Powdery mildew may not need treatment

You might be noticing powdery mildew (a disease that causes a grayish, powdery film on leaves) in your garden. Common plants that could be affected are lilacs, phlox, bee balm, zinnias and nannyberry viburnum.

One of my lilacs at home is losing its leaves but has buds set for next year, so I have no concerns for its health. This disease typically occurs in the late summer when the weather is warm and humid. Fungicides can help control powdery mildew if they’re applied before the infection becomes severe, but an application is typically not worth doing. It’s better to select mildew-resistant varieties or move the infected plants to an area of the garden that has better air circulation.

Divide perennials now

To divide perennials that bloomed in spring and summer, a sharp, flat spade works well. It’s best to do this work early in the month so the plants have time to establish before winter sets in.

It’s important to mulch the newly planted divisions and provide supplemental water for the rest of the gardening season as you would with any other plant that you may have purchased and installed.

Harvesting, preserving veggies

Continue to harvest vegetables as they ripen. Warm season crops like peppers and tomatoes should be picked as soon as possible. Full-sized pumpkins need to remain on the vine as long as possible to achieve their maximum size.

Continue to snip herbs to use fresh, or to dry or freeze. Try making some extra pesto and freezing it in an ice cube tray. Pop out the cubes when frozen, and store them in a plastic bag for use this winter.

Collards, kale and Brussels sprouts will have improved taste if they are exposed to frost before harvesting.

Don’t forget to maintain good sanitation throughout the vegetable garden. Remove diseased plants immediately, as well as those that have finished their growth cycle for the year. It’s best to compost only healthy plant material since most home compost piles will not get hot enough to kill disease organisms.

Check for watering needs

Monitor plants you have installed over the last two to three years, as they may benefit from some supplemental water during periods of hot and dry weather. My garden has become very dry recently; some shrubs I installed last year are starting to wilt.

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