Protect fall containers from season's first frost

The average first frost at the Chicago Botanic Garden is Oct. 15, though it is often later in the city. Tender plants can be protected from light freezes by covering them with sheets, plastic or boxes.

When night temperatures begin dropping below 40 degrees, it is time to bring in any tropical plants you are keeping outside. A gradual decline in temperatures over a period of time is best, as the plants will acclimate some to the cooling weather. 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 in a garden center, pick bulbs 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 good.

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. Plant them in clumps or drifts. Bulbs such as daffodils and Siberian 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.

Small bulbs such as crocuses 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.

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