by Tamara Scully
The location of your nursery, what you’re growing and how you’re growing it will influence the type of preparation your plants need to survive the winter season and emerge thriving in the spring. Protecting against freeze, frost, desiccation, rodents and winter injury requires planning.
Several things can be harmful to your nursery stock moving into the winter season. Plants have a period of dormancy which varies with species. A plant grown in warmer weather regions won’t acclimate to colder temps as well as the same plant grown in a cold weather environment.
Plants enter dormancy as they experience shorter day lengths and cooler nighttime temperatures. Changes in cell membranes, affecting water transport, occur. As plants prepare for winter conditions, nursery growers can take steps to assist in this natural process and not impede its progress.
In the nursery environment, some best practices implemented now will help plants move into winter with the best chance to survive and flourish next spring.
Excessive fertilization of any plant prior to winter dormancy can do more harm than good. All plants need enough nutrients to get through the dormant cycle. Inadequate nutrition means the plant isn’t able to bud and grow vigorously when it breaks dormancy. Too much fertility prior to the dormant period means new growth, which means winter damage is likely to occur. Once acclimated and dormant, however, a dose of balanced fertilizer can be added to containers. Controlled release fertilizers applied in autumn aren’t expressed in the winter if temperatures remain cold.
Irrigation needs of plants entering dormancy change. Dry growing media will freeze more rapidly than when kept moist. Plants cannot obtain water from frozen growing media. Before covering plants for the winter, watering them well will slow down the freezing of the container substrate while providing needed moisture to the plant. Keeping plants irrigated but not excessively moist as they enter dormancy provides protection from root injury, yet prevents root rot issues. More on calculating the optimal amount of irrigation in advance of winter can be at johnston.ces.ncsu.edu/wp-content/uploads/2012/08/AG-454-1-publication.pdf.
During winter, plants may need additional irrigation. Prolonged cold, sunny or windy conditions will increase the need for winter irrigation. As irrigation water freezes, it gives off heat to the surrounding area, keeping things warmer for the plants too.
Roots are killed at various temperatures, depending on the plant. Container-grown plants are exposed to lower temperatures than those grown in the field, even if the containers are protected from the environment in some manner. Depending on root hardiness, plants may require more or less protection. Grouping plants that require similar levels of protection together reduces labor needs and allows hardier plants to be arranged to buffer those requiring more protection.
Autumn pruning of trees and shrubs should be completed six weeks prior to average frost date to prevent new growth and avoid injury that may not heal properly and promote disease. Trim herbaceous perennials to just above the crown; trim back woody plants after initial dormancy has occurred, and new growth won’t be stimulated, prior to storing them for winter.
Rodents can be kept out of high tunnels or greenhouses by burying fine mesh screen wire at least six inches deep around the perimeter. The material should be bent outward at a 90º angle. Keep vegetation mowed in areas where plants are overwintered, and place traps or repellants under covers to prevent damage from mice and voles.
Pot plants as early as possible, allowing adequate time for root development to occur prior to acclimation. Weeds should be removed from pots prior to winter storage, and a pre-emergent can be applied. Fungicides can be used at this time as well, but must be dry before covering plants.
Even if roots do not die, desiccation can occur during winter, damaging the plant. Protecting plants with row coverings, in structures and/or by jamming them together with perimeter protection can protect from winter injury.
Once plants have fully hardened, they can be covered up for winter. If protected too soon, acclimation may be delayed and plants will ultimately be left more vulnerable to frost and freeze damage.
Container plants can be overwintered in unheated greenhouse structures, by jamming them together outdoors or by covering with winter blankets. Heated structures can also be utilized if plants are to be kept thawed and not allowed to freeze. A mixture of methods might work best, depending on infrastructure available, as well as the species of plants being grown and their individual winter hardiness.
Jamming involves moving plants close together in their overwintering location. Grouping plants closely together provides protection from the elements. Depending on the plant, the spacing can vary. Plants can also be laid on their sides, which works particularly well for taller plants. The perimeter around jammed containers can be lined with straw bales, sawdust or other mulching material spread over the pots. Newspaper, leaves or compost can be used as mulch. Some growers wrap the perimeter of the blocks with greenhouse plastic.
A thermal row covering, or winter blanket, can also be used to cover containers. These are available in various thicknesses, depending on the level of protection required. Some growers use a layer of plastic, but the plastic must not touch any foliage, as it would cause desiccation, and shade cloth is often used as the protective barrier. Winter row coverings need to be weighted down on the edges. Rodent prevention is needed undercover too.
Containers placed in unheated high tunnels can have additional winter coverings placed over them if needed. In unheated structures covered with white or clear polyethylene plastic, sides are rolled up or cut as the weather warms in spring to prevent early season excessive growth.
When using structures, whether heated or not, planning for snow and ice accumulation requires adequately preparing the structure to support the additional weight.
For those growing field-grown trees, shrubs or perennials, the soil itself offers plant roots winter protection. Comparatively, field-grown nursery crops require less preparation for winter than do those container-grown, but still require proper autumn handling – no excessive pruning, plus adequate fertilization and irrigation to prevent desiccation of the roots. Broadleaf evergreens are prone to desiccation from the wind and should be protected by windblocks or shade structures whether in the field or in containers.
Putting your plants to bed for the winter means healthy plants, ready for spring sales, are just a season away.