How to Manage Pests
UC 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 and there are no registered preemergent herbicides. 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, sowthistle, willowherb, fireweed, cheeseweed, and prostrate and spotted spurge. Controlling these weeds inside the greenhouse will also help reduce the reservoir of insects and diseases that are often associated with weeds.
Liverwort and Mosses. Liverwort and mosses can be found in many greenhouses where plants are highly irrigated. Their presence is exacerbated when there is high nitrogen in the upper soil surface, such as by top-dressing. These types of plants reproduce vegetatively and by spores and are easily spread throughout a greenhouse. They can compete with the crop for water and nutrients and also create a barrier on the potting media surface that restricts water movement into the container resulting in increased runoff. Decreasing the amount of water applied and avoiding top-dressing, as well as inspecting plants before they come into the greenhouse, can reduce the impact of liverwort and mosses.
Cultural Control. Sanitation is the best method for weed 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) and bittercress. 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. There are no preemergent herbicides currently available for use in greenhouses. Many of these herbicides are quite volatile at greenhouse temperatures and can move and/or accumulate in greenhouses to toxic levels for crop plants. 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 and under the benches, 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 the gravel or soil under the benches to reduce water collecting in low areas. 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.
UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries