Before planting it is a good idea to find your USDA Zone info to determine your garden schedule. The most important data that the zone info tells us is average last frost and the lowest expected temperature for your area.

One way of estimating when you can plant rosemary is to look at the USDA zone info for Thief River Falls.

Planting Calendar for Rosemary

Frost tolerance for rosemary:	Very tolerant of frost.
When to plant:	Up to 7 weeks before last frost.

Rosemary do well in pretty solid cold which tells us that you can plant them much earlier in the year than other frost tender plants.

The earliest that you can plant rosemary in Thief River Falls is March. However, you really should wait until April if you don't want to take any chances.

The last month that you can plant rosemary and expect a good harvest is probably August. Any later than that and your rosemary may not have a chance to really do well. If you are starting your rosemary indoors then you might be able to get away with starting them a couple of weeks earlier.

Last Frost Date

In Thief River Falls the average date of last frost happens on May 15. It might get as low as -35°F during the coldest months of winter.

Keep in mind that USDA zone info for Thief River Falls is an average and the actual date of last frost changes from year to year. Half of the time in Thief River Falls last frost occurs after May 15 so always be ready to cover your rosemary in the event of one of those late frosts.

USDA Zone Info for Thief River Falls

Here is the info for USDA Zone 3b.

Average Date of Last Frost (spring)	May 15
Average Date of First Frost (fall)	September 15
Lowest Expected Low	-35°F
Highest Expected Low	-30°F

This means that on a really cold year, the coldest it will get is -35°F. On most years you should be prepared to experience lows near -30°F.

Plants to Grow in Thief River Falls

You may be interested in your other planting guides for Thief River Falls.