Manage weeds in your greenhouse

Oxalis (one of them shows the seed pod forming).
Photos courtesy of Tina Smith.

When greenhouses are empty, there is an opportunity to thoroughly eliminate weeds to reduce problems for the next crop cycle. Weeds are a persistent problem, needing constant attention in both retail and wholesale greenhouses. Weeds create a poor impression to customers and are a primary source for pests such as whiteflies, aphids, thrips, mites, slugs and diseases. Common weeds in greenhouses, such as creeping woodsorrel (Oxalis corniculata), chickweed (Stellaria media), bittercress (Cardamine hirsuta), as well as others, can become infected with the tospoviruses, impatiens necrotic spot virus (INSV) and tomato spotted wilt virus (TSWV) and serve as a disease source. Weeds infested with thrips then vector the virus onto susceptible greenhouse crops.

How weeds arrive

Weeds and their seeds are brought into the greenhouse on infested plant material, tools, equipment, animals and people. Seeds can be moved by wind (dandelion, horseweed, groundsel), irrigation water (chickweed) and by seeds being naturally propelled (woodsorrel, bittercress). Annual weeds reproduce primarily by seed, with several generations occurring per year. Once growing in the greenhouse and allowed to flower, weeds produce an enormous amount of seed, and some weeds such as woodsorrel and bittercress propel seeds up to 12 feet throughout the greenhouse.

Weed management in greenhouses involves preventative measures, sanitation, using weed fabric, hand-weeding and using postemergence herbicides.

Preventing weeds

Prevention and sanitation are the first steps to managing weeds. Keep weed seeds and rhizomes out of greenhouses by using clean growing media, clean plant material and controlling weeds outside of the greenhouse. Keep growing media covered and be sure to control weeds around the media storage area.

Regular mowing around the greenhouse will keep the majority of the vegetation from flowering and producing seed. If weeds do get in, they should not be allowed to flower and produce seed.

Using weed block fabric

The use of a physical barrier such as weed block fabric helps to prevent weeds from establishing on greenhouse floors. It is best to leave the fabric uncovered so it can be easily swept. Fabric that is covered with crushed stone or other material will collect fallen growing media and create a favorable environment for weed seedlings to germinate.

Tears and worn spots that expose the ground should be repaired or replaced. Spilled growing media should always be cleaned up so that weed seeds don’t have a place to collect and germinate.

Controlling existing weeds

Existing weeds can be controlled by hand or by using herbicides. Preemergence herbicides are applied to the soil to prevent the emergence of seedlings. There are currently no preemergence herbicides labeled for greenhouse use.

A broadleaf brush killer applied around the perimeter of the greenhouse drifted into an empty greenhouse in late fall. Vapors in spring caused damage to tomato plants along the outside wall.
Oxalis.

Postemergence herbicides are applied after the weeds have emerged. Several postemergence herbicides can be used under greenhouse benches and on the floors. Contact herbicides are best applied to small seedlings. Large weeds will be burned, but not killed.

Natural-based herbicides

In addition to chemical herbicides, there are a few bioherbicides, natural-based herbicides that can be used by organic growers. Specific natural-based herbicides include acetic acid, citric acid, citrus oil and clove oil (eugenol). These materials are postemergence, nonselective, contact herbicides that work in various ways, but basically disrupt cell membranes and cause plants to desiccate. They work best on young plants and multiple applications are usually needed to control reemerging or perennial weeds. Products are sold under several trade names and some are approved by OMRI (Organic Materials Review Institute). 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.

Chemical herbicides

Few chemical herbicides are labeled for use in greenhouses due to the potential for crop injury or death. Injury can occur from spray drift if fans are operating at the time of application. Injury can also occur from herbicides that are volatile (change from liquid to gas). Auxin-type herbicides, such as turf herbicides containing 2,4D dicamba and MCPP, are very volatile and vapors can easily buildup within an enclosed greenhouse and injure desirable plants. These herbicides can cause distinguishable injury symptoms that include cupping and strapping of plant foliage. Always be sure the chosen herbicide is labeled for use in the greenhouse and carefully follow label instructions and precautions. Use a dedicated sprayer that is clearly labeled for herbicide use only. If using herbicides outside, around a greenhouse, avoid using volatile herbicides that can easily enter the greenhouse ventilation system.

Herbicide injury

Symptoms of herbicide injury include discolored, thickened or stunted leaves. Sometimes the growing point of young seedlings is injured. Some herbicides may cause foliage to turn white, while others caused leaves to become distorted, cupped or strapped. The possibility of herbicide-injured plants recovering depends on the sensitivity of the plants to the contaminating herbicide and the dose of herbicide the plant received. In most cases, injured plants will not grow out in time for sale, or symptoms may be so severe they cannot be sold.

An herbicide drifted into this greenhouse from a nearby field of vegetable crops, causing injury to a crop of ivy geraniums.

Depending on the nature of the herbicide and amount of herbicide that entered the house, activated charcoal might be used to neutralize herbicide activity in soil when mistakes occur. The efficiency of deactivation depends on the soil’s organic matter content and physical condition, the herbicide’s activity and the crop’s sensitivity. According to the label, D•TOX Flowable Charcoal (20 percent activated carbon) can be applied for spills at a rate of 1 to 2 gallons per 150 square feet. The product can be applied undiluted, or diluted up to 1 gallon in 3 gallons water, or as required for proper spraying. Using activated carbon is a poor substitute for a well-planned weed control program.

Herbicides for use in an empty greenhouse

Glyphosate is a nonselective, systemic, postemergence herbicide. It does not have residual control or preemergence activity. Trade names include Roundup ProDry, Roundup Pro, Roundup Pro Concentrate and Touchdown Pro.

Herbicides for use when crops are present

  • Selective, postemergence herbicide for the control of grasses only, works by contact. Clethodim (Envoy, Envoy Plus)
  • Nonselective, postemergence, systemic herbicide. Glufosinate-ammonium (Finale)
  • Selective, postemergence, systemic herbicide for the control of grasses only. Fluazifop-P-butyl (Fusilade II)
  • Nonselective contact herbicide. Diquat dibromide (Reward),
  • Nonselective, postemergence, contact herbicide. Pelargonic acid (Scythe)

Weed control outside the greenhouse

In addition to mowing, herbicides may also be used outside of greenhouses. Before spraying weeds around the greenhouse with any herbicide, close windows and vents to prevent spray drift from entering the greenhouse. Avoid using auxin-type herbicides, such as those labeled for broadleaf weed control in turf or brushkillers, or herbicides with high volatility near greenhouses. Effective preemergence herbicides with low volatility include orzyzalin (Surflan), flumiozazin (SureGuard), prodiamine (Barricade) and pendimethlin (Pendulum). They can be tank-mixed with postemergence herbicides listed above.

Prior to mowing or using an herbicide around greenhouses, use a knockdown insecticide such as horticulture oil on the weeds to kill insects and prevent them from leaving the weeds and entering the greenhouse through vents. Then, use a postemergent, nonselective herbicide to kill existing vegetation.

The author is an extension specialist with the University of Massachusetts and a program leader responsible for outreach activities in greenhouse integrated crop management and diagnostics.