Soil Temperature Is Critical To Garden Success
Soil temperature is more important than air temperature when planting seeds or seedlings. You can have in the spring a warm spell of temps in the 70’s while the soil temp is still in the 40’s. Every vegetable has a preferred soil temp for seeds or transplants. A soil thermometer is essential for determining the proper planting time. Planting too early, before the soil has had time to warm up, can lead to seed rot, slowed germination, poor growth and disease.
For example, cucumber seeds usually take less than a week to germinate in a soil of 70 degrees F. They could take two weeks at 60 degrees F. Tomato transplants need a soil temperature above 60 degrees F for growth. Setting pepper plants out before the soil temperature is 70 degrees F could stunt their growth for the entire growing season.
Correct temperature readings need to be observed on three consecutive mornings. Readings should be taken at a depth of 1 - 2 inches for seeds and 4 - 6 inches for transplants.
Use the following guide for minimum soil temperatures for seeds and transplants:
60 F - tomatoes, cucumbers, snap beans
65 F - sweet corn, lima beans, mustard greens
70 F - peppers, watermelons, squash, southern peas
75 F - okra, cantaloupe, sweet potatoes
Crops that will germinate in the coolest soils (down to 40 degrees) include arugula, fava beans, kale, lettuce, pac choi, parsnips, peas, radicchio, radish and spinach seed.
With a soil temperature above 50 degrees, Chinese cabbage, leeks, onions, Swiss chard, and turnips can be planted.
When the soil warms to 60 degrees, warm season and many cool season vegetables can be sown, including beans, beets, broccoli, Brussels sprouts, cabbage, carrots and cauliflower. But be forewarned – beans will not tolerate any frost and may have to be planted again if the temperature goes below freezing.
Wait until the soil warms to above 70 degrees to plant warm season vegetables including tomatoes, eggplants, peppers, cucumbers, squash, corn and melons. Tomatoes, eggplants and peppers are slow-growing and take many weeks to grow to the stage where you can plant them out in the garden, so you might want to purchase these as starts from your local garden center. On the other hand, squash, cucumbers and corn grow quickly and are easier to start from seed.
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More On Soil Temperature
I realized recently that I have never actually kept track of when the soil froze in my garden and lawn. I don’t consider it a major life failure, but it is curious to be someone who for 25 years has written about what happens when the ground freezes and never kept track in his own little territory.
- digital soil thermometer
So I have set up my highly technical monitoring system so in a few weeks or a month, I can officially declare that my ground is frozen. I've 3 degrees to go as of December 6th. When you get to my age, the little things take on more importance.
My fancy tech tool is my handy T-shaped digital soil thermometer which by the way makes a terrific stocking stuffer for any yardener not owning such a handy little tool. Google “digital thermometer” for any number of sources for about $15.
What’s the big deal about the soil freezing? Well, there are many gardening books that tell us to wait until the soil freezes before we lay out the winter mulching material. Here we are in December in Michigan and the ground is not yet frozen. Friends, I have no interest in going out there in mid-December or early January and put down mulch in my garden. I did that job a month ago when I was not worried about frost-bite.
For me, the freezing of the soil around the root system of any plant means that the plant is no longer growing in the root area, and that plant can no longer take up water through its roots. So by my way of thinking, I want the mulch on the garden long before the soil freezes to actually delay that time when the plant becomes more vulnerable to unexpected stresses. My plants still have some time. My garden soil temperature this morning was 37 degrees where there was no mulch and 39 degrees under the mulch. I am measuring the temperature at about 4 inches deep.
The major role for my mulch during the entire winter season is to minimize the effect of warming and cooling that can occur if we get an unexpected warm spell in the middle of January. I want to keep that soil temperature as steady as I can by buffering the changes in temperature above the soil. Three to four inches of organic mulch can do that job. Major soil temperature fluctuations can cause a perennial, for example, to be pushed right up out of the soil.
Come spring, I want that mulch to stay on because I know it will slow the heating up of the soil. When it does get warm, the soil should not be exposed to quick and unexpected deep freezes. Using my handy soil thermometer, I don’t do anything in my garden until the soil has dried out sufficiently and the soil temperature is at least 45 degrees. When spring air temperatures sometimes soar into the 70’s and even 80’s the temptation to get out into the garden is enormous; cabin fever is a real phenomenon. If that soil temperature is still below 45 degrees, I go to the movies.
Salt is Bad
Back to current weather conditions, the ice and snow has arrived and I see folks spreading that salt on their sidewalks and driveways not realizing how much damage that salt can do to their lawn and to plants growing in soil that is going to be saturated with salt by the end of the winter. Please use any of the many salt substitutes to deal with ice problems. Your plants will thank you in the spring.
Don't Tread On Me
Finally, my annual plea to stay off the lawn once that soil is in fact frozen. When the roots are frozen and you walk over dormant grass plants, you can do serious damage to the crowns of those plants that sit just above the surface of the soil. If you feed the birds from feeders out in the lawn, approach those feeders from different directions when you are filling them. To walk over a frozen lawn over and over again in the same area is to create a need to overseed some dead grass next spring.
