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After Dark

Soil temperature makes a difference

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Story By Patti Nagai

It may seem obvious at first thought; cool is when it’s cool outside, warm is when it’s warm, but when it comes to knowing what to plant when, there’s a little more to understand. One of the best gardening tools to gauge planting time is a soil thermometer.

Plants have to deal with whatever Mother Nature brings them. As gardeners, we can help by paying close attention to not only the day and night air temperatures, but also the soil temperature.

When we are in T-shirts and shorts in May because it has warmed up to 70 or 80 degrees, our soil is still quite chilly. Since plants depend on roots for water, nutrients and growth regulation signals, it is important for gardeners to know the soil temperature before planting seeds or transplants.

Cool season crops grow well in soil temperatures of 40 to 60 degrees, but warm season crops prefer soil temperatures above 65. In our part of Wisconsin, soil is often still in the 50s during May, but the best guide for your garden is to measure the temperature yourself. Raised beds and containers tend to warm faster, and you may find big differences in different parts of your yard.

Some plants love cool soil and thrive during cold nights, others will tolerate chilly air but will not tolerate cold soil, and those true lovers of summer will not tolerate cold soil or cold air. That’s where the “cool season” and “warm season” classification can be helpful.

Cool season crops grow in spring or fall, but some do better in fall when soil is warm and air is cool. Examples include Brussels sprouts, carrots and sugar snap peas, which grow faster in warm soil, but taste better when harvested in the freezing cold.

Cool season crops for spring planting include broccoli, cabbage, kale, turnips, beets and bok choi. Leafy greens, including lettuce, spinach and Swiss chard, do well in cool soil and cool air, and there are new “heat tolerant” cultivars of each that still hold onto their sweet flavors in the heat of summer. The risk with cool season crops is when days get longer and hotter, flavors become bitter and many “bolt” into flower.

Warm season crops need warm soil and warm air to grow their best. If the soil is cool, there is risk of seed and root rot, and also chilling injury which could greatly decrease growth. Wait until the soil is above 60 degrees to plant beans, squash, cucumbers, tomatoes, peppers and eggplant. They will grow better and produce more when the soil is warm.

When August rolls around and the abundance of tomatoes and squash is overwhelming your table, don’t forget to plant those cool season crops for a late fall harvest. There is nothing sweeter than a home grown carrot pulled from the garden after a hard freeze; and Brussels sprouts will taste their best when the leaves are falling. But wait, let’s enjoy spring and summer first!

Patti Nagai is the horticulture educator for Racine County UW-Extension.