Soil Temperature and Planting Tomatoes, Peppers and Other Warm Season Crops
The month of May is the time to start planting our tomatoes and other warm season veggies outside in our
gardens, weather permitting. So often we concentrate on only the air temperature to decide when to plant
the warm season crops but the soil temperature is actually just as important.
What is the temperature of your soil right now?
-To measure the soil temperature, use a soil thermometer.
-I prefer using a compost thermometer because they are much longer, usually around 24″ and can be used
not only to check the temperature of my compost pile but also the soil in my vegetable bed before I plant
heat loving veggies like tomatoes.
-Push it in deeper into your bed as the tomato plant won’t be in the top 3″ but more likely planted deeper
where the soil is cooler. I find the short soil thermometers just aren’t long enough to measure the soil
temperatures more than about 5 inches and quite often I plant tomatoes much deeper. I got my compost
thermometer online.
How to warm up the soil sooner
-To warm up soil sooner, you can put black plastic over the bed to pre-warm the soil. I use black plastic
garbage bags that I tack down with rocks. That way I can reuse the bags later instead of buying a roll of
black plastic.
-Leave it on for 1-2 weeks and take the temperature to see when the soil warms up to the optimum
temperature. Many warm season vegetables could benefit from planting in warmer soil.
Here is a chart I found giving the optimum soil temperatures for planting vegetables. I’m listing it as a pdf
(soil temperatures for veggie seeds ) so you can print it out as well.
You can also plant some transplants in wall of waters to help heat up the plant and soil early. I always use
them to plant tomatoes and peppers when I first plant. I take the WOW off later after they outgrow them or
it is the monsoon season. I also use row cover to protect some warm season crops.
Now that we’ve talked about how important soil temperature is, let’s start with most people’s favorite
veggie which is actually a fruit-the tomato and when to plant them.
Tomatoes
-If you haven’t started your tomato seeds by now-DON’T. It’s too late, so go support your local nurseries
and buy your plants to plant later this month.
-Tomatoes should be planted when the soil temperature reaches a minimum of 60°F in the daytime.
- If you plant too early in cold soil, tomato transplants will not be happy. Root development is very slow
and the roots have difficulty absorbing nutrients.
-The plants could show phosphorus deficiency. This shows up as stunted plants with purple leaves on the
underside. If your plants get this, top-dress them with some powdered rock phosphate and water in.
-Nothing is gained from planting too early in the ground. This may account for why we always seem to get
the bulk of our tomatoes in August and not earlier when planted outside no matter when we plant. The
tomatoes will just sit there until the soil temperature is optimum.
Here’s my recipe for planting tomatoes
When I plant them put them in a hole where I add some compost, worm castings, Yum-yum mix, a small
amount of mushroom compost (optional), a tablespoon of dry milk and a tablespoon of Epsom salts to get
them going. Then I create a well around them so the water is collected instead of running off. Then I put
my drip line around the plant in the well and then place my wall of water around the plant. I try not to water
overhead onto the plants to avoid fungal diseases but water from below or with a drip.
Peppers/Eggplants-As soon we are past freezes at night, you may think you should put out those pepper
plants that you have inside your house. Not so fast! Patience!
-Peppers are heat-loving plants (even more so than tomatoes) and our nights will still be too cold for them
to put out here in Santa Fe in May unless we have an unusually warm May.
-Hold your peppers inside for another week or so past that magical date of May 15th because if you put
them out now they will just sit there and sulk. Why? Because the soil is too cold.
I’ve had to replant some in years past because they stall out and never recover. I’ll be putting mine out
AFTER JUNE 1st and in wall of waters to add some protection with our cold nights. Peppers don’t like
temperatures below 60°F at night so for now they can sit and bask in the sun in front of a window. Don’t be
in a rush with them or you may have to start over!
With other warm season crops you can direct seed in the ground or put out after May 15th:
Beans
Carrots
Beets
Cucumbers
Corn
Squash-summer and winter squash
Pumpkins
Melons
GET BUSY-20 DAYS TILL MAY 15th!