Q: Please help me interpret my seed packet. They often say things like, "sow after last frost" (lol, Duluth) — so of course, we start them indoors. And they also say things like, "65 Days (to maturity/harvest)".

When does that 65 days start? Does it start when I sow them? When they sprout? When they leaf out? When the actual last frost is? When I put them outside? When the temperature stays above 55? When I put them in the ground?

Thank you for your help!

A: What great questions. Let’s start with “last frost.” The National Climactic Data Center keeps records on when the last frost in spring and the first frost in fall happen. From this they calculate a probability that a freeze will occur, and they make up a chart you can find online for wherever you live by clicking on the nearest marker on the map at dnr.state.mn.us.

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The table that pops up is a little hard to read, but what it tells you is that at the Duluth airport, there is a 10% chance that the temperature will hit 32 after June 4.

Thought about a different way, this means there’s a 90% chance the temperature won’t hit 32 after June 4 near the airport in Duluth. A little earlier in spring, chances are much higher. Reading across that row, you can see that there’s a 50% chance that there will be a frost after May 17.

If you click on a marker a little closer to the lake, that date where the probability of a frost falls to 10% is a little earlier: In Two Harbors, it’s May 24. Farther north, of course, it’s later. In Ely, the date is June 9.

But 32 degrees is really cold for some plants, so you may want to look up one row and see when there’s a 90% chance that the temperature won’t get as low as 36 degrees. At the Duluth airport, that date is June 21! And while it’s useful to know when plants may freeze, it’s also important to pay attention to soil temperatures. Some temperatures aren’t cold enough to kill plants, but not warm enough to allow them to thrive. Tomatoes, for example, will just huddle miserably in cool spring weather and won’t start growing vigorously until it warms up.

The bottom line is: Unless you’re a gambler, you want to wait until June, maybe even mid-June, to plant tender annuals such as tomatoes in northern Minnesota.

So when do they mature, if the seed packet says 65 days?

If you are planting seeds directly in the garden, the countdown starts the day you plant them. If you are starting seeds indoors, the countdown starts the day you transplant your seedlings to the garden.

As with so many things in gardening, that number is an estimate. Your mileage will vary depending on weather, soil, and other factors.

It’s worth looking at that chart again to figure out whether the number of days between last frost and first frost gives your plant enough time to get to maturity. It says that at the Duluth airport, there’s a 90% chance that we’ll get 82 days where the temp stays above 36, and 109 days where it stays above 32. I like to subtract some time from that for plants that don’t do well on chilly spring or fall days. So if your seed packet says 110 days, or even 90 days, you probably want to grow something else.

Written by U of M Extension Master Gardeners in St. Louis County. Send questions to features@duluthnews.com.