Now is the time to start planting our tomatoes and other warm season veggies outside in our gardens. So often we concentrate on only the air temperature to decide when to plant these crops but the soil temperature is actually just as important. 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 (and pepper) seedlings sulk and will not be happy. Root development is very slow and the roots have difficulty absorbing nutrients. The plants could show phosphorus deficiency which 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.
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 to check both the temperature of my compost pile and the soil in my vegetable bed before I plant tomatoes. Remember to 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 but I recently saw some at Payne’s Nursery here in Santa Fe.
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 from Farmerfredrant giving the optimum soil temperatures for planting vegetables. I’m showing it here but also listed it as a pdf (soil temperatures for veggie seeds ) so you can print it out as well.
giantveggiegardener 
Nicely done, Jannine! Not at all ‘technical’ – instead very practical!
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[…] Soil temperature is Important When Planting Tomatoes (giantveggiegardener.com) […]
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This explains so much about our tomatoes… Also, the chart is wonderful, I am saving that for later!
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Impressive write-up! I agree on that and Thanks a lot for sharing the chart. Sure comes in handy.
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I live in Michigan and it takes longer for the soil to warn up. The chart is very helpful
Don
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I’m looking for a 24” compost/garden thermometer – an affordable one. Harris Seeds has an affordable 19” thermometer. Any ideas?
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Are you in Santa Fe area? If so, Paynes Nurseries has carried them. Call and ask, they can bring it out for curbisde pickup/If you are not here in this area, go to amazon. The 24 inch compost thermometer is invaluable and I not only use it for my compost pile to see how hot its getting, I also use it for seeing what the soil temperature is before I plant. Many seeds packets say what temperature soil should be before planting. Different parts of your garden will have different temps too. A great tool.
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all the blah blah about 60 degrees minimum for tomatoes and then your chart shows fifty degrees smh
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This chart was for starting seeds inside for germinating, not planting transplants outside. Nothing is carved in stone and there are many charts out there with some differences on seed starting temps. I would not start them inside at 50 degrees as that chart mentions. I should of caught that. What temp do you start seeds inside? I assume you do since you are critical…
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What about full grown plants with very few tomatoes and even less foliage/greenery so it was suggest that the soil isn’t shaded therefore heating up the ground to much and too deep in our over 100 degree days. Any idea’s. The plant with lots of leaves and branches is the Rugar Heirloom, which was the one that responded to the fish emulsion (along with 3 of the Better Boys) but the other 5 Better Boys are 2 stalks with small leaves that haven’t increased in size or present new leaves for weeks now.
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Some questions-
Do you have any lower leaves turning mottled green-yellow? If so, you could have Early Blight, a soil fungal disease that can come when water splashes the EB soil spores up on the lower leaves.
Are you watering enough?
Is the Rutgers plant getting more water?
Do you have straw under the tomato plants to help conserve moisture in summer heat.
I always put straw under the plants to help keep the soil from splashing up on the lower plant leaves if you watering overhead.
Another suggestion is get 30% shade cloth from online. I use Johnny Seeds.com. Your plants swill benefit from a little shade in such heat.
Would love to see some pics to maybe help you…
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