Days may be cooler now, but the hot and dry summer weather has put a lot of stress on plants that were not given supplemental water. There are many trees and shrubs with leaf scorch in the landscape and some trees are shedding leaves early. Keep watering trees and shrubs if the dry weather continues.
General garden care
Powdery mildew (a fungal disease that causes a grayish, powdery film on leaves) is present now. This disease typically occurs in the late summer when the weather is warm and humid. Plants that commonly get powdery mildew include lilac, phlox, bee balm, zinnia and nannyberry viburnum. Norway maples have been affected earlier than normal this year and more severely.
Fungicides can help if they are applied before the infection becomes severe, but it is generally not worth treating for powdery mildew this late in the season. Other options include selecting more mildew-resistant varieties or moving the infected plants to an area of the garden that has better air circulation. If plants' leaf surfaces get enough air flow to stay dry, the fungal spores are less able to multiply.
Continue to harvest vegetables as they ripen. Snip herbs to use fresh, to dry or to freeze. Warm-season crops such as peppers and tomatoes should be picked as soon as possible, but collards, kale, and Brussels sprouts will have a better taste if they are allowed to be hit with frost before they are harvested.
Full-sized pumpkins need to remain on the vine as long as possible to achieve their maximum size.
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 is best to compost only healthy plant material; put diseased stems and foliage in the landscape waste.
Divide perennials that bloomed in spring and summer as needed. It is best to do this early in the month so plants have time to establish before winter sets in. Mulch the newly planted divisions.
Peonies are dependable, long-lived, hardy perennials. Their neat foliage stays green from spring until frost after their large, showy late-spring blooms. Peonies do best in full sun; although they can tolerate partial shade, they will flower less. September is a good time to plant peonies as well as divide and transplant existing ones. Lift the large, fleshy root carefully from the soil and use a sharp tool to cut it into smaller pieces. Be careful not to make these pieces too small; each section should have at least three "eyes," the reddish growing buds that emerge from the top of the roots in spring and fall. When you replant, set the divisions just 1 or 2 inches below the surface of the soil.
Mulch is important for fall plantings. Use a 2- to 3-inch layer for trees and shrubs and 1 to 2 inches for perennials, keeping it away from the crown (base) of the plants.
Trees and shrubs
Fall is a great time to plant and transplant trees and shrubs. As long as you can provide supplemental water to your new plants, there should be no problem planting this fall even if dry conditions continue.
In autumn, warm soil, moderate temperatures and rain help transplanted trees and shrubs re-establish their root systems. In general, there is no need to prune trees and shrubs when you transplant them.
Planting evergreens early in the fall will minimize chances of winter burn, which often is caused by a lack of water in the plant's tissues. Evergreens need plenty of time before the ground freezes to develop new roots and plenty of water for the roots to absorb. Try to get them planted before the middle of October and continue watering them weekly or as needed until the ground is frozen.
Winter burn also can be a problem for any evergreens planted over last three years if they go into winter dry, so water them too.
Fall is a good time to apply granular sulfur to the soil around plants such as rhododendrons and azaleas that prefer acidic soil conditions. Avoid contact with the sulfur by wearing latex gloves and keeping the dust out of your eyes. Apply the granules to the soil according to the package directions and gently scratch them in.
Sulfur works slowly in the soil and repeated applications may be necessary from year to year. One application in spring and another in fall should work well.
If you have deer in your neighborhood, it is time to protect the trunks of smooth-barked trees such as young maples from deer rubs. In late summer and early fall, deer often use the trunks of smooth-barked trees to rub the itchy velvet off their antlers. This can damage the bark and even kill trees if the damage goes all the way around the trunk. To help protect the tree, wrap the trunk with hardware cloth, chicken wire, plastic snow fencing or tree wrap to a height of 5 feet. Tree wrap gives the least protection and likely will not be adequate if your garden is near good deer habitat such as a forest preserve; hardware cloth is best. Be sure to remove any tree wrapping in early spring.
Contact a professional arborist if you have any ash trees in your garden that you want to protect from emerald ash borers. These insects continue to spread and affect more ash trees in the Chicago area. They will eventually kill any ash trees that have not been treated but will not attack other types of trees.
A number of signs may indicate the presence of emerald ash borers. The canopy of the ash tree will thin or die back. Emerald ash borer usually attacks the tops of trees first and then proceeds downward. Look for woodpecker damage on the outer back of the tree; as they feed on the larvae under the bark, the birds will leave vertical sections of bark with a scraped or shiny look. This is easy to see from the ground. New vigorous shoots arising directly from the trunk are another sign of emerald ash borer activity.
When you see an ash tree with these three conditions, it is very likely that there is a heavy infestation of emerald ash borer.
Insecticide treatments can fend off the insects, but only if they are applied while the tree is still relatively healthy, as the insecticides must be transported within the tree's tissues to reach the insects. It is probably too late to save a tree if it has lost more than 50 percent of its canopy. An arborist can help you decide the best course of action.
On lawns: Johnson's advice on fall lawn care will be published later in September.
• Tim Johnson is director of horticulture at Chicago Botanic Garden, chicagobotanic.org.