May is a volatile month, so watch the weather forecast for your location and elevation. Although the city’s average last frost date (32 degrees) is May 10-15, it’s not unusual to have a hard freeze (28 degrees) in late May.

General maintenance

• Clean and sharpen garden tools.

• Change oil in your lawnmower, sharpen the blade and replace the air filter, if needed.

• Clean potting containers with a solution of one part bleach to 10 parts water to eliminate disease transference. If you had diseased containers last year, do not reuse soil.


• Remove faded flowers from spring bulbs, but don’t remove leaves until they have yellowed because they provide energy for next year’s blooms.

• Start feeding roses a balanced rose food every six weeks and continue through the growing season.

• Begin planting gladiolus every two weeks mid-May through June for a continuous series of blooms.

• Stake peonies and delphiniums.

• After fear of frost has passed, begin planting tender annuals.

• Plant annual seeds outdoors the last week of May.


• To maintain a healthy lawn, fertilize late this month or early in June using a fertilizer with high nitrogen content for a quick boost.

• Check your irrigation system to ensure its reaching all areas of your lawn and gardens.

• Seed any dead or bare areas of your lawn.

Trees, shrubs and perennials

• Prune early spring-flowering trees and shrubs such as lilacs and forsythia after flowers fade.

• Pruning of many evergreen and conifers starts now.

• Plant container-grown and balled and burlap trees and landscape shrubs.

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• Plant raspberries, blackberries and potted roses. If you’re new to Colorado and not sure what will grow here, look for perennials marked “Plant Select” at your local nursery. These plants are grown for our region and will thrive in our challenging climate.


• On warm days, starting a week or two before planting, begin hardening off seedlings in a sheltered location starting with just 15 minutes a day and increasing daily by 15-minute increments.

• Thin seedlings of early planted crops such as carrots, lettuce, spinach and beets.

• Asparagus stalks can be harvested right at the soil level when they are larger than 3/8-inch diameter.

Late May when soil is as least 50 degrees:

• Transplant tomatoes and bury them deep to the topmost pair of leaves so they will root all along their stems. Tomatoes produce best if they are well rooted.

• Plant eggplant and pepper plants.

• Plant corn, bean, pumpkin, squash, cucumber and early-maturing melon seeds.

When you have questions, Colorado State University Extension has research-based answers. The Help Desk opened April 15 at 17 N. Spruce St. Hours are 9 a.m. to noon and 1 to 4 p.m. Mondays and Wednesdays. Call 520-7684 or email