How to Grow Tomatoes in Colorado

Few states can claim as diverse a climate as Colorado. With U.S. Department of Agriculture plant hardiness zones that range from 3 to 6, it's hard to know what to plant when. With warm season crops such as tomatoes (Lycopersicon esculetum), which are tender perennials in USDA zones 10 to 11 but are grown as annuals throughout the U.S., timing is everything. By choosing the best time to plant, the right cultivar for your location and proper planting techniques, Colorado gardeners can produce tomatoes that rival any grown in backyards around the country.

Colorado's elevation is part of the state's charm but causes the biggest disparity between growing seasons regionally. Along the Front Range, on the eastern side of the Continental Divide, gardeners have on average about 150 frost-free days in the growing season. Since tomatoes are very sensitive to cold weather, gardeners in areas such as Denver, Ft. Collins, Boulder and Colorado Springs, should wait until the first week of May to set tomato plants out. With this timeline, fruit will begin maturing in late July until the first frost, depending on the cultivar.

Meanwhile, some high-elevation mountain towns like Leadville or Crested Butte have only 25 to 30 frost-free days to grow warm season crops. At these higher elevations, tomatoes -- which typically require an average of 60 to 80 days to produce fruit -- are not a good use of garden space. Check out your county's first and last frost predicted dates before investing in tomato seedlings.

With over 2,000 cultivars to choose from, lots of tomatoes grow well in Colorado.

Hybrid varieties have been carefully selected for traits such as disease resistance or pest tolerance. Hybrids tend to deliver higher yields than heirloom varieties. "Celebrity" is a vigorous cultivar that produces medium-sized fruit. "Big Boy" produces both large fruit and high yields for slicing or sauces.

Early-maturing cultivars such as "Early Girl" and "Early Boy" are also well suited to Colorado. They are medium-sized fruits that are good for slicing and develop within 65 days.

Cherry tomatoes or pear-shaped cultivars are often faster maturing than full-size tomatoes and make up in yield what they lack in size. Hybrids such as the tiny "Super Sweet 100" are perfect for tossing on salads. They can also be grown in large containers, which makes them easy to pull inside or cover should temperatures take an unseasonable drop.

Tomatoes need full sun and loamy soil. Plant outside when daytime temperatures remain above 60 degrees Fahrenheit and nighttime temperatures do not drop below 50 degrees F. Seedlings should be between 6 and 8 inches tall and about the size of a pencil in diameter. Space at least 2 feet apart for good air circulation.

Depending on precipitation, plants should be watered in regular intervals, with moisture soaking at least 6 inches into the soil. Weekly watering is usually sufficient unless conditions are extremely dry and then more frequent applications are required.

Fertilize once a week with a liquid fertilizer such as a seaweed or fish emulsion to help encourage transplants to develop; dilute about an ounce per gallon of water, but follow the directions on the label. Stop fertilizing in early August, recommends master gardener Judy Sedbrook on the Colorado State University Cooperative Extension website, as this will encourage vegetative growth at the expense of flowering and fruiting.