With most areas of Colorado enjoying an average of 300 days of sunshine per year, vegetables are sure to receive the light they need to flourish. But Colorado is also the highest of the 50 states with a mean elevation of 6,800 feet above sea level. As such, the growing season for vegetables in many areas of Colorado can be much shorter than other parts of the United States.
Nearly all of eastern Colorado is a plateau that stretches from the Rocky Mountains in the west to the borders of Nebraska and Kansas. The plateau gradually slopes from about 5,000 feet in Denver to just over 4,000 feet in the east. Almost the entire region is in plant hardiness zone 5. The growing season averages 150 days per year with the risk of spring frost usually ending in the first week of May.
The western half of Colorado is covered for the most part by the ranges of the Rocky Mountains. Along with the great range in elevation, plant hardiness zones vary from 2b at high altitude to 7a near the Utah border. In Denver and the surrounding high plain area, the growing the season averages 150 to 155 days per year. This is in stark contrast to high-altitude communities such as Telluride, where there are just 60 days between last and first frost.
Mountainous regions have many microclimates that can produce wide variations in temperature and precipitation from one valley to the next. Gardeners must be very aware of their local climate conditions to determine their growing season. The Arbor Day Foundation publishes very detailed plant hardiness zone information that many consider to be more accurate than that of the USDA. Look up your hardiness zone by zip code on the Arbor Day Foundation website.
Extending the Growing Season
Many high-altitude gardeners in Colorado extend the natural growing season by building cold frames and hotbeds. A cold frame is simply a protected enclosure set into the ground with a transparent cover. Hotbeds are similar but are also heated from the bottom. These techniques allow planting to begin 4 to 6 weeks earlier than the last frost. Plants can be protected from light fall frosts by covering them with blankets or frost cloth spread between raised stakes.
The Colorado Master Gardener Program is an excellent resource for both novice and experienced gardeners. Established to promote all types of home gardening, over 1,800 volunteers in 36 county-based offices provide detailed information regarding the local growing season, recommended plants and gardening techniques. Contact your local county extension office for more information.