Cooperative Extension Service
The University of Georgia College of Agricultural and Environmental Sciences
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Weed Control in Greenhouses

Mark A. Czarnota, Extension Horticulturist

Introduction
Major Weeds in the Greenhouse
Weed Control
Table 1. Herbicides labeled for use in greenhouses

Weed control in greenhouses is of vital importance to keep your plants saleable and free from insects, bacteria, viruses, mycoplasma and, of course, weeds. Many people neglect to realize the importance of controlling weeds in and around the greenhouse. As well as being unsightly, weeds can harbor insects when no other plants are available in the greenhouse (Figure 1). They can also harbor diseases such as viruses, bacteria and mycoplasma that can be transferred to desirable greenhouse plants.

Figure 1. Leaf back of Mulberry weed (Fatoua villosa) taken from a greenhouse with two spotted spider mite castings (A) and immature thrips (B).

Weeds that exist either inside or outside the greenhouse can cause problems. Within the greenhouse, plants thrive with high temperatures, steady light levels and an ample supply of water. Some plants that are normally considered annuals can grow year-round. Perennials that normally flower once or twice a year can go through several flowering cycles. These conditions are very conducive to supporting growth of greenhouse crops as well as weeds. In the greenhouse, weeds can exist anywhere a growing medium will support them (Figures 2 and 3). Greenhouse floors that are not -- or only partially -- concreted are common places for weeds to germinate and grow. It is not hard, however, to find weeds growing anywhere growth medium builds up. Once a severe infestation occurs, weed control can be labor intensive, expensive and cause disruption of a saleable plant.

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Figure 2. Hairy bittercress (Cardamine hirsuta L.) under a greenhouse bench.		Figure 3. Longstalked phyllanthus (Phyllanthus tennellus L.) growing in the cracks of a greenhouse.

Major Weeds in the Greenhouse

Regardless of the types of crops grown in greenhouses, a few major weeds will usually be encountered. The big three are woodsorrel (Oxalis spp.), bittercress (Cardamine spp) and spurge (Euphorbia spp.). There are many reasons for the development of these weeds:

• Adaptability to various soil, air and light conditions.
• Fast life cycle (short seed-to-flower production).
• Prolific seed production.

Woodsorrel

Of the several species of woodsorrel, probably the two most common are yellow woodsorrel (Oxalis stricta) and creeping woodsorrel (Oxalis corniculata). Both species have very similar weedy growth characteristics:

• Progression from seed to flower in fewer than 3 weeks.
• Production of large amounts of seed.
• Dispersion of seeds up to 12 feet via structures called elators (Figure 4).
• Resembles clover, and leaves are difficult to wet with herbicide solutions due to small, waxy leaves (Figures 5 and 6).

Figure 4. Fruits of creeping woodsorrel (Oxalis corniculata L.) showing elators (A) that forcibly expel seeds (B) up to 12 feet.

Both plants are perennial; however, the main difference between the two is that yellow woodsorrel spreads by below-ground rhizomes (stems that creep below the ground), and creeping woodsorrel spreads by stolons (stems that creep above the ground). In addition to being an unsightly weed, this plant has been known to harbor pests such as whiteflies and spider mites.

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Figure 5. Flowers and foliage of creeping woodsorrel (Oxalis corniculata L.).		Figure 6. Yellow woodsorrel (Oxalis stricta L).

Bittercress

There are several species of bittercress, but probably the most common is hairy bittercress (Cardamine hirsuta L., Figure 7). Others that may be encountered are smallflowered bittercress (Cardamine parviflora L.) and less-seeded bittercress (Cardamine oligosperma L.). As with all the bittercresses, hairy bittercress displays many weedy characteristics:

Figure 7. Plant of hairy bittercress (Cardamine hirsuta) showing fruit (A) and flower (B).

• Progression from seed to flower in approximately 3 weeks.
• Production of large amounts of seed.
• Dispenses seeds several feet by a special mechanism in the wall of the fruit (Figure 8).

Figure 8. Fruits of hairy bittercress (Cardamine hirsuta L.) showing fruit wall preparing to disperse seeds (A).

Spurge

Low-growing or creeping spurges (Figure 9) are represented by approximately 11 species. They can be identified by the milky sap produced when a leaf or stem is broken off. The two most common spurges encountered are prostrate spurge (Euphorbia humistrata) and spotted spurge (E. maculata). Most of the species encountered in the greenhouse are annuals. Other weedy characteristics include:

• Progression from seed to flower in approximately 3 weeks.
• Production of large amounts of seed.
• Germination in late spring/early summer.

Figure 9. Spurge (Euphorbia sp.).

Phyllanthus

Also known as chamberbitter, leaf-flower and niruri, phyllanthus (Figure 10) is a prolific seed producing summer annual. In the same family as spurge (Euphorbiaceae), it has very similar growth requirements. As many as six species have been noted in the United States, but in nurseries the most commonly encountered species is Phyllanthus tenellus Roxb. Although Phyllanthus spp. are in the same family as spurge, it does not produce milky sap when a stem or leaf is broken.

Figure 10. Longstalked phyllanthus (Phyllanthus tenellus Roxb.). Flowers arise from the leaflet axil (A).

Other Weeds

Other weeds that may be encountered in the greenhouse are listed below.

Liverworts

Liverwort (Marchanthia polymorpha L.) is a primitive plant that reproduces by spores. Plants are either male or female. Infestations in a greenhouse are common on consistently wet media in pots and trays and in wet areas under greenhouse benches. Liverworts can harbor aphids. (Figure 11)

Figure 11. Liverwort (Marchantia polymorpha L.) showing gemma cup (A), a means of asexual propagation.

Mulberry Weed

Mulberry weed (Fatoua villosa L.) was introduced to this country in the 1960s and has spread over most of the eastern United States. It is becoming a nuisance weed in the container nursery industry. It can develop from seed to flower in fewer than 2 weeks. (Figure 12)

Figure 12. Flowers and leaves of mulberry weed (Fataua villosa L.).

Dodder

There are many species of dodder (Cuscuta spp.) that cause weed problems in the horticulture industry. Dodder is actually a vine-like parasitic plant (Figure 13). Once it has attached itself to the host plant, it loses its root system. Dodder derives its food from the plant it is parasitizing and has no need for leaves or chlorophyll. For this reason, the plant is essentially leafless and yellow to orange. Dodder is closely related to morningglories and is considered an annual. The best control method is to remove infected plants or plant parts.

Figure 13. The parasitic vine, dodder (Cuscuta spp.), parasitizing another plant. A = orange, yellow dodder vine; B = cluster of dodder flowers.

Asters

Asters are a family of plants (Asteraceae) composed of numerous species, many of which can be present in greenhouses. The main reason for this is wind-blown seeds. As with dandelion, many asters are equipped with an umbrella-like structure that allows the seed to take to the air and travel for miles. Some asters that might be encountered in the greenhouse are dandelion (Taraxacum officinale), common groundsel (Senecio vulgaris), wild lettuce (Lactuca spp.) and sowthistles (Sonchus spp.) [Figure 14]

Figure 14. Aster species growing in a container.

Grasses

Grasses are a large family of plants with many weedy representatives. Grassy weeds can enter on contaminated seed and plug materials. Seeds also can come from adjacent fields, woods and just about any weedy areas outside the greenhouse. Some grasses that may be encountered are annual bluegrass (Poa annua), barnyardgrass (Echinochloa crus-galli), goosegrass (Eleusine indica) and crabgrass (Digitaria spp.) [Figure 15]

Figure 15. Annual bluegrass (Poa annua L.) growing in a container.

Weed Sources

Other weeds may be encountered, depending on growing conditions and locale. Many weed problems, however, originate from seed sources contaminated with weed seed and plugs that are contaminated with weedy plants. Other sources of weed seed are fields, woods and weeds growing adjacent to greenhouses. Any plants (woody or herbaceous) that produce wind disseminated seed can make their way into the greenhouse and cause a weed problem.

Weed Control

Sanitation

Sanitation is critical to keeping weeds out of the greenhouse. Order your seed and propagation material from a reliable source. If your seeds germinate with unwanted weeds, and you receive propagation materials (plugs) with unwanted weeds, make the supplier aware of his weed problem so he has a chance to correct it. If possible, consider other suppliers.

Be aware of your stored growing media. Keep growing media covered and control weeds around the media storage area. If your mix contains any soil (rare these days), make sure it is either stem sterilized or fumigated. Keep greenhouse floors as clean as possible. Clean up spilled media in potting areas (Figure 16). Clean up growing media left over by tipped pots and dropped trays. When building new greenhouses, consider using concrete for the entire floor. Even though the initial investment is high, over time you will spend more money keeping non-concrete areas clean and weed free (herbicides and weed mats are expensive).

Figure 16. Clean up any spilled media. It can provide an excellent growth medium for unwanted weeds.

Weed Control Outside the Greenhouse

The likelihood that a weed will become a problem in the greenhouse increases the closer the weed is to the greenhouse. Although it is impossible to completely control nature, you can do certain things to decrease the chances a weed will become a problem in the greenhouse:

• Regular mowing of grassy areas around the greenhouse. This will keep the majority of the vegetation from flowering and producing seed.
• If feasible, eliminating any vegetation adjacent to the greenhouse.
• Mowing of vegetative areas around above-ground water sources. Certain weed seed can be easily transferred by water.

Weed Control Inside the Greenhouse

Despite your best efforts, weeds may still be a problem in greenhouses. At this point, four options are available to control these weeds:

• Physically remove the weeds.
• Dispose of infected plant material.
• Remove crops from the house and allow weeds to dry out and die.
• Use chemical weed control.

Physically removing weeds and disposing of infected plant materials.

Most weeds piggy back their way into a greenhouse. Small pots and flats containing weeds are usually the start of greenhouse weed problems. Obviously, you want to remove any weeds you see growing in your greenhouse. Remove weeds growing in pots or flats, under the benches or in areas with media buildup as quickly as possible. If flats or pots are severely infested with weeds, remove the infested plant or group of plants to an isolated area where you can treat them or dispose of them. Most of the greenhouse weeds have very fast life cycles and can produce a large amount of seeds in a short window of time.

Drying out the greenhouse.

If you have a 1- or 2-month period when your greenhouse is empty, you can dry out the weeds in the greenhouse as a means of control. If your greenhouse is water tight, you can kill the weeds by simply allowing no water to enter the greenhouse. This will control most if not all live weedy vegetation. Unfortunately, seeds will not be controlled, and these seeds can become a problem as soon as the greenhouse is back into production.

Chemical weed control.

When the weed problems become too large and costly for your staff to manage, many people turn to chemical weed control. Herbicides are chemicals used to kill plants. There are two types of herbicides -- preemergent and postemergent. Preemergent herbicides control only ungerminated seeds, though some will control recently germinated plants. Most postemergent herbicides will control only emerged plants and have little or no preemergent activity. The preemergent and postemergent herbicides can be selective or non-selective.

Selective herbicides are herbicides that kill one group of plants but not another. A good example is sethoxydim (Poast® and Vantage®), which controls grasses without injuring broadleaf plants. Non-selective herbicides control all vegetation to which they are applied. Unfortunately, few herbicides can be used in the greenhouse. All of the herbicides available are broad spectrum postemergent herbicides (Table 1). No preemergent herbicides are registered or labeled for greenhouse use. All the greenhouse herbicides are postemergent, non-selective herbicides and cannot be used over the top of desirable crops. This may change in the near future, as many crops (corn, soybeans, rice, etc.) have been genetically manipulated to tolerate glyphosate and glufosinate. At the time of publication, however, no such ornamental plants are commercially available to the greenhouse industry.

Few herbicides are available for use in the greenhouse because of liability concerns. Manufacturers steer clear of registering herbicides in greenhouses because of volatility potential of herbicides in an enclosed environment.

To increase the herbicide arsenal available to greenhouse growers, consider moving weed infested pots and flats outside. Plants growing outside in containers are considered containerized ornamentals, and numerous preemergent herbicides are available. Also, if a greenhouse is covered with a removable plastic film, you can remove the film, and the plants inside can now be considered outdoor grown containerized ornamentals. When using any herbicide, it needs to be effective at controlling the targeted weed and safe on the ornamental to which it is applied. For these reasons, it is vitally important that you read and understand the herbicide label. Reading the label will greatly reduce herbicide failure as well as prevent injury to desirable plants. Also check the label for any restrictions on re-covering the greenhouse. Most herbicides require at least 2 weeks between herbicide treatment and re-covering the greenhouse structure.

Herbicide precautions.

When using herbicides around a greenhouse, avoid using postemergent herbicides that can become volatile and easily enter the greenhouse ventilation system. Postemergent turf herbicides containing 2,4D, 2-4DB, dicamba, MCPP and MCPA can easily volatalize and move into the greenhouse via the ventilation system. Once inside the greenhouse, these herbicides can cause very distinguishable injury symptoms that include cupping and strapping of plant foliage (Figure 17).

Figure 17. Leaf cupping of a blackberry plant caused by dicamba.

If you detect herbicide injury on greenhouse plants, several things can be done to save plants and get the greenhouse back into production. First, determine where the herbicide injury is coming from. Herbicides or herbicide vapors may be coming from treated plants brought into the greenhouse, from previous herbicide contamination in the greenhouse, or from herbicide vapors drawn into the greenhouse by circulation fans. If you can determine the source of the herbicide injury, you can do several things to eliminate the source of contamination and possibly save damaged plants.

If someone makes herbicide applications near the greenhouse, there's a good possibility that, at the time of that application, circulation fans brought herbicide vapors into the greenhouse. Try to contact the person who applied the herbicide, and ask them to stop herbicide applications close to the greenhouse. If the applications cannot be stopped, consider turning off fans when these herbicides are being applied.

If plants are continually injured when placed into a contaminated greenhouse, herbicide vapors probably are coming from within the greenhouse. You can employ certain measures to decontaminate the greenhouse.

First, remove the desirable plants from the greenhouse. Water wash the entire greenhouse interior with a high pressure sprayer. Avoid any motors, electrical panels and other mechanical devices when water washing. There is unpublished information that indicates that a strong base (e.g., household bleach and ammonia) included in the wash mix (1:10 base to water) helps neutralize herbicides. If this wash is performed, make sure you follow it with a pure water wash. Depending on the herbicide and amount of herbicide that entered the house, you may want to use activated charcoal to help neutralize the herbicide. Before washing, coat the greenhouse floor with a 1- to 2-inch layer of activated charcoal. Allow the charcoal to remain for 1 to 2 weeks, and then remove it. Follow this with a high pressure water wash.

The possibility of herbicide injured plants recovering depends on the sensitivity of the plant or plants to the contaminating herbicide(s) and the dose of herbicide(s) the plant received. Determining the herbicide that contaminated the greenhouse can be done by certain laboratory techniques, but this will do little to help the herbicide injured plants recover. You may try to save injured plants by pruning out as much of the injured plant material as possible. This will produce new growth. By producing new growth, the plant can further dilute the herbicide from its system. Pruning may or may not save the herbicide injured plants, and your only option may be to discard the injured plants.

Several herbicides are available for use in the greenhouse (Table 1). These herbicides are postemergent herbicides primarily intended for use under greenhouse benches and not for use in flats or containers. Do not use any other herbicides in greenhouses. You may be tempted to use total vegetation control herbicides. These herbicides are not labeled for use in greenhouses, and they can persist in soils for several months, possibly injuring plants through harmful vapors.

Attention!

Pesticide Precautions

• It is the responsibility of the pesticide user to observe all directions, restrictions and precautions on pesticide labels. It is dangerous, wasteful and illegal to do otherwise.
• Store all pesticides in original containers with labels intact and behind locked doors. KEEP PESTICIDES OUT OF THE REACH OF CHILDREN.
• Use pesticides at correct label dosage and intervals to avoid illegal residues or injury to plants and animals.
• Use pesticides carefully to avoid drift or contamination of non-target areas.
• Surplus pesticides and containers should be disposed of in accordance with label instructions so contamination of water and other hazards will not result.
• Follow directions on the pesticide label regarding restrictions as required by State or Federal Laws and Regulations.
• Avoid any action that may threaten an endangered species or its habitat. Your county extension agent can inform you of endangered species in your area, help you identify them and, through the Fish and Wildlife Service Field Office, identify actions that may threaten endangered species or that habitat.

Trade and brand names are used only for information. The Cooperative Extension Service, University of Georgia College of Agricultural and Environmental Sciences does not guarantee nor warrant published standards on any product mentioned; neither does the use of a trade or brand name imply approval of any product to the exclusion of others which may also be suitable.

Bulletin 1246/May, 2004

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