Greenhouse Weed Control December 2010

Maintaining weed-free growing conditions is an essential part of producing 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 diseases. Low growing weeds help maintain moist conditions providing a favorable environment for fungus gnats and shore flies. 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.

Description: Thrips feeding damage, click for a larger image
Thrips feeding damage
Leanne Pundt photo
click for a larger image

 

Description: chickweed showing faint INSV symptoms, click for a larger image
Chickweed showing faint impatiens necrotic spot virus symptoms
Leanne Pundt photo
click for a larger image

An integrated weed management program can  help  effectively manage weed populations. This approach includes preventative measures including sanitation and physical barriers, and control measures including handweeding and the selective use of postemergance herbicides.  

Prevention  Weed seeds are easily blown into the greenhouse though vents and other openings. 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,  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).

Description: Bittercress, an annual weed, going to seed.  Click for a larger image
Bittercress, an annual weed, going to seed
Leanne Pundt photo
click for a larger image

Prevention is  the grower's first line of defense..

Sanitation

Keep weed seeds rhizomes and other propagules out of the greenhouse by using sterile media," clean " plant materials, and controlling weeds outside the greenhouse. Clean up spilled growing media both inside and outside the greenhouse. When moist, it provides an ideal environment for the germination of weed seeds. 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. 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 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  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 re-seeding of weeds.  

PRECAUTIONS FOR USE OF HERBICIDES IN THE GREENHOUSE

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:

  1. spray drift may occur if fans are operating at the time of application, and
  2. herbicides can volatilize, changing from a liquid to a gas.

Herbicide vapors are easily trapped within an enclosed greenhouse and can then 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. (See photo). Symptoms may be similar to those caused by nutritional imbalances, viruses or air pollution.. Proper diagnosis is needed to determine the causal agent. In many cases, symptoms are so severe that the injured plants cannot be sold.

 

Description: Herbicide injury to young seedlings.  Click for a larger image.
Herbicide injury to young seedlings
Leanne Pundt photo
click for a larger image

Types of Herbicides

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

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 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® and Matratec ® are best applied to small succulent seedlings.. Reward ® and Diquat E Pro 2L are  also contact herbicides but have 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 generics) and Finale® are best applied to actively growing weeds when temperatures are above 50oF. However, Roundup®   (and generics) 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, often 7 herbicides can be used when crops are in the greenhouse: Scythe®, Envoy®, Reward®, Finale®, Fusilade® II,  Matratec ®and Diquat E Pro 2L (See Table 1). Scythe ®, Finale® and Matratec ®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. 

Natural-based Herbicides

In addition to chemical herbicides, there are a few  natural-based herbicides that can be used by organic growers in and around greenhouses.  Some of these products contain various plant oils as the active ingredients (Matratec® and Sporatec®)s These materials are postemergence, non-selective, contact herbicides. They work by disrupting plant cell membranes causing plants to desiccate.  Natural based herbicides work best on young plants. Multiple applications are often needed to prevent reemerging  of annual weeds and for harder to control  perennial weeds. Some are OMRI (Organic Materials Review institute) listed. Growers who are seeking organic certification should consult with their local certifying agent to confirm that a particular product is permitted. Although bioherbicides are natural-based, they are not without risks. They may burn skin and eyes or cause nausea or other health affects. All label directions and precautions should be followed. 

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 

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 before weeds set seed to help limit the entry of weed seeds. 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.  

Precautions on the use of herbicides

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. Soil residual and post emergence herbicides may be carefully used surrounding the greenhouse. Select effective herbicides with low volatility. SureGuard® (flumiozazin), Barricade® (prodiamine), Pendulum® (pendimethalin) and Surflan® (oryzalin) are often used to prevent weed emergence. They may be tank mixed with post emergence herbicides such as Roundup®(and related generic products) 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 LABELED FOR USE WITHIN A GREENHOUSE

 Trade Name

 Common Name

Tox**

 EPA Reg. No.

Manufacturer

 

 Mode  of Action

 REI *
(hrs) 

Diquat E Pro 2L

Diquat dibromide

C

79676-75

Etigra

Non-selective. Works by contact. Cell membrane distruptor.

24

 EnvoyPlus

 Clethodim

 

C

 

 59639-132

Valent USA

Selective, postemergence herbicide. Works by contact.  Inhibits lipid synthesis.. No residual activity.

 24

 Finale®

 Glufosinate-ammonium

 

W

432-1229

Bayer

Non-selective, postemergence herbicide. Systemic. Works by disrupting cell membranes.

 12

Fusilade® II

 Fluazifop-P-butyl

 

 

C

 

 

100-1084

Syngenta

Selective, postemergance herbicide. Systemic. Inhibits cell division by blocking an enzyme involved in lipid biosynthesis.

12

Matratec®

Clove Oil

 

C

25(b) exempt

Brandt

Non-selective, post emergence herbicide. Works by contact.

 

Reward ®

Diquat dibromide

 

C

100-1091

Syngenta

Non-selective. Works by contact. Cell membrane disruptor.

24

Scythe®

Pelargonic acid and related fatty acids

 

W

62719-529

Dow Agro Sciences

Non-selective, postemergence herbicide. Works by contact.

12

Sporatec®

Rosemary oil, clove oil, thyme oil

C

25(b) exempt

Brandt

Non-selective, postemergence herbicide. Works by contact.

 

Glyphosate Pro II™4

Glyphosate

 

 

C

72112

Prokoz, Inc.

Non-selective, postemergence herbicide. Systemic. Inhibits enzyme found in plants essential to form specific amino acids. No residual soil activity.

4

Glyphosate T&O

Glyphosate

 

 

W

73220-6

Quaili-Pro

Non-selective, postemergence herbicide. Systemic. Inhibits enzyme found in plants essential to form specific amino acids. No residual soil activity

12

Razor ®

Glyphosate

C

228-366

NuFarm

 

4

 Roundup Pro®

 Glyphosate

 

 

C

 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 Concentrate®

Glyphosate

 

 

C

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

 Touchdown Pro®

Glyphosate

 

C

 

 

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.

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TABLE 2. HERBICIDES AND THEIR CHARACTERISTICS

 Trade Name

 Common Name

 Target Weeds

 Rate/1 gal.

 Comments

For use when crops are in the greenhouse

 EnvoyPlus

 Clethodim

 Annual and perennial grasses

 0.44 to 0.85 fl .oz

 Apply to actively growing grasses only under greenhouse benches. Does not control sedges or broadleaf weeds.

 Finale®

 Glufosinate-ammonium

 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

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 E Pro 2L

Diquat dibromide

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 mammailian toxicity.

Matratec®

Clove oil

Annual and perennial broadleaves and grasses

 

6.4 fl. oz

General weed control in and around greenhouses and under greenhouse benches. Most effective on small weeds that are actively growing. Avoid contact with desirable vegetation.

Scythe®

Pelargonic acid and related fatty acids

Annual and perennial broadleaves and grasses

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.

Sporatec®

Rosemary oil, clove oil and thyme oil

Liverwort and mosses

1 to 2 fl. Oz.

Avoid contact with desirable vegetation. Make a directed spray on the liverworts by wetting heavily.

For use in an empty greenhouse between crops and outside greenhouses

Roundup Pro®

Glyphosate T& O

Glyphosate Pro™ 4

Razor®

Touchdown Pro

Glyphosate

Annual and perennials

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 ®

Glyphosate

Annual and perennials

0.5-2.1 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.

Pesticide Labels and MSDS Information
Crop Data Management Systems
www.cdms.net


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

Information on our site was developed for conditions in the Northeast. Use in other geographical areas may be inappropriate.

 

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.

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