

Pest Management

The following fact sheet was prepared by Leanne Pundt, Extension Educator, University of Connecticut.

Managing Weeds In And Around Greenhouses

Introduction

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 source of insects such as aphids, whiteflies, thrips, and other pests such as mites, slugs and diseases. Many common greenhouse weeds such as chickweed, oxalis, bittercress, jewelweed, dandelion and ground ivy can become infected with tospoviruses including impatiens necrotic spot virus (INSV) and tomato spotted wilt virus (TSWV) while showing few, if any visible symptoms. Thrips can then vector the virus to susceptible greenhouse crops. Weeds can also carry other plant damaging viruses that are vectored by aphids.

An integrated weed management program will help to effectively manage weed populations. This approach includes preventive measures, sanitation, physical barriers, handweeding and the selective use of postemergence 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 bitter cress (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.

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 weeds (broadleaf, or grass), life cycle (annual, biennial or perennial) and location. It is critical to remove weeds from greenhouse pots, benches and floors before they flower and produce seed. For example, a single plant of bittercress can produce 5000 seeds, that germinate in as little as 5 days and can propel the seeds over 9 feet from the plant. Yellow woodsorrel and creeping woodsorrel also expel seeds by force throughout a greenhouse.

Physical Barriers

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

Controlling Existing Weeds

To control existing weeds, the following methods may be used: 1) hand pulling or 2) using a postemergence herbicide. These measures do not prevent reseeding of weeds.

Precautions on the use of herbicides

Few herbicides are labeled for use in a greenhouse due to the potential for severe crop injury or death to desirable plants. This injury may occur in a number of ways including: 1) spray drift occurs if fans are operating at the time of application, and 2) volatilization (changing from a liquid to a gas). Herbicide vapors are then easily trapped within an enclosed greenhouse and injure desirable 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.

Symptoms of Herbicide Injury

Some of the symptoms of herbicide injury include discolored, thickened, or stunted leaves. Sometimes, the growing point of young seedlings is injured, severely stunting their growth. Symptoms may be similar to those caused by nutritional imbalances, viral diseases 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.

Types of Herbicides

Herbicides are generally classified according to their mechanism of action (contact or systemic) and how they are used (preemergence and postemergence)

Avoid use of preemergence herbicides in the greenhouse

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 can continue to vaporize, causing crop damage. Currently, there are no preemergence herbicides labeled for greenhouse use. (Editors 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 postemergence herbicides can be used under greenhouse benches and on the floors (See Table 1).

There are of two different types of postemergence 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 ® are best applied to small succulent seedlings. Reward ® 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® and Finale® are best applied to actively growing weeds when temperatures are above 50 ° F. Note: Roundup® can only be used in an empty greenhouse between crops. Finale® is similar to Roundup® as it is a non-selective, systemic herbicide. However, Finale® produces symptoms more rapidly (often within 2 to 4 days) than Roundup® (for 7 days or more on most perennial weeds). Systemic movement to deeper roots and rhizomes is not as complete as with Roundup® so it is possible that some deeply rooted perennials may regrow after application with Finale®

Currently, only 5 herbicides can be used when crops are in the greenhouse: Scythe ®, Envoy®, Fusilade ® II, Reward ®, and Finale ®. (See Table 1) Scythe ® and Finale® are non-selective herbicides

They should not come in contact with desirable crop foliage. Irrigating crops too soon after applying an herbicide can wash it off the target weeds under the benches and reduce its effectiveness.

WEED MANAGEMENT OUTSIDE THE GREENHOUSE

Managing weeds outside the greenhouse is important to: 1) prevent weed seeds from being blown into the greenhouse; 2) prevent perennial weeds such as bindweed, quackgrass, etc., from growing under the foundation of the greenhouse; and 3) help reduce the unwanted entry of winged insects into the greenhouse.

Prevention and Sanitation

Maintain a 10 to 20-foot weed free barrier around the greenhouse. A geotextile fabric can be used outside the greenhouse to prevent weed growth. Mow beyond this area to help limit the blow in of weed seeds. Or maintain a boundary of grasses, such as a mix of chewings, hard and creeping fescues. Thrips tend to not reproduce well on these grasses. Fescuses are also not yet known to be hosts of tospovirsues.

Precautions on the use of herbicides

Herbicides may also be used 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. Soil residual and post emergence herbicides may be carefully used surrounding the greenhouse. Select herbicides with low volatility that will control target weeds. SureGuard (flumiozazin), Barricade (prodiamine), Pendulum (pendimethalin) or Surflan (oryzalin) are often used to prevent weed emergence. They may also be tank mixed with post emergence herbicides such as Roundup or Finale to also control existing weeds . 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 AND THEIR CHARACTERISTICS

  If any information in this table is inconsistent with the label, follow the label.

Trade Name/Common Name /REI * (hours)	Mode of Action	Target Weeds	Rate/1 gal.	Comments
For use when crops are in the greenhouse:
Envoy Plus ™ (Clethodim) 24 hr. REI	Selective, postemergence herbicide. Works by contact. Inhibits lipid synthesis. No residual activity.	Annual and perennial grasses.	0.44 to 0.85 fl. oz.	Apply to actively growing grasses that are not under environmental stress beneath greenhouse benches. Does not control sedges or broadleaf weeds.
Finale ® (Glufosinate-ammonium) 12 hr. REI	Nonselective, postemergence herbicide. Systemic. Works by disrupting cell membranes.	Annual and perennial grasses, and broadleaves.	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.
Fusilade ® II (Fluazifop-P-butyl) 12 hr. REI	Selective, post emergence herbicide. Systemic. Inhibits cell division by blocking an enzyme involved in lipid biosynthesis.	Annual and perennial grasses.	0.75 fl.oz.	Does not control broadleaf weeds or sedges (nutgrass). Thorough coverage is important for good activity. Optimum weed control is achieved when young, actively growing weeds are treated.
Reward ® (Diquat dibromide) 24 hr. REI	Non-selective. Works by contact. Cell membrane disruptor.	Annual broadleaves and grasses.	See label	For general weed control beneath greenhouse benches. Avoid contact with desirable foliage as injury may occur. Do not use on food crops. High mammalian toxicity.
Scythe® (Pelargonic acid and related fatty acids) 12 hr. REI	Non-selective, postemergence herbicide. Works by contact.	Annual and perennial broadleave and grasses.	4.0 to 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. Use under benches and in walkways.
For use in an empty greenhouse between crops and outside greenhouses:
Glyphosate Pro II Roundup Pro® 4 hr. REI Touchdown Pro® 12 hr. REI (Glyphosate)	Non-selective, postemergence herbicide. Systemic. Inhibits enzyme found in plants essential to form specific amino acids. No residual soil activity.	Annual and perennials.	0.66 to 2.6 fl. oz	Use higher rates for hard to kill perennials. Used to control weeds in and around greenhouses for use under greenhouse benches and in walkways. 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| (Glyphosate) 4 hr. REI	Non-selective, postemergence herbicide. Systemic. Inhibits enzyme found in plants essential to form specific amino acids. No residual soil activity.	Annual and perennials.	0.5 to 2.1 fl. oz.	Use higher rates for hard to kill perennials. Used to control weeds in and around greenhouses for use under benches and in walkways. Desirable vegetation must not be present during application. Air circulation fans must be turned off to reduce drift. No residual soil activity.
Roundup Pro Dry® (Glyphosate) 4 hr. REI	Non-selective, postemergence herbicide. Systemic. Inhibits enzyme found in plants essential to form specific amino acids. No residual soil activity.	Annual and perennials.	0.4 to 1.5 oz.	Use higher rates for hard to kill perennials Used to control weeds in and around greenhouses for use under benches and in walkways. Desirable vegetable must not be present during application. Air circulation fans must be turned off to reduce drift. No residual soil activity.

• REI = Re-entry Interval

Some Helpful References:

Uva, R.H. J. C. Neal and J. M. DiTomaso. 1997. Weeds of the Northeast. Cornell University Press. Ithaca , NY . 396 pp.

Newcomb, L. 1977. Newcomb's Wildflower Guide. Little, Brown and Company. Boston , MA . 490 pp.

UMass Extension Weed Herbarium
www.uMassExtensionWeedHerbarium.org

7/08
Prepared by Leanne Pundt
Extension Educator
University of Connecticut