The blustery, roaring lionhood of March is proverbially balanced by some lamb-like gentleness at the tail end. This year March in Michigan deviated very little in terms of temperature and precipitation difference from historical averages. So, even though this is a fairly chilly close to the month that inaugurates spring, we were spared some of the harsher weather for which early March is so notorious.
While the soil is still very cold and wet in most places, it's not to early to start sampling soil temperature. Use an inexpensive meat thermometer and check the temp about 3 inches under the surface of your garden. Its a good idea to check a couple locations, once each in the morning and late afternoon or evening. 3 inches below the surface is deeper than most seeds would be planted, but not as deep as most roots go--it will give you a good ballpark figure to work with. Note your temperature readings in a notebook or file.
When soil temperatures are generally at or above 45F, you are A-OK to plant many cool-season crops directly outside in the garden with seed. This includes broccoli, cabbage, peas, and other crops that can also be started from seed indoors and then transplanted out later. If you're doing the transplant method, make sure to harden-off your plants before finally setting them out into the garden. How do you harden-off your transplants? Start leaving them outside during the day when air temps are steadily above 40F. As nights warm up and as plants get used to the outdoors, start to leave them out at night, but shelter them as necessary. These crops will take a light frost, but NOT a hard freeze.
Other crops are best to start out in the garden from seed, and skip the transplant process. Carrots, beets, radishes and other root crops and greens like spinach may be seeded in the garden as soon as soil temp is steady at a 45F.
Then what? The next major milestone for soil temperature is 60F (again, a meat thermometer is crucial as soil temp differs from the daily air temps--it is slower to change). When soil has warmed to 60F, that's when transplants like tomatoes, peppers and eggplants can go out into the garden. But note, if you were starting these warm-weather plants indoors from seed, you would want the soil temp closer to 70F or even 80F for peppers. The difference is that the SEEDS need really warm or hot soil to germinate, whereas the PLANTS can handle more moderately warm, 60F soil for growing.
Again, keep in mind that air temps may still dip significantly in mid-spring when soil is stabilizing at 60F. Protect your plants, or wait a little longer to set them out.
When soil temp is steady at 70F or higher, that's when you'll get really good results planting seeds like melons, squash, cucumbers, beans, corn, and other heat-loving seeds like okra. Starting melons, squash, or cukes inside can help give a jump on the season but is not always necessary.
Healthy sunflowers result from healthy soil. Wait to plant seeds until soil is really warm. Noel (6`3") provides a sense of scale for this beaut at Catherine Ferguson Academy in Detroit.
There are lots of other, more experiential ways people know when to plant. The moon is one factor, as are environmental cues. One common piece of folk wisdom that has adapted multiple interpretations revolves around when to plant corn. Native Michigan people learned to wait until nearby oak leaves are the size of a squirrel's ear.
As mentioned at the start of this article, weather can be quite different year to year, month to month and day to day. Whether you use soil temperature, folk wisdom or both, the clues provided by our surroundings NOW help to adjust for the vagaries of daily and seasonal weather.