Better air circulation prevents powdery mildew

Powdery mildew, a disease that causes a grayish, powdery film on leaves, is present now and not anything to worry about this late in the gardening season. Common plants that can get powdery mildew are lilacs, phlox, bee balm, zinnias and nannyberry viburnum.

This disease typically occurs in the late summer when the weather is warm and humid. Fungicides can help control powdery mildew if applied before the infection becomes severe, but they are generally not worth applying. In most cases, there is no need to apply any controls for powdery mildew at this time of year.

Other options include selecting more mildew-resistant varieties, or moving the infected plants to an area of the garden that has better air circulation.

• Fall is a great time to plant and transplant trees and shrubs. In general, trees and shrubs do not need to be pruned when transplanted to compensate for loss of roots during transplanting. Fall conditions, which include warm soil, moderate air temperatures and rain, help plants re-establish their root systems.

Evergreens benefit from planting early in the fall to minimize chances of winter burn. Try to get them planted before the middle of October and continue watering them weekly or as needed until the ground is frozen. They should not go into winter under stress from being too dry.

Mulch is also important to install for fall plantings — use 2 to 3 inches of mulch for trees and shrubs and 1 to 2 inches for perennials and ground covers. It is best to keep the mulch away from the crown (base) of the plants.

• Good soil preparation is important for a successful planting. It is best to amend the entire planting area or bed instead of individual holes. Evenly incorporate 2 to 3 inches of compost into existing garden soil.

• Peonies are dependable, long-living, hardy perennials. Their neat foliage stays green from spring until frost, and follows large, showy blooms. They do best in full sun, although they can tolerate partial shade. Flowering is reduced if placed in the shade. September is a good time to plant peonies, as well as divide and transplant existing ones.

Lift roots carefully and use a sharp tool to cut the large, fleshy roots into smaller pieces. Be careful not to make these pieces too small — each section should have at least three eyes. The eyes are reddish growing buds that emerge from the top of the roots. You will find them in spring and fall. Set these divisions an inch or two below ground.

• Be aware of the weather as you spray to control weeds in the fall. Herbicides are most effective when temperatures are consistently above 55 degrees and weeds are actively growing. Spot spray the weeds to minimize the amount of herbicide you use.

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