One of the most often asked vegetable questions during this early season is “How soon can I plant my tomatoes and peppers in the garden?” There are two reasons that the northeast Ohio gardener’s rule of thumb is “wait for Memorial Day” before planting out the tender annuals like tomatoes, peppers and green beans. The first reason is the possibility of a frost is almost eliminated by waiting until Memorial Day. Those tender annual plants like squash, tomatoes, green beans and peppers, cannot tolerate a frost event or even lower temperatures at all!
The other reason to wait is that the soil temperatures need to warm up. Regardless of what daily highs the ambient air temperatures reach, the key limiting factor for early season plant growth is soil temperatures. The simplest and best way to know when it is time to plant those “prize winning” tomatoes is to buy a soil thermometer and test the soil temperature at about 3-4 inches deep.
Brilliant sunshine, snow showers or cold rains, partly cloudy or completely overcast skies, all serve to impact our soil temperature coming out of the winter season. It is all about sunshine heating the dark soils, followed by the mostly topical heat layer being gradually moved deeper through the soil profile via conduction. This is the reason that soils covered with a mulch, are slower to warm up in the spring. This is due to the mulch acting as a barrier or insulator that prevents the sunlight from directly striking the soil to heat it.
Research studies have provided insight for us to know that tomatoes seem to have problems uptaking phosphorus when soil temperatures, are around or below 50°F. This lack of phosphorus in young, tomato transplants shows up as a purpling of the underside of the leaves of the plant. Germination of vegetable seeds is also contingent upon soil temperatures. This is why peas, radishes, broccoli and other veggies we call “cool-season crops”, have no problem germinating and thriving early in the spring with cooler temperatures.
Put squash, pumpkin, green bean or corn seeds in the ground under those same cool soil conditions and little to nothing happens! Germination is very slow in these cold, spring soils and the seeds are often overwhelmed by the soil microbes and other decomposers, becoming their source of energy or food. This is one of the reasons why they are referred to as “warm-season crops”. The threshold or minimal soil temperatures appears to be about 60°F or higher, which will provide the necessary soil environmental conditions needed by warm-season vegetable crops to grow.
Have some fun in your garden and buy a soil thermometer and track the temperature of your garden soils throughout the growing season. I think that you‘ll be astounded, amazed and fascinated by what you discover about the temperature trends of your garden soils!