Q. I’m new to Colorado and want to know the best time to plant annual, perennial and vegetable gardens.

A. Spring weather in Colorado can be extremely variable, with the average last frost date for the northern Front Range May 15. It is possible to have snow as late as Memorial Day, and cool temperatures can persist into early June. Although nurseries and garden centers have a wide selection of plants available at this time, beware of planting too early. Cold-hardiness depends on the specific plant – for example, pansies withstand cold weather much better than heat-loving tomatoes. If a plant survives a late cold snap, their vigor may be severely stressed as they wait for the soil to warm, and their summer growth may be compromised as a result.

It’s still a good time to plant cool-season vegetables, which can be planted once the soil is adequately dry; cool season vegetables can be planted as early as two to four weeks before the last frost. Hardy cool-season vegetables can survive daytime temperatures as low as 40 degrees, as well as light to moderate frosts. Consider planting lettuce, radishes and onions. Cool-season perennial vegetables, which return year-after-year, include rhubarb, asparagus, chives and horseradish.

Warm-season or tender vegetables require consistently warm temperatures for optimal growth. Some, like tomatoes, require minimum nighttime temperatures of at least 50 degrees to set flowers. Exposure to less than optimal temperatures for even short periods can significantly slow their growth and reduce fruit production. Other warm-season vegetables include peppers, melons, squash, pumpkins and beans.

Experts often say fall is the best time to plant perennials on the Front Range, as the soil temperature remains warm even after the air temperature cools down. But if you’re itching to plant perennials this spring, now is a good time to do a little research to determine those you can safely plant in May. Two excellent sources of information are the Colorado State University (CSU) Extension website, ext.colostate.edu and the CSU Plant Environmental Research Center (PERC) gardens located at 630 West Lake Street in Fort Collins (perennialgarden.colostate.edu/PERC.htm).

Many new annuals and perennials need to be hardened off before they are planted. Set plants in a lightly shaded area outside when you bring them home from the nursery. For the first few nights bring the plant inside. Then, leave the plant in a lightly shaded location all day and night for a few nights. Acclimate your plant over the next few days, making sure it has adequate moisture before planting it in its permanent location.

Q: My bulbs have all bloomed and now have unsightly foliage. Is it OK to remove the dying leaves?

A: Removing the dying foliage will reduce the likelihood that your bulbs will bloom next year. It is fine to remove the spent flowers, but the bulbs receive nutrients from the dying leaves that will enable them to flower next year. Before the leaves die back completely, you can add a little nitrogen fertilizer to give added nutrients. Once the leaves are totally withered, you can remove them. A good strategy to avoid the unsightly foliage is to plant bulbs among perennials so that as the bulbs fade, they are camouflaged by the foliage of the perennials. Just be sure to select perennials that don’t require constant moisture, as bulbs can rot in poorly drained, overly-wet conditions.

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This page updated:  August 5, 2008