Every spring, as temperatures start to rise, Twin Cities gardeners face the same question: “Is it finally safe to plant?” This can be a complicated decision! Read on to demystify hardiness zones, frost dates, soil temperatures, and the hidden microclimates that can make one corner of your garden significantly warmer or colder than another.
First, we should mention the USDA Hardiness Zones. This is the standardized tool used to determine which perennial plants are most likely to survive in a specific location. The US and Canada are divided into zones 1-13, with lower numbers being colder. USDA climate scientists set the hardiness zones based on the 30-year average of minimum winter temperature. Most of the Twin Cities area is in zone 5a.
The hardiness zones are important for choosing overwintering perennials, so make sure to keep them in mind when you’re looking at trees, shrubs, and other plants that come back year after year. For annuals, though – like your vegetable garden – what matters most is the last frost date and soil temperature.
Last frost dates can be tricky because they are a probability, not a guarantee. To estimate last frost dates, climatologists look at historical data and calculate the probability of freezing temperatures occurring after certain dates. According to the Minnesota Department of Natural Resources’ latest data for Minneapolis, there is a 50% chance of freezing weather after April 23, but only a 10% chance after May 7.
Keep in mind that some vegetables are a lot more cold tolerant than others. Hardy vegetables like spinach, peas, kale, radishes, and carrots will be ok through a light frost, while tender crops like tomatoes, peppers, and basil must wait until all danger of frost has passed.
Personally, based on my own risk tolerance, I plant my cold season vegetables as soon as the soil has thawed, but I wait to plant warm season vegetables until mid-May (when there’s only a 10-20% chance of lows under 36 degrees). I used to use season-extenders to get my tomatoes in the ground earlier, but I found that they didn’t grow much until the weather warmed up anyways.
Although hardiness zones and last frost dates are broadly helpful, your own individual yard probably has small pockets that can run several degrees warmer or colder – referred to as “microclimates.” If you have warmer areas in your yard, you may be safe planting earlier (and conversely, if you have colder areas, you may want to be more cautious when planting there).
There are a lot of factors that can affect microclimates. For example, low spots in your yard can accumulate cold air and be more prone to frost. Any surface paved with concrete or black asphalt will absorb heat during the day and emit it at night, making surrounding areas warmer. Your house itself also radiates heat at night –south-facing walls in particular. Shelter from wind provided by hedges, fences, or buildings also helps protect plants from the cold. So, an elevated spot near your house, facing south, might be significantly warmer than a low spot in the middle of your yard, facing north.
To map your own yard's microclimates, spend a spring morning watching where frost lingers longest after a cold night, and where it disappears first. Watch where snow melts earliest. Once you've identified these patterns, it can help you decide when and where to start planting.
Finally, pay attention to soil temperature. Most cool-season vegetables like lettuce, spinach, and peas germinate reliably once soil hits 40 to 50°F. However, warm-season crops like tomatoes and peppers need soil temperatures of at least 60°F to germinate and establish well. A sunny 70°F day in April may feel like summer, but the soil often remains far too cold for seed germination.
The Minnesota Department of Agriculture is a great source for soil temperature data, with records on their website updated daily. However, it’s also easy to measure soil temperature yourself. Just take a soil thermometer (or even a kitchen thermometer) and push it a few inches into the ground. In the Twin Cities, soil temperatures tend to reach 50 degrees sometime in late April, and reach 60°F around mid-May. Raised beds and sunny areas will likely warm up faster.
The bottom line is that there are many factors to think about when choosing annuals and deciding when to plant them. Watch the soil, watch the forecast, and spend a morning mapping the microclimates hiding in your own backyard, and you will be well equipped to start your garden off on the right foot this spring!
For more information, check out the University of Minnesota Extension Yard and Garden website.
Extension resources are written by experts and contain the latest and most reliable research-based information. Happy gardening!
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