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

Elevation sets the season

USU Extension’s Suggested Vegetable Planting Dates for Utah lists dates by town because conditions change so much over short distances. The Wasatch Front cities and the warmer southern towns can plant hardy crops in mid-March. High valleys such as Heber, Loa, Panguitch and Coalville wait until late April or May. In several of them, the most tender crops (tomatoes, peppers, squash and melons) are listed only with protection from a cover. At Woodruff, the guide gives no dates at all for warm-season crops. Check the city table above for your own frost dates.

Washington County is the exception. USU Extension sets its planting dates separately from the rest of the state. Within the county, elevation still rules. St. George, at about 2,600 feet, has a frost-free season of more than six months. Enterprise, at about 5,300 feet, gets only about three and a half months.

The St. George summer

USU Extension in Washington County describes an unusual limit: the middle of the season is too hot for most vegetables. From June, often into mid-September, St. George highs top 95°F nearly every day. That heat sterilizes the pollen of most vegetables, so tomato and squash flowers wilt and drop instead of setting fruit. Gardens at higher, cooler elevations do much better during this stretch.

The local answer is two seasons. Start cool-season crops in late winter and set out tomato, pepper, eggplant and melon transplants in early April so they bear before the heat. Then plant a fall garden. For fall timing, count back from the first frost using the days to harvest on the seed packet.

Soil: alkaline, salty and thin

Utah’s soils cause more trouble than its weather. USU Extension’s Preparing and Improving Garden Soil lists the main issues:

• High pH. Soils average about pH 8.0, the result of an arid climate and carbonate-rich parent material. High pH ties up iron, zinc and manganese. Iron chlorosis (yellow leaves with green veins) is one of the most common garden problems in the state.
• Low organic matter. Wasatch Front soils average about 1.5 percent organic matter, compared with 4 to 5 percent in wetter parts of the country. Many also have heavy clay subsoils and poor structure.
• Salinity. Salts build up easily. Fresh cow and poultry manure is often salty, compost and manure are high in phosphorus and potassium, and too much potassium raises salinity.

USU’s advice is to add 1 to 2 inches of organic matter every year. One inch only keeps pace with what Utah soils lose, and two inches of low-salt material builds them up. Amend clay with organic matter, not sand. Fall is the best time to work it in, but never till wet soil. Never add lime or wood ash, and use gypsum only on a sodic soil a lab has confirmed.

USU fact sheets differ in tone on lowering pH. Tips for Growing Garden Vegetables in Utah: Soil Preparation calls yearly elemental sulfur, at 6 to 10 pounds per 1,000 square feet, the most effective approach. The older Solutions to Soil Problems II. High pH warns that naturally occurring free lime and alkaline irrigation water make pH extremely hard to change. It suggests that living with slightly alkaline soil, and choosing crops that tolerate it, is easier and cheaper. Many vegetables, including asparagus, beets, cabbage, cauliflower, lettuce and spinach, are listed as adapted to neutral-to-alkaline soil.

USU Extension’s planting groups

USU sorts vegetables into four spring groups:

1. Hardy: peas, spinach, onions, radish, turnip and kohlrabi from seed, plus broccoli, cabbage and asparagus as plants. Plant as soon as the soil dries out in spring.
2. Semi-hardy: beets, carrots, lettuce, potatoes, chard and parsnips, plus cauliflower transplants. Plant a week or two later, about two weeks before the average last frost.
3. Tender: beans, sweet corn, cucumbers and summer squash. Plant around the average last frost, which USU notes is roughly when the first apple trees reach full bloom.
4. Very tender: tomatoes, peppers, eggplant, melons, pumpkins and winter squash. Plant about two weeks after the tender group, once the soil is warm.

Tomatoes are usually started indoors in early April. For fall harvest, USU lists midsummer sowings of beets, kale, lettuce, spinach and turnips.

Other points from USU’s soil preparation guidance:

• Grafted tomatoes, peppers and melons tend to be more vigorous on Utah’s high-pH soils.
• Blossom end rot is common even though Utah soils have plenty of calcium. The cause is uneven watering, so steady irrigation is the fix.
• In windy spots, fences, trees or shrubs protect plants.

Test before you amend

USU Extension recommends a soil test before adding compost or manure, since both can push phosphorus, potassium and salts too high. USU Analytical Laboratories in Logan tests garden soil. A test can also flag excess salts, high pH, low organic matter and drainage problems.