Sun’s Out - Trowels Out? Considering Soil Temperature Before Planting

In late winter/early spring, in many regions, we begin to get stretches of warm, sunny weather, which can awaken the hibernating gardener within us. We find ourselves wandering the aisles of our favorite nursery, considering the purchase of plants and seeds that “might do okay” if the weather holds. Perhaps a better investment during this tricky yearly phase is a soil thermometer.  

Monitor Soil Temperature

Typically featuring a 5-7 inch probe stem, this type of thermometer (analog or digital) can be found at many nurseries, box stores, and online retail companies.

Because a soil thermometer measures the temperature 2-4 inches below the soil surface, if taken between 9:00 and 10:00 am each day for 4-7 days, these readings, averaged, will give a more accurate indication of consistent soil conditions than can be gleaned sitting out on the patio during gloriously sunny March afternoons.

Another valuable resource for information on your area is the UGA Weather Network. This website enables you to enter your ZIP code for soil temperature and other weather conditions. 

Optimum Temperatures for Germination and Development

To apply soil temperature information, it’s important to research the needs of the plants we want to cultivate before we begin. For example, for germination, most tomato seeds need 6-8 days at 75-85°F, or 14-16 days at 60°. Bean, squash, and cucumber seeds require a minimum temperature of 60° or higher to germinate, and they germinate more quickly and reliably at soil temperatures of 70° and higher. If the seeds are exposed too long to cold, wet conditions, they may rot before the plant can successfully begin to develop. On the other hand, leafy greens, brassicas, and root vegetables, as well as many legumes and herbs, can germinate and thrive in cooler conditions, with soil temperatures as low as 45°F.

Apply Pre-Emergents at the Right Temperatures

Another important phase of landscape care that depends upon soil temperatures is pre-emergent application to dissuade weed growth. Pre-emergents such as broad-leaf herbicides should be applied before the roots of lawn grass and perennial flowers enter an active growing phase, which is typically 65° and above. Applying pre-emergents now, in February and early March, will help stop new weed growth without harming desirable plants, the roots of which have likely established themselves well below the pre-emergent layer (herbicides settle and form a “weed shield” about 1” below the soil surface). If we wait to apply the pre-emergent until temperatures have become consistently higher than 65°, weed roots will likely already have grown deeper than the 1” mark, and it will be too late for the herbicide to do its magic. (By the way, pre-emergents can be applied again in fall, when the soil has cooled, to prevent winter weeds. These herbicides last in the soil for roughly 2-4 months, depending on soil conditions.)

Slip into Gardening Mode, Strategically

So if your inner gardener is itching to get started on this year’s landscaping tasks, the best “early steps” are to purchase a soil thermometer and perhaps apply a pre-emergent in strategic areas. There’s no reason we can’t go ahead on nice days and get our hands dirty! But a bit of patience and forethought now can certainly boost our chances of a successful growing season later.