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Preparing Plants for the End of Summer (Yes, Fall is Coming)

Jenny Cu
Creative Commons/Flickr

Hard to believe with this summer’s hot weather, but it’s time to start thinking about preparing trees and shrubs for fall. And KUNC Gardener Tom Throgmorton says it's especially important for those living in the mountains.

Summer speeds by in the high country. But the intense sun and cooler nights make flowers colors vibrant.

The last Spring frosts can happen in mid-June. The first fall frosts come as early as mid-August. That is as little as sixty frost free days. Warm season vegetables like tomatoes and peppers don’t grow well in that short of a season. They need to be grown in containers or hot micro-climates.

Cool season vegetables do great in the high country. Broccoli, cabbage, lettuce and salad greens prefer cool temperatures. Try sowing spinach and other greens now. They will germinate in a few weeks for fall salads. The spinach plants will stay dormant through the cold winter. As the temperatures warm next spring the plants grow quickly. They’re ready for an early spring harvest.

Annual flowers seem to glow in mountain plantings. Old stand-bys like petunias, marigolds and zinnias are a blast of color until the first hard frosts. Pansies, snapdragons and dianthus are great for cutting and tolerate frosts. Ageratum, alyssum, lobelia and violas are great low, cool season accents.

Folks growing shrubs at elevations above 8,500 feet should look for natives. Native maple, birch and alder are large garden accents.  Snowberry and thimbleberry are medium sized shrubs.

Non-natives to try are the Canadian shrub roses. These roses are on their own root so if they freeze to the ground, the same plant will come back and bloom the next season. With an insulating cover of snow, the roses usually don’t die-back. But watch for mold and rot on the plant as the snow slowly melts in the spring.

Native trees like aspen, spruce and pine are high country mainstays. Crabapples, hawthorns and chokecherries are non-natives to add to the landscape. Up to 8,500 feet - try tatarian maple for a broad, medium sized tree.

This time of year woody plants need to be prepared for fall. If they keep growing, their soft new growth can be nipped in the first freezes. Don’t fertilize trees and shrubs after July. That goes for the lowlands too. Fertilizer pushes soft, new growth. Limit pruning. Pruning stimulates growth. Cut back on water in mid to late August. Less water means less new growth to get nipped. Once trees or shrubs go dormant keep them moist until the soil freezes.

Summer zips by, especially in the high country. Picking the right plants keeps mountain gardens blooming.


Tom has been offering garden advice on KUNC for almost two decades. During that time he has been the wholesale sales manager at Ft. Collins Nursery, Inc. Since January of 2005 he has been the owner and operator of Throgmorton Plant Management, LLC., a landscape installation and maintenance company as well as a horticultural consulting firm. He lives in northern Ft. Collins with his wife and two kids.
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