Many of the gardening questions people ask master gardeners in autumn deal with perennial plants and tend to start with the words “when,” “which ones” and “how much.”
The leaves and stems of perennials die back during the winter, but the crown and roots remain alive. It is from these tissues that the plant starts growing in the spring, when the weather is more hospitable. Typically the dead top growth is not an asset during the winter months and the majority of it should be removed.
Plants with attractive or nutritious seed heads are an exception. They can provide winter interest in the garden and food for wildlife. Leaving a small amount of stem — 3 to 6 inches — when cutting back perennials can be a benefit in areas such as ours that don’t receive a lot of consistent snow cover. When it does snow, these short bunches of stems catch and hold the snow, increasing the chance that the plant will get the moisture it needs during the winter.
If the foliage that has died back has any sign of disease, it should be put into the trash rather than composted. Iris or hosta leaves with spots, hawthorn tree leaves with rust spots, lilac leaves, phlox or monarda that succumbed to powdery mildew all fall into this category. Peonies also benefit from fall sanitation practices to prevent diseases.
In summer, organic mulches are used to reduce evaporation. If the mulch applied during earlier months has broken down or has been raked or blown away, an application of mulch for the winter is in order. As during summer, a layer of mulch helps reduce evaporation during warm, dry spells and from drying winds. It also insulates the soil and helps maintain a more stable soil temperature. The winter freeze-thaw cycles are especially hard on newly planted perennials, shrubs, trees and bulbs. In extreme cases the crown of the plant can be heaved from the soil. More often, the expansion and contraction of the soil damages the tender root hairs that take up water for the plant. When layering on mulch for the winter, it should be applied after the ground freezes.
Winter watering is important in successful overwintering of plants. Snow acts as an insulator in the garden. Unfortunately there are often extended periods of time where the Front Range does not have snow cover, and the soil is exposed to drying winds and sun. A good rule of thumb is to water every four to six weeks when there is no snow cover or precipitation. Water on days when the temperatures rise above 40 degrees and the ground can accept the moisture.
Colorado State University Extension in the City and County of Broomfield provides unbiased, research-based information about 4-H youth development, family and consumer issues, gardening, horticulture and natural resources. As part of a nationwide system, Extension brings the research and resources of the university to the community. The Broomfield County Extension office is at 1 DesCombes Drive, Broomfield, 80020. For information, call 720-887-2286.
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