As the weather cools down, it is easy to stop paying attention to watering. But after a long, hot, dry growing season, it is a good idea to keep providing supplemental water to your garden, especially if dry conditions continue throughout the fall. Focus your watering on plants you have installed over the last three years so that they do not go into winter with drought stress. It especially important to water newly planted evergreens all the way through October and November.
The average first frost at the Chicago Botanic Garden is Oct. 15, though it is often later in the city and may be earlier further inland. Tender plants can be protected from light freezes by covering them with sheets, plastic or boxes. Containers can be moved into a garage for overnight protection. But when night temperatures begin dropping below 40 degrees, any tropical plants you have brought outside for the summer should come back indoors.
Continue to harvest vegetables. If a hard frost threatens, pick all tomatoes, including the unripe ones, and store them in cardboard boxes or paper bags in the basement. Many will ripen.
After a killing frost, remove dead plant debris from annual and vegetable beds. Sanitation is especially important if you have had disease problems in your garden. Remove all diseased foliage or fruits and do not add infected materials to your compost pile, as most home compost piles do not get hot enough to kill disease organisms.
Shredded leaves make good mulch for your garden beds. Ground-up leaves also will decompose more quickly in a compost pile.
A serious gardener might rent a shredder, which results in a fine leaf mulch. It is a very noisy machine that should always be used with ear protection, however, and using it is time-consuming.
For the average gardener, a lawnmower with a bag to catch the leaves works fine. Simply rake the leaves onto the lawn and run the mower over them. The mower does not cut up the leaves as finely as a shredder but works a lot faster.
In general, it is not necessary to mulch established perennial borders. Leave dried perennials standing for winter interest and leave a light layer of leaves in the bed to provide some winter protection. Cut the perennials back when they start looking bad.
You only need to mulch perennials if they were planted this year. The freeze-and-thaw cycle in early spring can push newly planted perennials out of the ground.
Many plants have had fungal diseases this season. The spores of fungi overwinter on infected plants’ leaves and stalks, so it is important to remove any diseased foliage from the garden. It is best to put diseased plants into the landscape waste and not into your compost pile. Most home compost piles do not get hot enough to kill disease organisms.
Downy mildew, a fungal disease, may have affected your impatiens late in the season. Several beds of impatiens at the Chicago Botanic Garden in Glencoe were killed by this disease.
Symptoms of downy mildew include new leaves that are small or discolored (yellow or pale green); stunted growth overall with yellowish foliage; flower buds that fail to form; leaves that curl downward; and a fluffy white fungal covering on the lower surfaces of leaves. As the disease progresses, the plants will become barren stalks with one or two yellow leaves left. This disease affects common impatiens (Impatiens walleriana), while New Guinea impatiens (Impatiens hawkerii ) are highly resistant.
If you have had this disease in your garden, remove all infected plants and wait two to three years before planting common impatiens again. That will give the fungal spores time to die off for lack of a host.
Terra cotta containers need to be stored out of the elements for winter to stay dry. This porous pottery absorbs water and freeze-and-thaw cycles of winter can crack it if containers are left outside. When the plants are finished for the year, dump out the growing medium and store the pots in a garage or shed out of the rain and snow.
Warm fall days are great for installing outdoor holiday lights, even if you leave them unlit until December. Most homeowners wait to install lights until late November or early December, when the cold makes it more difficult. The Chicago Botanic Garden staff starts installing thousands of lights in mid-October. Wrap branches of your trees with strings of lights to accent the tree’s form.
Buckthorn is an invasive tree common throughout the Chicago area. Fall is a good time to remove it because it tends to remain green later than other deciduous trees and shrubs, making it easy to spot. Cut the stalks at ground level and quickly treat the stumps with a systemic herbicide to kill the root system.
When buying spring-flowering bulbs in a garden center, pick ones that are plump and firm with no mushy spots. Small nicks, loose tunics or blue/gray mold do not affect the development of bulbs. But bulbs that show white mold or are soft and lightweight with a strong moldy smell probably are not good.
Most bulbs should be planted after a hard frost, from mid- to late October until the ground freezes. If you cannot plant your bulbs right away, store them in a well-ventilated area that is cool but above freezing. Artificial heat will dry out the bulbs, while high temperatures may destroy next spring’s flower inside. Keep bulbs out of reach of rodents and away from ripening fruit, which produces ethylene gas that can harm them.
Bulbs rarely look good alone or in rows. Plant them in clumps or drifts. Small bulbs such as crocus should be planted in large groups of at least 30 to 50 so they are prominent in the landscape. Incorporate bulbs into the perennial border in groups of seven to 15 bulbs or more.
Perennial bulbs such as daffodils and squill can be naturalized, or planted to look as if they are growing wild. One way to do this is to toss handfuls of bulbs and plant them where they land.
Daffodils are one of the hardiest, most adaptable and pest-resistant bulbs for Chicago-area gardeners. They naturalize beautifully and are available in many sizes and bloom times. Planting varieties with a range of bloom times will give three to five weeks of steady bloom.
Deer, squirrels, chipmunks and rabbits do not eat daffodils. They also avoid ornamental onions. Tulips, however, are a deer and rabbit favorite. In my garden, squirrels and chipmunks have left winter aconite and snowdrops alone while devouring all of my crocus.
Proper conditions are important for success with spring-flowering bulbs. They prefer to have moist soil in early spring and fall and to be dry in the summer when they are dormant. They do not like wet sites or heavy clay soil. Usually, a bulb should be planted three times as it is wide. But if your soil is on the heavy side, plant bulbs less deeply than normally recommended.
Continue cutting your grass throughout the fall as needed. Cold weather will eventually stop grass growth. Make your last cut of the year at a lower height of 2 inches. Take care to stay off the lawn when heavy frost is present.
Schedule a time to have your sprinkler system drained for winter. Water left in the system can freeze and crack sprinkler heads and pipes. Compressed air often is used to blow water out of the system.
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Ÿ Tim Johnson is director of horticulture at Chicago Botanic Garden, chicagobotanic.org.