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How to know whether to cut perennials back in fall or spring

Keep your garden interesting and attractive through the winter by waiting until early spring to cut back perennials. Some perennials (like hosta) quickly decline after a hard frost, so I cut them back in fall as they flatten to the ground. I have a large planting of hosta under an oak tree at home that I do not cut back in fall or spring to save time. The oak and hosta leaves serve as mulch. A good general rule is to cut back plants that do not look good late in the fall, but to wait to cut back remaining plants until early spring.

You may want to consider cutting more perennials back in fall if you have had problems with voles. Voles are compact rodents with stocky bodies, short legs and short tails that are sometimes mistaken for mice. They do not hibernate and are active throughout the year, mostly at dawn and dusk.

Voles primarily eat the stems and leaves of various grasses, but they also consume other vegetation. They eat the bark of trees and shrubs during the winter, which causes the most serious damage. Gardens with low-lying landscaping, such as arborvitae, spreading yews, junipers and cranberry cotoneaster have higher potential for vole activity. Voles are most prolific when they have abundant vegetation and cover. Damage is likely to be more severe during extended cold spells with deep snow cover. Eliminate weeds and dense ground cover around lawns to make these areas less able to support voles. Mow lawns and other turf regularly and cut back vegetation from the bases of trees and shrubs as winter approaches in areas where there is vole activity. Also, pull mulch back away from the base of trees and shrubs and keep snow cleared away, especially from the base of young trees. Reducing cover makes voles more vulnerable to predators such as hawks and owls. In general, removing cover is very effective in preventing damage done by voles.

Invasion of buckthorn

Buckthorn is an invasive tree that is common in Chicago-area gardens. It tends to hold green leaves later than other deciduous trees and shrubs in fall, so it is easy to spot late in the season. Cut the buckthorn at ground level and quickly treat the stump with an herbicide to kill the root system. Smaller trees can be dug out with a sharp spade.

Save your bulbs

Try pinning garden netting over large freshly planted beds of bulbs to discourage chipmunks and squirrels from digging up the bulbs. This is not feasible if you are drifting bulbs into shrub and perennial borders. Bulbs that are spread out over a larger area or near existing plants will be in the way of netting. Remove the netting in early winter when the ground has frozen or in early spring. A light layer of mulch over the netting will hide it. Spreading blood meal or a granular repellent over the bed may also work to repel chipmunks and squirrels. Though this is not feasible on a large scale, you can protect small pockets of bulbs by covering them with chicken wire underground to prevent them from being dug up. The bulbs will grow through the chicken wire in spring.

• Tim Johnson is director of horticulture at Chicago Botanic Garden, chicagobotanic.org.

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