An important consideration when selecting trees, shrubs, herbaceous perennials, and other plants for the home landscape is cold hardiness. In most cases, plants that are adapted to Iowa's cold winter temperatures don't require winter protection. However, a few commonly grown landscape and garden plants, such as modern roses, strawberries, and chrysanthemums, require winter protection to insure their survival.
Cold temperatures aren't the only potential threat to plants during the winter months. Rabbits can also cause serious damage to trees, shrubs, roses, and other plants.
Modern, bush-type roses (hybrid teas, floribundas, and grandifloras) require protection during the winter months. Exposure to low temperatures and rapid temperature changes can severely injure and often kill unprotected roses.
Hilling or mounding soil around the base of each plant is an excellent way to protect bush- type roses. Begin by removing fallen leaves and other debris from around each plant. Removal of diseased plant debris will help reduce disease problems next season. Then loosely tie the canes together with twine to prevent the canes from being whipped by strong winds. Next, mound soil 10 to 12 inches high around the base of the canes. Place additional material, such as straw or leaves, over the mound of soil. A small amount of soil placed over the straw or leaves should hold these materials in place. Prepare modern roses for winter after plants have been hardened by several nights of temperatures in the low to mid-twenties. Normally, this is late October to early November in northern Iowa, mid-November in central areas, and mid- to late November in southern counties.
Strawberries are also susceptible to winter injury. Temperatures below +20Ã…Â¡F may kill flower buds and damage the roots and crowns of unprotected plants. Repeated freezing and thawing of the soil through the winter months can heave unprotected plants out of the soil and also cause considerable damage.
The application of a mulch in the fall is the best way to protect strawberries. Excellent mulching materials include clean, weed-free straw and chopped cornstalks. (Leaves are not a good mulch for strawberries. Leaves tend to mat together and do not provide adequate protection.) Apply 3 to 5 inches of the material. After settling, the depth of the mulch should be approximately 2 to 4 inches.
Allow the strawberry plants to harden or acclimate to the cool fall temperatures before mulching the bed. Plants that are mulched before they have properly hardened are actually more subject to winter injury. In northern Iowa, strawberries are normally mulched in late October to early November. Gardeners in central and southern Iowa should mulch their strawberry plantings in mid-November and mid- to late November, respectively.
In windy, exposed sites, straw mulches can be kept in place by placing boards or wire fencing over the area. The fencing can be held in place with bricks or other heavy objects.
Though garden mums are also referred to as "hardy," they often don't over-winter well in Iowa. Alternate freezing and thawing of the soil during the winter months can heave plants out of the ground and cause severe damage or even death. Damage is most severe in poorly drained soils.
Gardeners can increase the odds of their mums surviving the winter by protecting them in the fall. Mulches should be applied in late fall after the plants have been exposed to cold temperatures, typically mid-November in central Iowa. Do not cut chrysanthemums back in the fall. While cutting mums back in the fall is a common practice, recent research found that unpruned plants survived the winter better that those pruned in the fall. Simply place several inches of mulch around the mums in the fall. Suitable mulching materials include clean, weed-free straw, pine needles, and evergreen branches. Leaves are not a good mulch as they tend to mat down and don't provide adequate protection.
During the winter months, rabbits often gnaw on the bark of many woody plants. Heavy browsing can result in the complete girdling of small trees and small branches clipped off at snow level. Apple, pear, crabapple, and serviceberry are frequent targets of rabbits. Small trees with smooth, thin bark are the most vulnerable. Other frequently damaged plants include the winged euonymus or burning bush, Japanese barberry, dogwood, roses, and raspberries.
The best way to prevent rabbit damage to young trees is to place a cylinder of hardware cloth (1/4 inch mesh wire fencing) around the tree trunk. The hardware cloth cylinder should stand about 1 to 2 inches from the tree trunk and 20 inches above the ground. The bottom 2 to 3 inches should be buried beneath the soil. Small shrubs, roses, and raspberries can be protected with chicken wire fencing.
While winter can be a difficult time for plants, those that are adequately protected should survive well. The time spent preparing your roses, strawberries, chrysanthemums, and other plants for winter should pay big dividends next spring, summer, and fall.
This article originally appeared in the October 13, 2000 issue, pp. 116-117.
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