Answer: I am pleased to provide you with information regarding over-wintering your nursery crops.
Winterization of your nursery crops depends on what they are planted in. Bare root crops and burlapped roots are the most susceptible to winter damage and obviously those in the ground are the least susceptible and need very little preparation. I recall you saying that your crops are overwintered in plastic pots, which is one of the most common planting methods for nursery crops. I found a newsletter article from NCSU regarding overwintering nursery crops. This information would be most applicable to your situation, see the article titled, “Preparing Nursery Plants for Winter”.
The key to overwintering is to acclimate your plants so that they are dormant and remain dormant during the winter. Protection from dessication from wind as well as occasional watering is also important. Once plants are dormant they are ready to be protected. Before they can be covered, however, they need to be properly prepared by following these guidelines:
• Containers should be well watered before going into cover.
• Containers should be placed on a clean, level surface that is not subject to flooding. Growers often use gravel and/or landscape cloth as surfaces for Quonset greenhouse structures.
• Foliage should not be overly wet right before covering.
Ideally, the best possible protection is to cover plants once they reach a state of dormancy, and then keep temperatures cold enough to maintain this dormant state until covers are removed in the spring. Once plants are covered up, they cannot be ignored. Houses must be scouted periodically to track environmental conditions, plant quality and any pests. Overwintering structures are natural havens for rodents, and it is necessary to use rodent bait stations to prevent excess rodent feeding on roots and bark. Watering needs must be monitored frequently, especially if temperatures inside the poly house warm up on sunny days.
Plants will dry out under cover, especially evergreens. Plants on the edge of the poly houses may be particularly susceptible to drying out. Before covering, consolidate plants as close as possible and water well. Moist media freezes slower and releases heat compared to dry media offering protection to the roots. The moisture level of the media should be checked during the winter and irrigated if necessary. This will also increase the relative humidity, which helps guard against desiccation. Structureless systems will not need watering if sealed properly.
Since your climate is sporadically warm, I advise you not to use clear plastic over the winter. Many nurseries overwinter their nursery crops with a white poly cover to prevent solar heat up and desiccation.
Visit this site for a table of minimum temperatures in preventing over-wintering damage with specific crops.
Below you will also find further information about overwintering your container ornamentals. I have referenced a fact sheet from UMASS Extension as well as another more applicable to your climate from the University of Kentucky Extension Service.
Overwintering Nursery Crops
Winston C. Dunwell & Robert E. McNiel
University of Kentucky
Department of Horticulture
UMASS Extension. (no date). Overwintering Container Grown Ornamentals. UMASS Green Info Fact Sheet.
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