Colorado Gardening Calendar for May and 2022 Denver Master Gardener Plant Sale

By Terry Deem-Reilly, CSU Extension-Denver Master Gardener since 2003

May is the month when gardening really goes into high gear! Here are the areas and tasks to tackle this month:


  • Veggies and herbs can be planted this month; be prepared to cover them if nighttime temperatures go below 40 degrees. It’s advisable to plant tomatoes when overnight temperatures are consistently above 50 degrees.
  • Start weeding now, in all areas of the garden. Get ‘em while they’re young!
  • Find hints for combatting vegetable pests in this list of CSU fact sheets.


  • Make sure that hoses and sprinklers are in good condition; apply an inch of water to the lawn each week, except during rainy periods. Denver Water has some excellent suggestions regarding lawn irrigation and general care from May 1st through September 30th, when watering restrictions are in effect.
  • Consider irrigating the lawn at night or in the early morning hours when evaporation is minimal. This can be a real turf AND water-saving measure in the hotter months, and it won’t promote diseases.
  • Set mower blade height at two inches – higher grass keeps the soil moist and promotes good root growth.
  • A thick thatch layer interferes with efficient watering and fertilization, so check for thatch and arrange for power-raking if the thatch is at least one-half inch thick.
  • Products like GrubGONE! that contain bacillus thuringiensis var. galleriae (Btg) as a control against Japanese beetle grubs must be applied to turf in May and early June to take effect. Check with a local nursery regarding availability of these products or order them online. Consult this Extension fact sheet regarding Japanese beetles for advice on dealing with this pest throughout the summer.
  • How lawns are watered, mowed, cultivated, and fertilized in their early growth will determine their appearance and health throughout the summer and fall (and perhaps for the next year as well). This fact sheet on lawn care outlines the best practices for tackling these tasks from now until fall.


  • Take a look at the garden to spot any “holes” that can be filled with a good-looking shrub, rose, or small tree – right now is when nurseries have the best selection. Be sure to call the Utility Locator Service at 811 before digging any large holes.
  • Begin watering existing trees and shrubs deeply once a week and check to ensure that plants are well-mulched. Here’s helpful information on selecting the correct mulch for your plants.
  • Start checking for pests; they will become more active as the weather warms and our spring rainfall commences (we hope). Consult this list of CSU fact sheets for information on specific insects and controls. When selecting pest controls, consider their effects on beneficial insects!
  • Now’s the time to apply copper spray to susceptible trees such as apples, pears, quince and crabapples, to prevent fire blight.
  • Finish spring fertilization and pruning of roses; make sure to apply two inches of mulch at the base of each plant and water newly planted roses twice a week for the first two or three weeks to promote root growth. Once established, roses will appreciate getting at least an inch of water weekly as temperatures rise.


  • Perennials can be hardened off and planted now; wait until at least the last average frost date in mid-May to fill annual beds and containers. Keep frost covers handy if we have one or two chilly nights before Memorial Day.
  • Summer bulbs can be planted now, so check them out at your local nurseries. It’s also a great time to divide summer and fall blooming perennials, find excellent info here.
  • Treat pollinators by seeding bare spots with their favorite annual plants, including borage, dill, zinnia, and/or cosmos. Plant as recommended by the seed packets and water. Seeds will germinate in a week or two; sprinkle the seedlings gently every few days and wait for the bees and butterflies to arrive! More pollinators mean more tomatoes, squash, fruit, etc., etc.

May certainly is a busy month in the garden! What’s on your to-do list?

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