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.