Preparing Nursery Plants for Winter
Most plants produce a flush of growth in the spring, and this
growth may continue throughout the season. Growth gradually slows in
the fall because of cooler temperatures and shorter days. To grow
plants more rapidly, we can extend the period of rapid growth by
fertilizing, irrigating, and controlling pests; however, plants are
less tolerant of low temperatures during this time.
Several physiological requirements must be met to successfully
overwinter nursery plants. To choose the appropriate winter
protection techniques, you must know how plants acquire cold
hardiness and how they are damaged by cold temperatures and winter
Part 1: WINTER ACCLIMATION
To harden plants or induce dormancy, the proper conditions must
occur at the same time. To induce winter hardiness and dormancy,
interaction between photoperiod (length of daylight) and temperature
are important. During overwintering, several internal processes
within plants affect cell membranes, energy storage, leaf coloration,
and abscision in deciduous plants. Fall cultural practices such as
applying fertilizer, irrigating, pruning, digging, and controlling
light can affect these internal hardening processes. Failing to
provide proper conditions by neglecting any one of these factors can
cause winter injury.
Nutritionally balanced plants have the best chance of withstanding winter conditions. If you use granular and liquid fertilizer programs that immediately supply soluble fertilizer to the plant, begin withholding fertilizer about six weeks before the average first frost date (Figure 1). If you use slow-release fertilizers, you may have trouble reducing the plant growth late into the season. Tender growth caused by high or fluctuating rates of nitrogen fertilizer late in the season may also prevent hardening. The source of nitrogen is also important. Nitrate nitrogen is readily available to plants for uptake; however, high nitrate levels may stimulate new shoot growth.
Most information on wintering nursery crops suggest increasing
potassium (K) levels to promote the cell permeability, which is
important in avoiding cellular freeze damage. Although information on
this practice is conflicting, maintaining adequate tissue potassium
levels is advised. Foliar analysis from the North Carolina Department
of Agriculture Plant and Soil Testing Laboratory indicate that
potassium levels in foliage should have an index of 50 to 75 for most
woody ornamentals and that a soil test index value of approximately
50 is adequate. If foliar or soil levels are well below these values,
winter hardiness may be improved by applying potassium.
If you wish to apply a complete NPK fertilizer in the fall, wait
until above-ground plant parts are fully dormant. After deciduous
plants have dropped their leaves, a fall fertilizer application is
usually safe. A moderate level of balanced fertilizer should not
cause plants to break dormancy or reduce hardiness.
Either too much or too little water during the later part of the growing season can reduce the winter hardiness of nursery crops. Regular irrigation during the growing season is necessary for maximum growth and proper nutrient availability. If fertilizer has not been released during the summer because of a lack of water, it may become available during September rains, creating a flush of growth that will not acclimate before cold weather.
In the fall, reduce the frequency of irrigation for
container-grown plants; however, apply enough water with each
irrigation to allow some water to reach the bottom of the container.
Plants subjected to very dry conditions during the fall are less able
to withstand severe winter conditions than those receiving reduced
irrigation even if ample water is provided during early winter.
Decreased survival is linked to reduced energy storage. Drought
conditions in the fall reduce root storage. As a result, plants may
not accumulate enough stored energy for bud break and shoot expansion
in the spring.
When you overwinter plants in the open, you must water them
occasionally. You may increase plant survival if you irrigate
containers before a cold period that is expected to drop temperatures
low enough to freeze the growing medium.
Using irrigation as a winter-protection technique over outdoor growing blocks is feasible only if the plants have shoot growth that has not quite hardened and temperatures are expected to drop near freezing. This technique is frequently used with peaches, apples, and strawberries in spring to protect flower buds from freezing. For nursery crops, this procedure can be used successfully in fall and spring to avoid damage to soft shoot growth. The irrigation must be applied before ambient temperatures reach 32oF and must usually be continued through several daylight hours the next day until the ice begins to melt. If discontinued sooner, freeze damage is likely to occur. However, icing in woody nursery crops also has disadvantages because the heavy coat of ice can break limbs (Figure 2). Unprotected plants (with soft shoot growth) that suffer an early-fall or late-spring frost generally lose the current flush of growth. If soft shoot extension is 6 inches or more, you may need to prune off dead growth. Apply a fungicide in either case. The following flush usually produces multiple shoots from each shoot apex.
Late-season pruning may stimulate bud break, resulting in new growth that does not harden off before cold weather. Avoid pruning within 6 weeks of the average first frost date. Extensive late-fall pruning also creates wounds that do not close until active growth begins in spring. This may increase the opportunity for decay organisms to become established in the wounds.
Both intensity and duration of light affect plant dormancy. In the shade, plants acclimate more slowly than in the sun. For this reason, mountain growers remove shade in September to help harden plants. For example, the portion of the stem that enters the soil or potting medium is the last part of the plant to attain full winter hardiness. Early frosts may cause bark splitting in this area of the stem (Figure 3).
Removing shade in the fall induces more rapid acclimation and
decreases the potential for splitting. In piedmont and coastal
nurseries, considerable growth occurs throughout the fall. Removing
shade from actively growing shoots may cause sun scald on succulent
shoots that are accustomed to shade. Sometimes, during a short period
of time after the new growth hardens but before the extended cold
arrives you may remove shade and increase hardiness. If you are going
to move plants to sheltered, shaded areas, they should be fully
hardened before they are moved. Mulch for winter protection only
after plants are hardened by initial frosts and shorter days.
Mulching may insulate the plants and reduce acclimation. In western
North Carolina, both of these practices are usually performed after
November 15. In other regions of the state, these steps are usually
completed just before Christmas.
Turn off supplemental lighting in the fall if plants are to be wintered in unheated areas. Shorter days are just as essential as reduced fertility, irrigation, and temperature if a plant is to harden properly.
As temperatures drop, plant growth slows and many nursery plants begin winter acclimation and dormancy. Cool temperatures and shorter days initiate the first phase of hardening, allowing plants to withstand a frost but not a hard freeze (Figure 4).
To become fully acclimated so they can tolerate the cold
associated with their hardiness zone, nursery crops require exposure
to temperatures between 32OF and 40OF followed
by temperatures slightly below freezing.
After plants become fully hardened, prolonged periods of warm weather can cause them to lose some degree of hardiness even if all other factors are favorable.
Not all plants can withstand the same degree of cold. They are usually ranked according to hardiness zone. Western North Carolina is generally ranked as Zone 6 or 7 in a normal winter, whereas the piedmont and coastal regions of the state are ranked as 7 and 8 (Figure 5). Local conditions such as air drainage, elevation, slope, and proximity to large bodies of water can influence temperatures within a small geographical area.
Some plants, such as hybrid rhododendrons, have their own rating
Minimum Temp. oF
Damage can occur when frost forms on the leaves of evergreen
plants such as hemlock, mountain laurel, azaleas, rhododendrons,
camellias, osmanthus, and others. If frost covered shoots are exposed
to bright sunlight, freeze damage or "burn" may occur. Foliage
usually turns bright yellow in a few days because of chlorophyll
degradation. This damage is usually easy to diagnose because the
inner leaves (those in the shade) are not affected. There is no
long-term damage from freeze bum. Once nominal growing conditions
resume in the spring, leaves will return to a normal green color.
Wind Burn and Desiccation
When plants lose moisture through leaves more rapidly than the
moisture can be taken up by the roots, permanent damage can occur. On
broadleaved evergreens, this moisture loss results in curled leaves
with dead brown tips or edges. On boxwood and conifers, foliage may
turn bronze before leaf tips turn brown or black.
Drying out or winter desiccation causes more loss than freeze
injury in uncovered nursery stock. Although this condition is
expected in very windy locations, cold, sunny days with minimal wind
can also cause severe desiccation. Wind injury is not always fatal;
however, plants may not be marketable in the spring. If the soil or
planting medium freezes, no moisture is available to leaves and
shoots. Thus, plants can be killed to the soil line and be totally
desiccated even though the temperatures would not have been low
enough to kill them otherwise. In the winter, dead plants around the
edge of unmulched seedbeds and transplant beds are often caused by
Part 2: Protection Techniques