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May Garden Calendar

By: Eileen Tully, Colorado Master Gardener
May 1, 2017 Updated: May 1, 2017 at 3:53 pm
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Usually I break May’s garden chores into three time frames – Early, Mid and Late. In previous years the average last frost date in town has been May 10 to May 16. But with the warmer weather this year the calendar has been adjusted to merge early and mid-month chores. Ah, but I probably should include the phrase, ‘this is Colorado and we could get a late spring frost.’

Now, and all during the month, plant perennials and small fruits such as raspberries and blackberries, container grown and balled-and-burlap trees, landscape shrubs and potted roses.

Make it easy on yourself: if you’re new to gardening in Colorado look for perennials marked “Plant Select.” These plants are grown for our region. Many of them will be labeled drought tolerant, but all new plants will require regular watering until they get established.

When shopping, don’t be tempted to buy transplants already in bloom, and look for compact, bushy plants with healthy green leaf color and a thick main stem.

Plant summer bulbs that can remain in the ground over winter like Crocosmia Lucifer and some varieties of Alium. Gladiolus, which must be dug up late summer, can be planted now and every two weeks through June for a continuous series of blooms.

Pruning begins now. If you want to shape and control growth of mugo pines, pinch or cut out 2/3 the length of the new growth “candles.”

On warm, sunny days when the wind is fairly in check, bring seedlings outside and harden them off in a sheltered location. Do this for a week before planting.

Even though we’ve had warmer weather I’m still recommending planting annuals mid-May – if you live in lower elevations.


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

Stake peonies and delphinium.

After early spring bulbs have bloomed, remove the faded flowers but don’t cut the leaves until they have yellowed because they provide energy for next year’s blooms. Leaves and stems should easily pull out of the ground.

Feed roses a balanced rose food every six weeks.

Late May – weather should be settled in. Soil should be above 55 degrees.

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

Plant warm season vegetables:

  • 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 seeds of corn, bean, pumpkin, squash, cucumber and early-maturing melon.
  • Transplant seedlings of eggplant and pepper plants.


After flowers fade on lilacs and forsythia the shrub can be pruned and shaped.

Annual flower seeds can be direct sown the last week in May.

Easy-to-grow seeds like nasturtiums are colorful and edible. Plant them around the edge of your vegetable garden; they disrupt insects from your vegetables. Marigolds, another easy to sow annual, are a good companion with tomatoes.

If you long for height in your garden check out Hyacinth Bean vine (lablab purpureus); it’s an annual vine with striking purple flowers and deep purple seeds pods. Cardinal Flower vine, another annual that’s easy to direct sow, has red trumpet-shaped flowers that attract hummingbirds. An heirloom flower, Black-eyed Susan (Thunbergia), will climb a trellis or trail out of a container.

To maintain a healthy lawn, fertilize late this month or early in June. Seed any dead or bare areas of your lawn. If you didn’t get to it earlier it’s never too late to aerate.

Three tips for successful aeration -

  1. Make sure the soil is moist, but not soggy, before aerating.
  2. Plugs should be at least three inches long.
  3. Leave plugs on the lawn.

When you have questions, Colorado State University Extension has research based answers. Get answers to your horticulture questions by calling a Master Gardener Volunteer at 520-7684 or emailing  Volunteers are available to help you Monday to Thursday from 9 to 12. For current garden tips visit For current classes visit


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