By Carl Wilson, Horticulture Agent, Colorado State University, Denver County
If you're going to garden in Colorado, you must know the basics. With that in mind, try this quiz. Answers are below. Questions 1. How much precipitation does the Front Range receive? a) 35-40 inches; b) 25-30 inches; c) 10-20 inches; d) 5 inches. 2. What USDA plant-hardiness zones encompass Colorado? a) Zones 1 and 2; b) Zones 3, 4 and 5; c) Zones 6, 7 and 8; d) Zones 8, 9, and 10. 3. What is the last average killing frost date in spring, and the first average killing frost date for Denver in the fall? a) May 10/Oct 11; b) May 30/Sep 15; c) May 30/Oct 31; d) April 15/Oct 30. 4. What trees are native to the Front Range? a) cottonwoods and aspen; b) juniper, cottonwood, blue spruce; c) cottonwood, hackberry, juniper, pinyon pine; d) blue spruce, cottonwood, aspen. 5. Supplemental landscape watering should be done at what times of year? a) spring and summer; b) spring, summer and fall; c) summer; d) all 4 seasons. 6. What plants are adapted to Colorado's dry climate? a) rhododendron and azalea; b) penstemon and creeping grape holly; c) Japanese maple and dogwood; d) yew and boxwood. 7. What additional landscaping factor is necessary to consider in foothills and mountain areas? a) fire; b) hail; c) low oxygen; d) drier air. 8. Which soil amendment is recommended in Colorado? a) lime; b) wood ash; c) compost; d) Epsom salt. 9. What weeds have developed into problems in Colorado? a) yellow toadflax; b) purple loosestrife; c) oxeye daisy; d) all of these.
Answers1. c. From a low of 11 inches in Pueblo to 18 inches in Boulder, we live in a semi-arid climate. Did you know that many home-well permits, now issued in the state, prohibit outdoor irrigation? Making the best use of Colorado's water is a necessity with population growth increasing the demand on water resources. If the average home irrigation system was tuned up, 40 percent of the water used easily could be saved. 2. b. Hardiness zones used for plant selection range from the warmer Front Range Zone 5, to cooler foothills Zone 4 and coldest mountain Zone 3. Plants in Zone 5 areas must tolerate winter low temperatures down to minus 20 degrees; Zone 4 to minus 30 degrees; and Zone 3 to minus 40 degrees. Choose plants wisely. 3. a. Denver has a 160-day frostfree growing season from mid-May to mid-October. Note that our average dates are not as meaning ful as frost dates on both coasts because they can vary by a month in either direction. Selected growing seasons for frost-sensitive annuals around the state include 189 days in Grand Junction, 94 days in Aspen, 123 days in Durango, 148 days in Colorado Springs and Greeley and 173 days in Pueblo. Untimely freezes are common in Colorado and cause plant damage to even large trees that are caught out of dormancy. 4. c. Cottonwoods and Western hackberry trees grow along Front Range rivers, junipers and pinyon pines on ridges. Foothills vegetation includes ponderosa pine, and Colorado blue spruce and aspen thrive in the mountains. Aspen often experience problems when planted in warmer Front Range areas because they are adapted to cooler mountain elevations. 5. d. Regular watering in the growing season is routine for most gardeners but fall and winter wa tering often is forgotten. Uneven distribution of rainfall over the year usually means that one or two fall and winter waterings can prove more valuable for plant survival and health than an extra summer water application. 6. b. In addition to cold hardiness, some plants perform poorly because they can't tolerate low humidity. Broadleaf evergreens, such as rhododendrons, azaleas and boxwoods, often winter-burn from drying winds even if they are cold-hardy. Some needled evergreens, such as yews, winter-burn also. Both penstemon flowers and creeping grape holly shrubs are western natives. 7. a. Firewise landscaping is a necessary consideration in forested foothill and mountain areas that are subject to wildfires. Creating defensible space, avoiding vegetative fire ladders and choosing plants with higher moisture content are a few of these consider ations. 8. c. Organic matter, such as compost, sphagnum peat and lowsalt manure, will improve both clay and sandy soils for plant growth. Colorado's alkaline soils are different from the acid soils found in the East and don't require lime or wood ash. Either would react to make an alkaline soil more alkaline. 9. d. Designated noxious weeds in the state now include all of these that were at one time sold in the nursery trade for ornamental planting in Colorado. Dame's rocket is another "flower" that joined the prohibited list in January 2001. These plants were introduced from elsewhere, have prospered too well and are taking over native habitats. Photograph courtesy of Judy Sedbrook. Back to Gardening Back to Home
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010