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How to Manage PestsUC Pest Management Guidelines
It is difficult to control weeds in greenhouses because greenhouse-grown plants are generally sensitive to herbicides and weeds are often hard to reach. Only a few weed species commonly present problems in greenhouses, and they are all closely associated with high moisture and nutrients; they also spread rapidly if they are allowed to become established. The most common weeds in and around greenhouses include annual bluegrass, lesser-seeded bittercress, creeping woodsorrel, pearlwort, common chickweed, moss, and liverwort. Others that may be present include cudweed, fireweed, cheeseweed, and prostrate spurge. Controlling these weeds inside the greenhouse will also help reduce the reservoir of insects and diseases that are often associated with weeds. Cultural Control. Sanitation is the best method for good control. Weeds may be brought into the greenhouse in potting mix or with cuttings, bulbs, or other plant material, or on dirty pots and tools. If weeds do get in, they should never be allowed to flower and seed. This is especially true of creeping woodsorrel (oxalis). Maintain trash cans in the greenhouse for weeds that are pulled during maintenance, so they can be readily removed before flowering. Hand-weed frequently (daily or weekly) so no weeds go to seed. If the floors are concrete, regularly wash or sweep away soil that drops to the floor so that weeds will not establish or seed. When crops are rotated, clean weeds out of the greenhouse. Irrigate with water that is free of weed seeds or fungal spores. If using raised or self-contained beds, sterilize soil before planting by either steaming or solarizing. These methods are described in "General Methods of Weed Management" (solarization) and in "Container Nurseries" (steaming). Herbicides. A preplant herbicide can be used where soil is not sterilized before planting. Preplant herbicides are used when the crop is not present, because they will kill the crop as well as germinating weed seedlings and established plants. Methyl bromide is the most broad-spectrum preplant herbicide, controlling more weeds and other organisms than any other preplant herbicide. However, it will not control seed of little mallow (cheeseweed), field bindweed, or many clovers. Metham or dazomet are other preplant herbicides, but these materials have a longer waiting period from the time of application to when the crop can be planted than methyl bromide. There are no preemergent herbicides currently available for use in greenhouses. Even though some herbicides may be labeled for use in a crop, it must specifically indicate to be used in greenhouses to be legal and safe. On the greenhouse floor, a postemergent herbicide treatment can be used to reduce weed populations and to keep the weeds from flowering and seeding. Try to have good drainage and level low areas that remain wet. Wet areas increase the chance of mosses and liverwort infestation. Air movement at the floor level will help dry off the floor and will also reduce the chance of infestations of weeds that favor wet areas. After a crop has been harvested, remove any weeds to keep them from seeding so new seeds will not be added to the seedbank in the soil.
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UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries |
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