I know I talked about planting times last week, this week I
am going to focus more on soil temperatures and how knowing the soil
temperature will help guide you on when to plant things. For some people a
device like a soil thermometer takes the guesswork out of knowing when to plant
and gives them more confidence.
You can take the soil temperature by using a soil thermometer, which are very inexpensive in garden shops. Push the thermometer into the soil to about 8 inches deep. If you have an old fashioned “stick” type air thermometer you can use it too, if you carefully insert it into the soil. Most candy and meat thermometers won’t have low enough measurements to be useful.
In this article I will also give a few phenological indicators that generally are consistent with soil temperatures. This means I will note what other common plants in your environment may be blooming when the soil temperature is at a certain point. Remember that there can be microclimates in every garden. If your neighbor a few houses down has a lilac in bloom it may still be too cold in your garden bed for certain plants.
When we decide to plant crops, we consider the soil temperature, air temperature and the plants preferred growing conditions. Sandy soils and darker colored loam soils warm up faster than heavy clay soils. Soil in raised beds also warms up faster. Wet, poorly drained soil stays cool a long time.
Heavy layers of mulch will delay soil warming. Pull them back in early spring so your soil can be warmed by the sun. You can replace them after the soil is warm enough for what you want to plant has been planted.
Vegetables fall into 3 categories for the proper planting time. The cool weather crops, mild weather crops and warm weather crops. The very first crops to plant are peas and leaf lettuce. These vegetables will grow in soil below 45 degrees and air temperatures just above freezing. Frost doesn’t faze them. They can be planted as soon as your soil is dry enough to work. Crocus, daffodils, and maples will be in bloom.
Dormant perennials hardy to your area, and dormant trees and shrubs can be planted now. Grass seed can be sown.
Next crops to plant are kale, chard, spinach, pac choi, radishes, onions, leeks, potatoes, and turnips. Soil temperatures of 50 degrees are high enough and daytime air temperatures in the 50’s and 60’s. Light frosts can still be expected. Depending on your location and the weather this can be late April- early May. Dandelions will generally be blooming and the trees starting to leaf out.
At this time blooming pansies can be planted in the flower bed too. You can still plant dormant perennials, trees and shrubs. Potted growing perennials with minimal leafing out can be planted.
Next crops to plant, with soil temperatures of 55-60 degrees are carrots, beets, cabbage, broccoli, cauliflower and Brussels sprouts as transplants or plant their seeds. Air temperatures should be 60’s and low 70’s. This is usually early to mid-May. Popcorn can be planted, and you can take a chance with an early planting of beans. If there are no frosts after the beans emerge, your beans will do well. Frost will mean you have to re-plant. Some people also like to gamble with a cold tolerant sweet corn around this time to attempt an early corn crop.
At this soil temperature some flowers that can be planted are calendula and snapdragons. You can direct seed most annual flowers, like zinnias and marigolds at this time. It will take 10-14 days for them to emerge from the soil and frost chances will be minimal. Most potted perennials and trees and shrubs actively growing can now be planted.
Bedding annuals and hanging baskets can now be planted or moved outside. If the danger of frost has passed houseplants and tender tropicals can now go outside.
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