The calendar says late April, the weekend forecast is warm and sunny, but there’s still snow melting off the trees and loitering in the shadows. With our off-again, on-again spring, how can a gardener possibly know when to plant?
There’s no foolproof formula, but a soil thermometer can help take much of the guesswork out of gardening in Colorado.
While air temperatures may easily vary over 50° in a single day, soil temperatures are much slower to change, and so are a more reliable indicator of how the season is advancing. More than once, cold soils have kept me from succumbing to the siren song of a balmy 72° day and planting too early.
It doesn’t really matter which kind of thermometer you use, just as long as it includes a range from about 40° to 100°F. Soil thermometers often have a protective metal sheath on one side to keep them from breaking, and are easy to read while still inserted in the dirt, but a meat thermometer can work just as well.
To be effective, thermometers need to be used properly. There’s more to it than just sticking the thermometer into the ground.
Take your soil’s temperature first thing in the morning, say about seven or eight o’clock. This ensures a more accurate reading; later in the day you may find yourself measuring how sunny it is rather than how warm the soil is on average. Remember, the plants are outside all day, not just at noon.
How far should you insert the thermometer? It depends on what you want to plant. If you’re sowing seeds, measure in the “seed zone”—about three to four inches deep. Yes, the seed may be closer to the surface than that, but the germinating plant immediately sends roots downward, so that’s where to put your thermometer.
If you’re setting out tomatoes or other transplants, measure where the root ball is going to be. (Since soils get rapidly colder the deeper you go, you can see why it’s not a good idea to bury transplants too deeply. Rather, lay them on their sides, gently bending the stem so that all the leaves are above ground.)
Now that you know your soil’s temperature, how do you decide when to plant? This table (from the Alabama Cooperative Extension System) gives the minimum, maximum, and optimal temperatures for germinating a variety of common garden seeds.
Note that lettuce, parsnips, and spinach will germinate as soon as the soil has thawed, but may not sprout at all in the high temperatures of mid-summer. On the other hand, sowing zucchini and other squash before the soil reaches at least 60° just gives the seeds a chance to rot. You can see why it’s important to plant when the soil is the right temperature for each crop you are growing.
Not surprisingly, the plants that prefer cool weather—such as cabbage and broccoli, lettuce, spinach, peas, carrots, etc.—also germinate in colder soils. These seedlings are able to handle a bit of frost, too. Even so, don’t plant too early. Wait until the average last frost date is no more than a couple of weeks away.
Warm weather plants—tomatoes, squashes, melons, okra, peppers, etc.—need warmer soils to germinate, and do their best growing when it’s truly hot outside. They shouldn’t go into the ground until a week or more past the average last frost. (Remember, it’s an average—half the time there will still be frost past this date!)
Some of these warm season crops take a long time to mature, much longer than our curtailed growing season allows for. Those should be started indoors, long before the soil outside is ready to receive them. Using a heat mat designed for warming seeded trays will speed things up. I use shop lights on my indoor seedlings and have found that placing the trays on top of the lights provides just the right amount of gentle heat for fast germination. Of course, once the first seed-leaves appear, I immediately move the trays to their normal spot under the lights!
If you don’t want to be bothered starting seeds, these like-it-hot plants are the best candidates for purchasing as transplants. Just be aware that melons and squash have brittle roots and need TLC when being taken from their pots.
When asked which are my must-have garden tools, I always include a thermometer along with the more obvious trowel and garden cart. Once you start sowing by the thermometer, it will become one of your essential tools too.
A L A B A M A A & M A N D A U B U R N U N I V E R S I T I E S
Soil Temperature Conditions for Vegetable Seed Germination
Joseph Kemble, Extension Horticulturist, Associate Professor, Horticulture, Auburn University; Mary Beth Musgrove, former Extension Associate.
© 2006 by the Alabama Cooperative Extension System. All rights reserved.