Eight Ways to Minimize Winter Injury
Reprinted from from the MAC Chapter Newsletter
Select varieties that are adapted to the climate. Choose plants that can withstand the minimum winter temperatures in your area and that are also able to fully acclimate under your growing conditions.
Keep plants as healthy as possible and avoid cultural practices that interfere with cold acclimation. Healthy plants cope with stresses better than unhealthy plants. Plant sugar content has been correlated with cold hardiness and plants can build up these sugar reserves better when they are kept free of insect damage, disease, water and nutrient stress, BUT do not over water or over fertilize, as doing so will damage the plant. It is best to avoid late summer or early fall applications of nitrogen fertilizer to slow growth in the fall. Late lush growth forced by fertilizer does not harden off well. Irrigation should also be cut back, though do not let the plants get too dry especially as they go into freezing weather or the leaves will show desiccation damage. Don't be in a hurry to prune as this can stimulate growth. It is better to wait until later December or January for dormant pruning. (Some light fertilizing can be done after frost without stimulating growth, according to some horticulturists.)
Mulch plants, particularly around the roots. Snow is a good insulator, providing you have it. Straw or sawdust can be used for mulch and will help control weeds, but extra nitrogen will be needed, as the material breaks down. Leaves can also be used, especially shredded leaves. (However, you must be careful to put the mulch down only AFTER the ground freezes or field mice will find the mulch a splendid winter haven with a vast supply of rhododendron roots for food.)
Cover up the plants. Synthetic films and fabrics are now frequently used for winter protection. An ideal cover should not trap sunlight and cause heat buildup which will kill the plants, as they would start growth and then die. The ideal cover reduces temperature fluctuations. Plastic netting or non-woven agricultural fabrics made of spun bonded polyester or polyolefin are both good. Polyethylene film, both clear and white, helps reduce freezing injury but can cause problems in early spring with heat buildup and forced growth.
Protect against sunscald. This injury occurs to the bark usually on the south west side on clear cold days. Shielding your trunks from direct rays of the afternoon sun can help prevent this problem. Snow fencing can be used to shade the trunks or waterproof paper tree wrap used around the trunk.
Reduce desiccation. When soils are frozen plants cannot take up water to replace what they lose through their leaves. Two ways to reduce desiccation injury are to ensure the plants do not go into dormancy in a state of water stress and to protect the plants from drying winds. Snow fence and other shrubs can protect your less hardy plants.
Minimize frost heaving. Young plants can be heaved right out of the soil due to alternate freezing and thawing. Freshly tilled soil is more prone to frost heaving than well settled soil. Organic matter can act as a cement to hold the soil structure together and reduce frost heaving. Mulching is also very valuable in preventing frost heaving.
Minimize trunk splitting and limb breakage. Splitting of bark is associated with sunscald, but a different type of injury can be caused by repeated freezing and thawing, that of splitting of trunks or breaking of limbs. Water gets trapped in bark inclusions in narrow crotch angles. It freezes and expands and forces the bark on limbs apart. During a thaw, more water seeps in and enables the crack to widen more as the water freezes again. Eventually the wood splits. Excessive snow load contributes to the problem. Proper pruning to encourage wide crotch angles and supports for young or structurally weak plants helps reduce breakage.