Last Frost Date in Washington County MN

Yellow daffodil flower covered with snow

The average last frost date in Washington County is May 15th. Ours is a very large county though, with slightly different dates depending on where you live in the county.

The northern cities like Forest Lake and Scandia have a last frost date of May 21st, and in the center of the county Stillwater has an earlier last frost date than even more southern cities of May 8th.

It Can Still Freeze After the Last Frost Date

While it is unlikely to freeze after the last frost date, it’s certainly possible. The way last frost date is calculated is that there is a 10% chance of a freeze happening after that date. So, on average, once every ten years you will get a freeze after the last frost date for your area.

Frost dates can also vary substantially based on micro-climate — one garden in your yard might freeze on a cold night while another might not. In general low spots freeze more easily (cold air falls, forming these frost pockets); areas along your house are less likely to freeze as easily; and areas protected from wind are less likely to freeze as easily.

For warmth-loving vegetables like tomatoes, peppers, or melons it’s often wise to wait a couple weeks after the last frost date for temperatures to be more consistently friendly to the tender plants. You can plant perennial flowers any time after the last frost date, but they will thrive better if planted two to four weeks later after the soil has had a chance to warm up (this is one reason why September is a great time to plant perennials).

Pond at sunset surrounded by snow-covered trees that reflect in the water

How It’s Calculated

The last frost date is calculated by the Minnesota Department of Natural Resources (DNR), which uses historical temperature data from 1981 – 2010 to calculate the freeze dates for the state, as well as first fall freeze dates. You can find the DNR data Here.

The years after 2010 have in general been much warmer than the period of time the DNR’s data is based on, so gardeners with a bit more risk tolerance could consider pushing a week or two earlier if the forecast looks promising.

It’s worth noting that while the DNR collects temperature data from hundreds of locations in Minnesota, technically there is only one collection point in Washington County, which is Stillwater. They do have data for Forest Lake just north of the county border (last frost date 5/21), and for St. Paul just west of Woodbury (last frost date 5/15) and for Rosemount, a ways southwest of Cottage Grove and Hastings (last frost date 5/17).

– Written by Brian Wood, Master Gardener