Gardening in Wyoming: zones, frost dates and when to plant

Plains, basins and mountains

According to Barnyards & Backyards, UW Extension’s magazine for rural landowners, Wyoming’s average elevation is about 6,700 feet, the second highest of any state. NOAA’s state climate summary puts the range at about 3,100 to 13,800 feet, which causes wide differences in temperature. The low point is along the Belle Fourche River in Crook County and the high point is Gannett Peak. The eastern part of the state is Great Plains, while the center and west alternate between mountain ranges and basins.

Far from any ocean, the state is mostly semiarid. NOAA notes that summer highs across most of Wyoming average 75°F to 89°F, but they run much cooler at high elevation. Dry air also lets temperatures drop sharply at night. Most precipitation falls in spring and summer, often in thunderstorms. Southeastern Wyoming lies in “Hail Alley,” the most hail-prone part of the country.

What limits Wyoming gardens

University of Wyoming Extension’s Growing Vegetables in Wyoming (B-1115R) lists the main problems plainly:

• Short seasons: growing seasons range from short to very short.
• Temperature: growing-season temperatures are often too low, sometimes too high, and frosts come at the wrong time.
• Wind: strong or steady wind damages plants, erodes soil and dries the garden fast.
• Dry air: low humidity speeds water loss from plants and soil.
• Soil: native soils are usually alkaline, low in organic matter, shallow, rocky and cold.
• Water: it may be scarce and of poor quality.
• Hail: a single storm can wreck a vegetable garden.

Extending the Vegetable Growing Season (B-1148R) puts growing seasons at 20 to 130 days, depending on location. Day, night and soil temperatures are often below what vegetables need for best growth. The frost dates for your city above give the outer limits, but a cool summer can still leave long-season crops unripe.

What UW Extension recommends

Choose crops that fit the season

B-1115R recommends quick-maturing, cool-tolerant crops such as radishes, leaf lettuce and onions, followed by cabbage, cauliflower, head lettuce, spinach, beets, carrots and peas. Choose varieties with short maturity times, or ones bred in the northern United States or Canada.

Elevation sets hard limits. Above 6,500 feet, B-1115R does not recommend cucumbers, eggplant, muskmelons, okra, winter squash, peppers, tomatoes or watermelons. B-1148R adds that where the season is shorter than about 100 days and cool, crops like sweet corn, tomatoes, peppers, beans, pumpkins and melons usually yield poorly or fail to mature. Where you do grow tomatoes, it recommends determinate (bush) types, because they keep setting and ripening fruit in a short season.

Pick and build a warm site

Both bulletins favor a gentle slope facing south, southeast or southwest. Soil there warms sooner, and cold air drains away instead of pooling. The south side of a building adds reflected heat. A windbreak on the side facing prevailing winds protects a strip downwind about 10 times its own height. Barnyards & Backyards’ article on microclimates adds that cold air flows downhill like water and collects in low spots. Windbreaks can also catch snow, which insulates plants and adds moisture in spring.

Add heat and protection

B-1148R lists many ways to gain warmth:

• Clear plastic: laid on the soil early in spring, it can raise soil temperature 10 to 15°F in a few days.
• Raised beds: beds 10 to 12 inches high, level or sloping south, warm faster than flat ground.
• Covers: cold frames, plastic tunnels and water-filled plant protectors shield young plants.
• Water jugs: a gallon jug of water half-buried on the north side of a plant releases stored heat on cool nights.

For timing, hardy crops such as peas, spinach, onions and cabbage can go in six to eight weeks before the average last frost. Semi-hardy crops such as beets, carrots, chard and potatoes can go in two to four weeks before it. Wait until soil reaches at least 50°F before transplanting tender crops.

Water and soil

B-1115R says irrigation is necessary anywhere in Wyoming and suggests about 1½ inches of water a week. Sandy soils need more frequent watering than clay. Drip lines keep foliage dry and cut disease. If your water is high in dissolved salts, occasional heavy watering flushes salts out of the root zone.

Native soils are usually short on organic matter. B-1115R recommends working in about 2 inches of well-composted organic matter to at least 6 inches deep, for both clay and sand, and warns never to add sand to clay. Hold off on organic mulches until the soil has warmed, around mid-June, and add nitrogen when you use them. Before you fertilize, get an early-season soil test from a reputable lab. Your local University of Wyoming Extension office can help you arrange one.