It's time to protect tender plants

Frost is just around the corner. The average first frost date at the Chicago Botanic Garden in Glencoe is Oct. 15, though it is often later in the city and sometimes earlier farther inland.

When nighttime temperatures begin dropping below 40 degrees, it is time to bring in any tropical plants that you are keeping outside. Plants acclimate best if there is a gradual decline in temperature over a period of time. 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.

In an emergency, tender plants can be protected from light freezes by covering them with sheets, plastic or boxes.

When you buy 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 that are soft and lightweightwith a strong moldy smell are probably not good.

If you cannot plant your bulbs right away, store them in a well-ventilated area that is cool but above freezing, out of reach of rodents. Do not store bulbs with ripening fruit, which can produce ethylene gas that damages them. Artificial heat will dry bulbs out, destroying next spring's flowers inside.

Most bulbs should be planted after a hard frost, starting in mid- to late October. They can be planted until the ground freezes.

Plants from bulbs rarely look good singly or in rows. Instead, 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. When planting bulbs with perennials, think ahead to how you will manage the bulb foliage, as the bulbs need to keep their leaves after they bloom until they go dormant in summer. Lots of browning bulb foliage can be intrusive in a perennial border, so choose plants with less foliage or blend them carefully with larger perennials.

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