It is a good time to clean empty greenhouses rather than to wait until just before starting spring production. Cleaning now helps to eliminate over-wintering sites for pests in unheated greenhouses, especially if the winter is unseasonably warm.
- Begin by removing all leftover plants, weeds and debris.
- Clean the floor of spilled soil and organic matter.
- Check areas around furnaces and alongside-walls and remove all those small weeds that are often overlooked.
- Repair tears in worn weed barriers. If replacing worn weed barries, do not place gravel on top of the weed mat for it traps spilled media and holds moisture creating an ideal environment for weeds, diseases, insects and algae.
- It is also a good time to correct any drainage problems and low spots in greenhouses.
The pre-emergence herbicide, indaziflam (Marengo®) is labeled for use on greenhouse floors in empty greenhouses. Follow label directions, wait 24 hours before introducing plant material into the empty greenhouse after applying this material.
Biocontrols will not clean up existing, out of control pest populations. However, if a greenhouse is cleaned of weeds, old plants and debris, biocontrols may help prevent pests from overwintering especially during a warm fall. Here are some biocontrol options for two-spotted spider mites, aphids, thrips and whiteflies.
Two spotted spider mites: Predatory mites such as Phytoseiulus persimilis, Neoseilus californicus, Amblyseius andersonii, Neoseilus (Amblyseius) fallacis and the predatory midge larvae, Feltiella acarisuga are commercially available. Rove beetles (predatory ground beetle) and Stratiolaelaps scimitus (a type of predatory mite) will feed upon spider mites that have moved off of plants to find crevices in the greenhouse where they go into dormancy.
Thrips: The predatory mite Amblyseius cucumeris can be used for thrips during September and October. Rove beetles and Stratiolaelaps scimitus (predatory mite) are effective against pupae in the soil. Both of these predators are active at cool temperatures (50F).
Whiteflies: Good weed management and the absence of plant material should eliminate whiteflies from the greenhouse, (assuming you are not growing poinsettias), however, whitefly parasites such as Eretmocerus emericus (for use against silverleaf whitefly) or Encarsia formosa (for use against greenhouse whitefly) can be released.
Tina Smith, UMass Extension and Leanne Pundt, UConn Extension
IPM Laboratories Inc. Newsletter via email, September 1, 2015
Biological Control of Western Flower Thrips (UConn Extension)
Biological Control of Two-Spotted Spider Mites (UConn Extension)
Eretmocerus emericus (Cornell University)
Encarsia formosa (Cornell University)