Focus on fall planting and transplanting

I have been seeing more powdery mildew (a disease that causes a grayish, powdery film on leaves) in gardens. While this fungal disease is typical in late summer, there’s no need for concern this late in the growing season. Instead focus on fall planting and transplanting.

Common plants that can get powdery mildew are lilacs, phlox, bee balm, zinnias and nannyberry viburnum. Fungicides can help control powdery mildew if applied before the infection becomes severe but is generally not worth doing. It’s a good opportunity to consider replacing any plants prone to powdery mildew with 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 for planting and transplanting perennials, 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 reestablish their root systems. Evergreens benefit from planting early in the fall to minimize the chance of winter burn — foliage turns brown. Winter burn can occur if evergreens do not get established before winter sets in. Try to get them planted before mid-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.

Water any plants you are planning to transplant a couple days before you do the work if conditions are very dry. This will reduce stress on the plants.

Good soil preparation is important for successful planting. I prefer to amend the entire planting area or bed instead of individual planting holes. One can amend the surface soil around a new tree or shrub but it's best not to amend the backfill. Evenly incorporate 2 to 3 inches of compost into existing garden soil.

Weeds can be sprayed in the fall with success if you are mindful of the weather. 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 that you use.

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