To the beginning gardener, the idea of tucking plants under cover in the fall may seem a natural and desirable way to keep them warm during winter. But the gardener soon learns that it is not cold that does the most damage to perennial plants, but unseasonable warmth. This, of course, does not apply to plants of questionable hardiness in any locality they are not considered here.
Plants of proven hardiness may be relied upon to survive cold weather, even benefit from it, both roots and tops. For plants in general, protection is needed to prevent the soil from thawing out; for evergreens in particular, protection is used to minimize the drying effect of winter winds and sunshine upon the leaves.
The top growth of plants, of course, is subjected to much colder temperatures than their roots. The soil in even our coldest areas rarely freezes deeper than 3 to 4 feet. We find the lowest temperatures in a hard frozen crust at the surface. Even that is rarely colder than zero and tends to insulate the soil below it, where the temperature rises rapidly the deeper you go. Air temperatures fall much lower and occasionally rise much higher than those in the soil. When this happens trouble starts.
Herbaceous perennials with tops that die down to the ground in winter have relatively shallow roots. When an unseasonably warm spell thaws a few inches of the frozen soil crust, puddles are created where water stands. This excludes air from the plant tissues below it, which may cause disease, decay, and death. If the water stands long enough, the plants will drown.
Heaving Of Shallow Rooted Plants
When alternate freezing and thawing spells are frequent, heaving may lift shallow-rooted plants entirely out of the soil, or raise them enough so that they lose contact with soil moisture. If this goes unnoticed until too late for correction, the plants will die.
Mulch to shade the soil over herbaceous plants, put in place after a frozen crust has formed in the fall, tends to stabilize soil temperature, obstruct thawing and save the plants from frost heaving. The shallower the roots of the plants, the more necessary the mulch.
Peonies and Iris suffer considerably from frost heaving. All plants maintaining green crowns through the winter – such as foxgloves, canterbury bells, primulas, and chrysanthemums – are highly susceptible to decay from surface water and excessive dampness.
The mulch protecting these particular plants must admit light and air to the green foliage otherwise, it will die and the plant will starve.
You can use something as simple as evergreen boughs and old baskets inverted over the plants to give protective shade while admitting both light and air. Where there is no green growth above the soil surface, materials such as dried leaves, straw, salt hay, dried lawn clippings, peat moss and strawy manure serve the purpose.
Mulching – Should You?
“Mulch” comes from a word root meaning decay. Apparently, strawy manure was the original mulch, and its use is ancient. Fifty years ago it was commonly spread over lawns in the fall, where it did more harm than good, contributing an annual sowing of weed seeds.
Lawns need no mulch over winter, but they do suffer almost every spring from frost heaving. A light roller should be used after the soil begins to dry out, to press the turf in contact with the earth.
A planting of bulbs of questionable hardiness will find the soil much warmer when mulch covers the ground above them. If it is heavy enough, it may cause the ground to thaw to the surface. Snow, said to be the ideal mulch, often does this when it lies several inches deep for many days.
Even in the best gardens, setting out of hardy bulbs is sometimes delayed until late fall. A mulch applied after planting and before the soil freezes will keep it from freezing and permit the bulbs to make roots much later than otherwise. Since the soft soil attracts hungry field mice the practice is not advisable except where late planting makes it necessary.
Roots of deciduous trees and bulbs hardy in your area need protection only when newly planted. Then, a mulch keeps the moisture in the soil to prolong the fall season during which new root growth can develop.
A drying out which would easily be survived by a deciduous plant during its dormant period may kill an evergreen, which transpires water throughout the winter. While the ground is frozen, roots are limited to moisture already in the soil. A permanent mulch on the surface around each plant is beneficial and should be replenished each fall. Before hard freezing, the ground should be soaked deeply several times, to insure a reserve water supply.
Evergreens exposed to drying winds and direct sunlight may need shading by artificial means. Decades ago burlap-covered frames were often set up with this aim in view. Large baskets can be set bottoms up over small plants.
The foliage of evergreens presents another difficulty; it often catches enough snow along extended branches to break them. Strong twine wound around the tree, holding the branches close to the trunk, will prevent such damage.
Bush roses are often protected by piling garden soil 6 to 12 inches high around their stems. In colder areas, the spaces between plants are filled with strawy manure or compost, and the bed covered with a one-foot layer of leaves, straw or similar material.
The wood of rose bushes rarely kills back below the point reached by the piled up soil. In the spring, dead wood is cut out and stems chosen for development are cut back to selected buds. This method produces low-growing plants.
If taller roses are desired, the roots of a plant may be loosened on one side, and the plant bent down to the ground, where the stems are covered heavily with soil.
Climbing rambler roses are usually hardy without protection, but the canes may be laid on the ground and covered with soil, straw or evergreen boughs after pruning out old canes.
Rocks Require Different Protection
Winter protection in rock gardens, where true rock plants grow, requires great care. No heavy mulching materials such as manure, dead leaves, peat-moss, etc., should be spread over the plants since the remedy is apt to cause more trouble than the weather.
Evergreen boughs and other materials allowing air and light in are the solutions, especially for plants that retain green top growth over winter. These provide shade from winter sun and reduce frost heaving and puddles resulting from warm weather. Beware of coverings that harbor field mice, though.
Standing Water A Garden Hazard
A vital point in all winter protection is good surface drainage for the whole garden – lawns as well as borders. Standing water during winter thaws is always a hazard to plants.
It is important to provide low places where such water naturally collects, with means of draining it away. This may involve digging temporary drainage ditches in some cases avoiding those icy puddles around prized plantings is of primary importance for next year’s garden!