Guide to the Colorado Growing Season
(Denver area)


In the Denver area, climate information going back to 1900 shows that April has, on average, one day with a minimum temperature of less than 32 degrees (F). The range is from 0-4 such days. May has an average of zero days with a minimum temperature less than 32 degrees, but a May freeze has been recorded at least once.

So, plants that tolerate light frost can be safely planted in April, those which like cool temperatures will grow well in May, but plants which prefer warm temperatures and soils are generally not planted until later in May, or even in June. If you provide protection in the form of "wall-o-water", spun row cover, or cloches, these dates can be shifted 2-4 weeks earlier. Soil in raised beds will also warm up earlier than the normal ground. Harvest times shown depend on the age of your transplants - larger plants will fruit earlier.

The first fall frost is occasionally in September, but usually in October. By November, frosts are common. Again, providing protection from the cold can stretch your growing season considerably, but soil temperatures have to meet the ranges shown below.

Cool weather crops
April May June July Aug Sept Oct Nov
Broccoli
Cauliflower
Brussel sprouts
Ideal temps: 65-85 F, min. 50. (transplants)
8-9 wks to harvest. Second planting, mid-Sept for Nov harvest
Lettuce
Leaf and bibb
Ideal temps: 55-75 F, min. 40. (from seed or transplants)
7-8 weeks from seed to harvest. Can be planted earlier; most varieties bolt once summer heat begins. Second planting, mid-Sept for Nov harvest
Peas
Sweet peas
Ideal temps 65-85, min 50. (from seed)
Some pea types need cooler temps. Approx. 10 weeks to harvest frm seed.
Parsley
Ideal temps: 60-85 F, min. 50. (seed or transplants)
Continual harvest, matures in 8 wks from seed. Some butterflies use parsley for their larvae.
Root crops
Beets, Carrots, Radishes
Ideal temps: 65-85, min. 50. (from seed)
Radishes: Can take cooler temps, 4 weeks to harvest. Carrots and beets about 8 wks to harvest. Plant a few seeds every two weeks for successive harvests. Check specific varieties for heat resistance.
Spinach
Ideal temps: 60-80 F, min 42. (from seed)
6-8 wks to harvest from seed. Most varieties bolt once summer heat begins. Second planting, mid-September.
Swiss chard
Ideal temps 60-80 F, min 50. (from seed)
Again, successive plantings. 6-8 wks from seed to harvest. Not as susceptible to hot weather as lettuce and spinach, but may fail mid-summer if the temps are too hot.
Warm weather crops
April May June July Aug Sept Oct Nov
Beans
Bush or pole
Ideal temps: 70-85 F, min. 60. (from seed)
Plant seed at last frost (mid- to late May, cold tolerance varies with variety). Bush beans harvest in 8 wks. Pole beans harvest continually from about 9 wks until frost.
Corn
Ideal temps: 70-85 F, min. 60. (seeds or transplants)
Plant at last frost (mid- to late May). Harvest in 10-12 weeks from seed.
Cucumbers
Ideal temps: 75-95 F, min. 60. (from seed or transplant)
Plant in warm soil. 8 wks from seed to first harvest, then continual till frost.
Eggplant
Ideal temps: 75-85, min. 58. (transplants)
Plant in warm soil. 8 weeks to first harvest, then continual till frost.
Melons
Ideal temps: 80-95 F., min. 68. (from seed)
Plant in warm soil. Up to 12 wks from seed to first harvest, then continual till frost.
Peppers
Ideal temps: 75-85 F, min. 68. (transplants)
Plant in warm soil. 8 wks to first harvest, then continual till frost.
Squash, summer
Ideal temps: 70-90 F, min. 60. (from seed)
Plant after last frost. 8 wks from seed to first harvest, then continual till frost.
Squash, winter
Ideal temps: 70-90 F, min. 60. (from seed)
Plant in warm soil. 10-12 wks to first harvest, then continual till frost.
Tomatoes
Ideal temps: 68-80 F, min. 60. (transplants)
Plant in warm soil. Often started earlier with protection such as "wall of water". 8-10 weeks to harvest, harvest season varies by variety. "Indeterminate" types will fruit until the first frost.
A note about flowers: I haven't included annual flowers in this chart yet, but you can interpret flower planting times if you know what kind of weather they prefer. Warm-weather flowers like marigolds and zinnias will die in freezes, so don't plant them until the danger of frost is gone. Pansies, on the other hand, can often be planted in the fall and will live through the winter, blooming again the following spring. Snapdragons are in between, enjoying cool weather and surviving a few mild frosts without trouble. To understand planting times, it always helps to know your plants.

City Floral Greenhouse has a lengthy seed planting chart for Denver, if you want to read more.

Echter's Greenhouse describes the frost hardiness of various plants, focusing primarily on annual and perennial flowers.

Temperature information is adapted from this chart at Fedco Seeds.


Last update: February 3, 2009
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