Greenhouse and Nursery - Moving Plants Outdoors

With greenhouses full and rain delaying plant sales, many Delaware greenhouses and nursery growers are moving cold tolerant plants outdoors to free up space. While risk of frost diminishes after April 20, it is not entirely risk free. The following is an article on the subject.

Growers can free up greenhouse space, by safely moving cool tolerant plants outdoors. This space can be then be refilled with another crop. In addition to saving growing space, plants grown at cool temperatures “harden off” or acclimate allowing retailers to display plants safely outdoors.

How well plants finish outdoors depends on the genetic cold tolerance of the plants, the air movement around plants, how well plants are acclimated and how well the plants are maintained, once outdoors. Tropical and subtropical warm crops such as alternanthera, angelonia, New Guinea impatiens, lantana, vinca, celosia, cleome, coleus, cosmos, gomphrena, ipomoea, melampodium, portulaca, sunflowers, zinnias, tomatoes, peppers and squash plants are naturally sensitive to cool temperatures and are not a good choice for early spring outdoor yards.

Herbaceous perennials and cool tolerant annuals, however, can be grown outdoors, but 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.

Plants will adapt best to cooler temperatures when they have been started at optimum growing tempertures and have a well-established root system. Once plants get to their desired size they should be exposed during the day to colder temperatures and moved back inside if frost is a risk. Monitor the weather forecast and avoid moving plants 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.

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 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 will work more slowly in cool media. It may take longer to see results of an application or an application may be less effective.

Be prepared to cover plants if temperatures go below 28°F. An over-wintering thermo blanket such as polyethylene or polypropylene foam can be used for this purpose.

Adapted from the current edition of the New England Greenhouse Update. http://www.negreenhouseupdate.info/greenhouse_update/index.php