Regional Gardening Reports :: National Gardening Association

In the Garden:
Upper South
March, 2012
Regional Report

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Using a soil thermometer helps you make the best decision when planting vegetable seeds.

A Key to Spring Planting Success

Vegetable gardening season has started! How do I know? At this point in my life, I can go by past experiences and observing the weather. Other criteria used by gardeners include the phases of the moon or related natural phenomena, such as planting corn when the leaves on an oak tree are the size of mouse ears. Often, gardeners use the "count back" method, that is, planting crops a certain number of days before or after the traditional frost-free date. Any of these methods work reasonably well, but what if there was a way to be more accurate? The solution is to take your soil's temperature, using a soil thermometer that costs about $10.

Even if you've have successfully started vegetable seeds in the past, why not add an element to your arsenal that helps the greatest number of seeds to germinate and the resulting plants be their strongest and best? Plants grown from seeds that germinate as quickly as possible will be less stressed and better able to thrive and be their most productive.

How To Use a Soil Thermometer
For early-season and small-seeded crops, insert the thermometer 2 inches into the soil, while with warm-season and large-seeded crops, you'll want to insert the thermometer 4 inches into the soil. Take the temperature readings in the morning and late afternoon for several days in a row and average them. Of course, it's always important to read and follow the thermometer packaging for specific directions. Often, a soil thermometer will also include a chart listing crops and planting temperatures. As with any tool, you'll need to take care of your soil thermometer, protecting it from breakage.

Planting Times By Using Soil Temperature
Fortunately, most seeds have an optimum temperature range for germination, so that gives you some leeway in planting in case other factors aren't in alignment. Still, at this time of year, we're looking for the minimum temperatures when we can plant. The following are some general guidelines.

Plan on planting arugula, kale, lettuce, bok choi, parsnips, peas, radish, and spinach seeds when soil temperatures are at least 40 degrees F. For Chinese cabbage, leeks, onions, Swiss chard, and turnips, your goal is a soil temperature above 50 degrees F. To start beans, beets, broccoli, Brussels sprouts, cabbage, carrots, and cauliflower, soil temperatures at or above 60 degrees is needed. For those summer crops like tomatoes, eggplants, peppers, cucumbers, squash, corn, and melons, you need to wait for soil temperatures above 70 degrees F.

Partner Soil Temperature with Soil Moisture and Weather Conditions
Even when the soil temperature is in the correct range, if the soil is too wet, which is particularly an issue in spring, then you'll have to wait. A simple way to test soil moisture is to take a handful of soil and squeeze it. If water drips from your hand, then obviously it's too wet. If the soil forms a ball that doesn't readily crumble when you poke it, you should wait to plant. Soil with proper moisture should compress into a ball but crumble when gently prodded.

Even if soil temperatures are in the right range for what you want to plant, especially in spring, there is always the possibility of cold weather returning. Even if seeds germinate and plants are growing, there is the possibility of losing them. It's a good idea at this time of year to plan for ways to protect your young plants, such as using row covers or low tunnels.

Other Ways to Use Soil Temperatures for Gardening Success
Still not sure if you want to invest in a soil thermometer? Then consider that owning a soil thermometer will also help with other aspects of gardening. For instance, when planting spring-blooming bulbs in the fall, wait until the soil temperature falls below 60 degrees F. For planting shrubs in the fall, choose a time several weeks before soil temperatures fall below 40 degrees F. Or for crabgrass control in the spring, wait until soil temperatures reach 55 degrees F for at least four days in a row before applying pre-emergent herbicides. While we're on the subject of lawns, cool-season grass seed germinates best when soil temperatures are at least 50 degrees F.

So why not add a soil thermometer to your garden tools this year? Discover all the ways that it can make your garden more successful than ever.

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