It is that time of year for extreme temperatures and weather conditions. Monitor the weather forecast and avoid moving cold tolerant annuals and herbaceous perennials outdoors if a hard frost (colder than 28°F) is predicted at any point within at least three nights of when plants would be put outside. Avoid placing plants in low-lying areas because frost will more likely settle in these areas. Be prepared to cover plants if temperatures go below 28°F.
While herbaceous perennials and cool tolerant annuals, can be displayed or grown outdoors, they need to be acclimated or hardened off first. Cool tolerant crops include pansy, annual phlox, alyssum, osteospermum, nemesia, calibrachoa, verbena, diascia, bidens, antirrhinum (snapdragon), mimulus, lobelia and petunias.
Begin with just a few plants and experiment with cultivars, making adjustments as needed. Some varieties of the same type of plant have varying degrees of cold hardiness. Learning which plants perform best will take some experimenting.
Plants will adapt best to cooler temperatures when they have been started at optimum growing temperatures and have a well-established root system. Once plants get to their desired size, lower the greenhouse temperatures to 45° - 55°F for at least a week to acclimate them for outdoors.
Botryis blight and lack of fertilizer are two common problems which occur with outdoor growing. To prevent Botrytis, water plants in the morning and let the foliage dry before night. Plants growing outdoors may require less water and less frequent irrigation and this means that plants are fertilized less often. Although plants are being grown cool, it is important to continue to fertilize plants and maintain proper fertility. Pay particular attention during rainy periods, (like this weekend) and monitor plants for both Botrytis and nutrient leaching.
Cool media temperatures also increase the risk of root rots caused by Pythium, Rhizoctonia and Thielaviopsis. Regularly monitor roots for disease and apply fungicides early. Fungicides work more slowly in cool media. It may take longer to see results of an application or an application may be less effective.
Without some sort of protection, you are always taking a chance when growing outdoors in early spring and occasionally plants will get damaged. By choosing cool tolerant plants, acclimating plants, closely watching weather forecasts and providing extra protection when needed, you will minimize the risk, grow higher quality plants and provide an extra turn in production.
Fact Sheet: Moving Cool Tolerant Plants Outdoors: Risks and Benefits Link to: Proven Winner outdoor growing guidelines
Re-posted from April 17, 2014
Tina Smith, UMass Extension and Leanne Pundt, UConn Extension
