Overwintering can be easier than you think

Containerized perennials held over winter need protection from cold temperatures. Unlike field-grown plants, roots of container-grown plants are not buffered from cold temperatures by the soil, and the only insulation is a thin wall of plastic. Besides root hardiness, factors such as soil moisture, temperature fluctuations and root development in the container also affect plant survival.

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Perennials are susceptible to root injury over a wide range of growing medium temperatures. Generally, protection is needed in areas where the average temperature is 20°F or lower.

Different techniques can be used for overwintering, depending on the expected minimum temperatures, the length of time temperatures are expected to remain low, temperature fluctuation, container size and plants’ root hardiness. Some experimentation may be necessary to determine which method of overwintering works best.

Preparing plants for winter

Potting up. Successful overwintering begins when plants are potted up by late September or early October to allow them to become established for several weeks before cold weather arrives in late November. The more established the root system, the better the chance the plants will survive winter. The smaller the container, the more protection will be needed. Poorly established and pot-bound plants tend to overwinter poorly.

Trimming. Many perennials have top growth that dies back with the onset of cold weather, while others retain their foliage throughout winter. Plants that die back should be trimmed back to the crown and cleaned up before covering. Evergreen perennials such as Phlox subulata should be protected from direct contact with overwintering coverings either by laying pots on their side or by placing a structure over them.

Watering. Before covering, water the growing medium well to prevent desiccation, but allow the foliage to dry. Check soil moisture periodically throughout winter and avoid overwatering, which promotes root, crown and foliar diseases. In an unheated poly-covered house, water may be needed as often as once every two weeks. Ground beds covered with thermoblankets hold moisture and probably don’t need watering.

Pest control. Some growers treat plants with a broad-spectrum fungicide before covering. Overwintered perennials also need some type of rodent control. Greenhouses can be made rodent tight by burying fine mesh screen wire such as hardware cloth around the perimeter under ground and bending it outward at a 90-degree angle, leaving it at least 6 inches deep. Mowing and cleaning up natural vegetation eliminates protected areas for rodents.

Two basic methods for overwintering are maintaining plants below freezing or above freezing.

Maintaining plants below freezing

Thermoblankets. One of the simplest techniques to keep plants at a constant temperature below freezing is by placing a sheet of insulating material over containerized plants during the coldest months outdoors. Thermoblanket systems are relatively inexpensive.

In the production yard, perennials are placed pot-to-pot in an upright position on the ground; tall plants are leaned over, just like laying shingles; and plants with foliage are laid on their side. Cover plants with a thermoblanket such as 1/4-inch flexible polypropylene foam or polyethylene foam laminated to white UV-treated polyethylene film. Pull the blanket tight over the containers and secure the edges about 12 inches beyond the pots with concrete blocks. Follow the manufacturer’s recommendations.

In areas where air temperatures are likely to drop to minus 10°F, or where the site is exposed to high winds, consider using a double-foam layer, placing plants inside an unheated greenhouse under foam or putting plants in a minimally heated greenhouse.

Thermoblankets tend to trap and retain moisture, which may eliminate the need for irrigation. Problems under the blankets are difficult to detect because the blankets are opaque.

Pots tend to warm prematurely during winter thaws or near the end of storage. This can cause shoot growth that can be injured by the return of freezing temperatures. Venting may be needed during unexpected periods of warm weather.

Thermoblankets are usually so tight that ventilation is practically nonexistent. Check periodically for disease development.

Sandwich method. Place plastic sheeting or spun-bonded fabric directly over plants and then cover with a 6- to -12-inch-deep layer of fluffed straw. Cover the straw layer with another layer of white plastic sheeting. This insulating technique is especially effective in Northern regions where temperatures remain consistently low all winter. Plants are so well-insulated that they are slow to warm in spring and remain frozen until the cover is removed.

Unheated poly-covered houses with foam blankets. Containers are stored in narrow hoop houses covered with white polyethylene film. White poly reduces fluctuating temperatures and maintains lower temperatures than clear poly film. However, the lack of light in spring can cause plants to stretch. If clear poly film is used, ventilation is needed.

Once temperatures consistently drop to 25°F-30°F, pots inside unheated structures can be covered with a thermoblanket to further protect plants from extreme cold. This technique creates a microclimate 8°F to 10°F warmer than the air above the blanket inside the structure. Place the thermoblanket over the plants only during extremely low temperatures to avoid excessive moisture buildup.

Maintaining plants above freezing

Heated greenhouse or poly-covered house with roll-up sides. This is the best method for temperature control, especially if minimal heat is used. It also provides more control over moisture levels, plant growth and heat buildup. Greenhouses or poly houses covered with two layers of clear poly film are heated in the fall to 50°F until all of the plants are well-rooted, then the temperature is slowly lowered to 35°F where that temperature is maintained throughout winter. Clear poly is used in this case to allow the maximum amount of sunlight exposure to the crop. Foliage stays drier. In spring, plants break dormancy earlier allowing for early shipping.

Roll-up sides maintain the same temperature inside as outside, whether during a winter thaw or in the spring when warm sunny days cause a rapid rise in the greenhouse temperature and push the plants along too quickly. Rolling up the sides is also useful for hardening off plants in spring and dropping them for protection if unexpected cold weather or snow arrives.

Other methods

Other techniques have been used to protect containerized herbaceous perennials during winter, including retractable-roof greenhouses, covered cold frames, earthen pits, sunken frames, root cellars, barns and sheds, covering with evergreen boughs or fleece (thick felt) and deep snow.

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- Tina Smith