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It's time to cut back many perennials to prepare for winter. Remember from last week's column that first you consider which plants you are going to leave standing. The reasons are: for the birds, for winter interest, to provide maximum crown protection or to mark their spaces to be identifiable next spring. Cutting back -- usually to about 3 inches above the crown -- is optional for some plants, but very important for others and here's why:

Over-wintering diseases

A few perennials should always be cut back and the leaf litter picked up because particular diseases or insects plague them. Most important is to cut back peony foliage, which may be infected with botrytis, and destroy it rather than compost. Don't be too eager to do this in summer. Peonies need that foliage for photosynthesis -- to help them prepare for future production -- so leave the healthy foliage alone until Oct. 1 or so then cut back to 3 inches.

Powdery mildew, common on Monarda (bee-balm) or many other perennials during a moist season, leaves millions of spores behind, and it's good advice to shear back and remove diseased foliage. Know, however, that fungal spores are everywhere and ultimately it's the air circulation, plant selection and weather patterns that bring mildew to your garden.

Another fungus disease, black spot, affects roses (depending upon humidity, etc.), and clean-up of foliage is essential. Remember, though, that pruning roses is an entirely separate horticultural art; nobody recommends you whack them back.

Insect pests hiding out

To minimize insect reinfestations, remove and destroy above-ground plant parts where the pests over-winter in some form. This includes bearded iris foliage (to get the iris borers), and daisies, asters and many herbs which are riddled with four-lined plant bugs. In the case of Hostas, pull back mulch, remove deteriorating foliage, and lightly stir up the soil surface under the plants to expose the slug eggs. Ideally, wait until Hosta leaves have totally withered and then pull them off. (It's better than cutting, which leaves wounds where diseases may enter, and better than letting the litter lie.) Later, re-mulch perennials after the ground freezes.

To avoid wild spreading

If you grow some of the plants that are known for vigorous re-seeding, this is your chance to save yourself! Do cut off the seed-heads (gently, so that you don't become the instrument of re-seeding), and trash them before they make a million more Artemesias, Euphorbias or perennial Geraniums. On the other hand, you will delight in the re-seeding of wonderful perennials, so select thoughtfully.

Finally, remember some plants look awful this time of year, and it gets worse later. Daylilies, a staple all summer for the great foliage, are a mess in spring so do it now. The floppy flower stalks of Stachys (lambs'-ears), or the discolored ratty remnants of some coneflowers, Helianthus or Centaurea montana . . . let your eyes be the judge.
Sally Cunningham is an educator in Consumer/Community Horticulture with Cornell Cooperative Extension of Erie County.