Moving Plants Outdoors

May 1, 2007

Growers are cautioned that cold night temperatures are expected this week in most areas. Watch weather reports for your area and plan to cover susceptible plant material growing outdoors.

Some growers are moving cool tolerant plants outdoors earlier in the season to utilize outdoor growing space during early spring and free up space in greenhouses. Safely moving plants outdoors is going to depend on the cold tolerance of the plants, where you are located and how well plants are hardened off. Even perennials, if grown in a greenhouse and not hardened off properly will be injured by frost.

Some cold tolerant plant material includes perennials and selected annuals such as Pansy, Alyssum, Snapdragon, Phlox "Intensia" series, Osteospermum, Nemesia, Calibrachoa, Verbena, Diascia, "Supertunia" series, Bidens and Dianthus. Pleasant View Gardens, Loudon, New Hampshire reports that they have had annual plant material growing outside for about a week and a half without problems so far this year.

Here are some guidelines from Pleasant View Gardens' "Cool Tolerant Annual Program":

  • Get vegetative growth to desired size before cooling down plants (plants grow very slow under 50°F)
  • Run greenhouse cool (45-55°F) for 1 week to harden off plant material before moving outdoors.
  • Keep an eye on the weather forecast - don't move plants outdoors if a hard frost is predicted colder than 28°F) at any point within 3 nights of when plants would go outside. If weather is to be mild move plants outdoors.
  • Cold frames work great (non heated but covered)
  • Avoid low-lying areas as frost settles in these areas.
  • Cover with Remay type product (5 mil thickness) to protect when needed. If temperatures go below 28°F, heavier cover may be needed.
  • Water in the morning, then let plants dry down and go dry into the night.
  • If frost does occur, overhead irrigate in the AM before the sun hits the plants.
  • Good plant spacing is critical for air movement.
  • During rainy periods, watch for both botrytis and nutrient leaching. It is important to be able to fertilize plants. Also, flowering is delayed by colder average daily temperature.

Keep in mind that you are always taking a chance when growing outdoors in early spring and occasionally plants will get damaged. By choosing cool tolerant plants, hardening-off plants, watching weather forecasts and providing extra protection when needed, you will minimize the risk, grow higher quality plants while providing extra growing space.

Note that plant selection and hardening off plants is also good advice for retail customers.

Tina Smith