Utah growing zones and frost dates

Utah gardeners lean on two different numbers, and they answer different questions. The USDA hardiness zone is built from your average coldest winter night, so it tells you whether a perennial, shrub or tree is likely to survive the winter.^[1] The average frost dates tell you when it is safe to put tender vegetables in the ground.^[2] Two yards can share a zone and still plant tomatoes weeks apart, which is why the frost table further down matters as much as the zone.

The 2023 edition of the USDA Plant Hardiness Zone Map replaced the 2012 map. Zones come from the average annual extreme minimum winter temperature, split into 10-degree Fahrenheit zones and 5-degree half zones.^[1] Across Utah the zones run from 4 to 9.^[3]

The zone for any address comes from the USDA map data distributed by the PRISM Climate Group at Oregon State University.^[4] Our city pages and the region guides below show a zone for every Utah city on the Ledger. We matched each city to its three nearest ZIP codes in the 2023 data and show a range, like 6a–6b, when those disagree. Treat it as a starting point and look up your own address on the USDA map.

USU Extension warns that hardiness maps are not very detailed and do not accurately show zones across much of the West, because elevation and microclimates change so quickly.^[5] The example USU gives is Cache County, which the USDA map shows as mostly zone 5b while local weather records for 1990 to 2019 point to mostly 4b.^[5]

The practical rule from USU: the zone is an average, not an absolute minimum. If your area often runs colder than average, pick a lower-numbered zone, and do the same when losing the plant would be costly or when you are unsure.^[5] A cold-air pocket at the bottom of a slope, a bench with a longer season and a sheltered south wall can each shift a yard by a full zone.

Frost-free seasons swing a long way. USU Extension lists St. George with 205 frost-free days on average and Lake Town with 87.^[2] USU also notes that last-frost dates can differ by many days within a few miles, with benches and built-up areas usually running longer and low spots where cold air pools running shorter.^[2]

USU sorts vegetables by how much cold they tolerate. Hardy crops such as peas, spinach, onions, broccoli and cabbage can go in as soon as the soil is workable, typically 45 to 60 days before the average last frost. Semi-hardy crops (beets, carrots, lettuce, potatoes) follow one to two weeks later. Tender crops (cucumbers, sweet corn, most beans) go in on the average last-frost date, and very tender crops (squash, melons, tomatoes, eggplants, peppers) wait at least a week after it.^[2] The vegetable guide turns this into dates for your town.