Fall gardening checklist: Bulbs, tropicals, frost prep
The main gardening season passed incredibly quickly from my perspective. The average first frost at the Chicago Botanic Garden is Oct. 15, though it is generally later in Chicago, so it’s time to be thinking about fall garden maintenance.
Tender plants can be protected from light freezes by covering them with sheets, plastic, or boxes. When night temperatures begin dropping below 50 degrees Fahrenheit, it is time to bring in any tropical plants that you are keeping outside. A gradual decline in temperatures over a period is best as the plants will acclimate some to the cooling weather and the plants can stay outside later in the season. You may want to move the tropical plants in for a night if there has been a long spell of warm weather and a sudden dramatic drop in temperature is predicted.
When you buy bulbs at a garden center, choose 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. Bulbs with white mold or that are soft and lightweight with a strong moldy smell are probably not viable. If your bulbs cannot be planted right away, store them in a well-ventilated area that is cool but above freezing, out of reach of rodents and away from ethylene-producing materials such as ripening fruit. Artificial heat will dry bulbs, while high temperatures may destroy next spring’s flower in the bulb.
Most bulbs should be planted after a hard frost starting mid- to late October and before the ground freezes. Fall-flowering bulbs such as autumn crocus (Colchicum) should have been planted as soon as they arrived in early to mid-September. Bulbs rarely look good alone or in rows and many gardeners do not plant enough for a good show. Plant them in clumps or drifts. Bulbs such as daffodils 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. Small bulbs such as crocus should be planted in large groups of at least 30 to 50 so they are more prominent in the landscape. Incorporate bulbs into the perennial border in groups of seven to 15 bulbs or more.
Consider the management of bulb foliage when planting in perennial borders as the bulbs need to go dormant before cutting back the foliage. Lots of browning bulb foliage can be intrusive in a perennial border so choose ones with less foliage or blend carefully with larger perennials. Consider applying a granular repellant to the soil above the newly planted bulbs if you have issues with chipmunks and squirrels digging up the bulbs.
It's important to continue watering any newly installed plants. They will require less frequent watering as the weather cools and plants begin going dormant for winter.
• Tim Johnson is director of horticulture at Chicago Botanic Garden, chicagobotanic.org.