Frost Dates: First and last frost dates for Simpson, MT

As spring unfolds, the risk of frost gradually decreases each day until your tender plants are finally safe. The probabilities shown in the chart help you make smart decisions about when to plant.

While frost typically forms at 32°F, tender plants can be damaged even when temperatures are slightly higher. Air temperatures are often measured several feet above the ground, but temperatures at plant level can be several degrees colder. Protected areas like porches help by blocking radiational cooling. Given these variables, we consider 36°F the "caution zone" for frost-sensitive plants.

Key dates for your spring planting:

• By August 5, you have a 90% chance of being frost-free — this is your "safe date" for planting tender crops
• Around July 27, there's a 50/50 chance of frost — check weather forecasts carefully if planting now
• Before July 21, there's still an 80% risk of frost — too early for tomatoes and other tender plants unless you're prepared to protect them

If the 10-day forecast shows no temperatures below 40°F, you might safely plant earlier than the above dates suggest. Many gardeners watch for this weather window to get a head start on the growing season.

As summer ends in Simpson, MT, nighttime temperatures begin to drop, and your risk of frost increases with each passing day. Knowing when to expect frost helps you harvest sensitive crops and extend your growing season with protection methods.

The chart below shows the probability of experiencing specific temperatures on various dates. Here's what this means for your garden:

These probabilities help you time your end-of-season harvests and know when to apply protective measures like row covers, cloches, or cold frames to extend your harvest.

• Lake City, CO 22.2 mi
• Silverton, CO 22.7 mi
• Vallecito Dam, CO 27.4 mi
• Ouray, CO 30.9 mi
• Telluride, CO 39.9 mi
• Wolf Creek Pass, CO 42.0 mi
• Rico, CO 47.6 mi
• Cimarron, CO 53.1 mi
• Blue Mesa Lake, CO 54.1 mi
• Ft Lewis, CO 58.5 mi