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May and June Garden Checklist for Northern Colorado

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May is the month that our garden senses really start to warm up. We can hardly wait for the moment to view our garden fully planted! Of course, Mother’s Day weekend and the average last date of frost for Northern Colorado fall closely together around May 15. As we all know, it is very common to have freezing temperatures after mid-May. So this is the time of year to focus on spring cleanup, soil amendment and planting hardy perennials, trees and shrubs. Use Soil Pep and Root Stimulator to get vigorous root growth. What about annuals? Plant and be prepared to cover.


  • Clear dead canes and flowers that you left for the birds to enjoy.
  • Pull the mulch from around your plants, away from the crowns, and let the sun and air warm the plant.
  • Prevent weeds by adding a layer of newspaper and straw (not hay) in your garden. In the fall, turn this in to the soil and by the following spring watch for straw regrowth. This will look like tall green grass. Jump for joy, get out the tiller and under it goes to become fabulous green manure... a wonderful boost for the soil!


  • Allow spring blooming trees and shrubs to completely finish their bloom cycle before pruning.
  • Prune summer and fall flowering shrubs now.
  • Protect your fruit trees and crabapples from fireblight with Ferti-Lome Landscape & Garden Fungicide.
  • Deep water and then fertilize trees and shrubs to give them a good boost to start the growing season.
  • Once the temperatures warm up for the season, apply a nice layer of mulch to help keep the roots cool, slow weed growth and hold in valuable moisture.
  • Monitor codling moth by setting moth traps.


  • Spring is an ideal time to plant perennials. This gives them a long season ahead to develop their root systems before cooler fall temperatures set in.
  • Deadhead throughout the season to keep your plants beautiful and inviting for butterflies, bees, hummingbirds and beneficial insects.
  • Use Espoma Plant Tone fertilizer or slow-release Osmocote as an important spring feeding.


  • Existing rose plants may be bursting with leaves and flower buds at this time. Make sure to protect any tender, new growth when temperatures dip below 40 degrees.
  • Watch for powdery mildew. Avoid the chance of this occurring by watering in the morning.
  • Use Triple Action or Eight to control infestations of mites, aphids and other pests. For organic pest control, use Neem oil.


  • Cool season veggies such as lettuce, spinach, swiss chard, chives and radishes can be planted now. Also kale, broccoli and cauliflower thrive in spring's cooler weather.
  • Harden off any seedlings you’ve started indoors by slowly acclimating them to cooler temperatures. Place the plants outside, in the shade for a few hours each day, gradually increasing until weather temperatures are steadily warm.
  • Once planted, use a Wall o Water to shield heat-loving vegetables from cool nights.


  • Clean up spring bulbs by cutting back browned stems and leaves.
  • Plant summer bulbs in May.


  • Most sprinkler systems can now be turned on for the season. Inspect for damaged pipes and sprinkler heads.
  • Early May is a great time to start a new lawn. Begin by amending the soil and incorporating healthy topsoil or clay buster to break up our heavy Colorado clay soil.

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