Nursery and Greenhouse - Overwintering Perennials

The following is information on overwintering perennials from the New England Greenhouse Update and a factsheet from the University of Massachusetts.

It is time to prepare perennial plants for overwintering. Top growth has died back on many perennials while on others, it will remain over the winter. Prepare plants for overwintering by cutting back dead growth and removing dying debris to prevent Botrytis infection.

Next, irrigate if needed, but allow any foliage to dry prior to covering to prevent Botrytis. Check soil moisture periodically and water if needed, but avoid over-watering plants throughout the winter. If plants are kept too wet, root rot diseases will occur.

The final step prior to covering plants is to provide some type of rodent control. Many growers use commercially available baits while others have reported that human hair or cut up deodorant soap works.

Soil temperatures should be in a range of 30 to 34º F for most perennials. Soil temperatures that are cooler than 30º F may kill some sensitive species. Use a soil thermometer to verify that growing media are in this range.

The key to overwintering containerized perennials is to avoid wide fluctuations in temperature. When plants are allowed to freeze and then thaw, there is a greater risk for plant loss.
Heated greenhouses with roll-up sides are one option that works well for many growers. To use this method, perennial houses are heated to 50°F until all of the plants are well rooted, then the temperature is slowly decreased to 35°F so the plants will vernalize and flower in the spring. The roll-up sides are useful in the spring to regulate temperature and prevent plants from growing too quickly.

One of the simplest techniques growers have been using to keep plants at a constant temperature is placing a sheet of insulating material over containerized plants during the coldest months of winter. Commonly used thermo blanket materials include Microfoam (¼" flexible, polypropylene foam), The Winter Blanket (polyethylene foam, laminated with 3-mil UV resistant P/E) and Guilbond (1/4" closed cell polyethylene foam laminated to white UV treated polyethylene film).There are also fleece materials available that work. Containerized herbaceous perennials that are ready for storage can be placed pot to pot in an upright position on the ground in the production yard and simply covered with a material having insulating qualities. Tall plants can be leaned over just like laying shingles, exposing as much foliage as possible to light. Coverings are usually secured by tucking the edges underneath the containers or by weighing down the edges with heavy objects such as crushed stone or rocks. The thermal blanket sheets should be oriented in a north-south direction. With some thermal blankets, sheets of 4 mil white polyethylene plastic are placed over the blanket; other types do not need this extra layer of plastic. Follow the manufacturer's recommendation. At the end of the winter season, store thermal blankets in a dark area so they will last two or more years.

Another common way growers overwinter plants below freezing is by using unheated quonset polyhouses. These structures are usually constructed with wooden frames to which uniformly bent galvanized pipe are attached. The pipe provides the basic structure that supports one or two (air-inflated) layers of white (not clear) polyethylene that are secured to the wooden baseboards to which the bows are also secured. Under average conditions, temperatures within these structures do not reach levels that result in injury to plants that are stored inside. However, during extended periods of extreme cold (more than two consecutive nights when ambient temperatures go below 0º F), plant injury may occur.

Quonset style polyethylene structures are a rather expensive alternative if one considers only the initial construction costs. However, well-constructed frames and bows will last ten or more years before any major renovations are needed. In addition, thousands of plants can be covered with plastic in a matter of hours. For large operations, this offers considerable efficiency as compared to other methods of overwintering. Finally, these structures allow the grower to inspect plants and to remedy any problems that may be noted.

Plants which are considered marginally hardy when grown in containers should get additional protection with thermal blankets or a heater inside the structure. An entirely different technique such as the plastic-straw-plastic cover is described later in this article. For structures with two layers of covering, growers use white polyethylene for the outer layer and clear or white for the inner layer. This will help control the amount of light and heat in the structure. Even so, plants in houses covered with white plastic can begin to grow too early during periods of unusually warm weather. To avoid premature budbreak, orient the polyhouse in a north-south direction. North-south oriented polyhouses tend to be cooler than east-west oriented structures. If more than one type of plant is to be overwintered in the same structure, place the most cold-hardy types near the walls. This is the coldest part of the polyhouse. Plants should be spaced pot to pot for the best insulation.

Information from Tina M. Smith, Extension Specialist, UMass Extension, Amherst, MA