Keystone, SD 57751 Frost Dates
Frost dates for Keystone, SD, ZIP 57751
Typically, the last spring freeze lands around 05/15, roughly 260 days out. The first fall frost typically arrives around 10/01, about 138 frost-free days later.
The USDA Plant Hardiness Zone Map puts Keystone in Zone 5a (-20 to -15°F average annual minimum); frost dates are NOAA's 1991-2020 station normal, not this year's forecast.
What to do before the first frost
Zone 5a around Keystone has about 5 weeks left before the first fall frost — that's the usual window to get a cold frame up if you want to keep harvesting past it.
How much risk comes with planting early here
05/15 is the middle of 30 years of NOAA records, not a promise. Plant by 05/01 and you're betting against frost in roughly 9 years out of 10. 05/29 is the safer bet, historically only a 1-in-10 chance of frost that late.
How Keystone compares
About 138 days separate Keystone's last and first frost, putting it 37,454 of 40,283 ZIP codes nationally. Within SD, it ranks 332 of 378.
Learn more
zone vs. frost date explains why Zone 5a and Keystone's frost date answer different questions, and the frost range explains how to read the range around Keystone's dates instead of trusting one.