Garden Tips: Check soil temperature before planting

At long last, warmer weather is here and spring arrives officially tomorrow. Nic Loyd, Washington State University meteorologist, recently said, “At least for now, it appears that spring-like weather is here to stay.” In his 8- to 14-day outlook for March 18-24, Loyd predicted above average temperatures and near to above average precipitation.

Woo hoo! As anxious as we all are to get out in the garden and get things planted, we need to remember that the last average date of frost in the spring for most of our area is May 1. WSU’s AgWeatherNet shows that the soil temperature at a depth of 8 inches is still below 50 degrees in this area. This is the result of cool weather and the absence of warming sunshine.

It is convenient to have this information at my fingertips on my computer or smartphone (http:// weather.wsu.edu). You can also get up-to-the-minute data on temperature, wind, evapotranspiration, accumulated rainfall and relative humidity, as well as the frost risk from day to day

If you prefer a hands-on approach, use a soil thermometer to check the exact soil temperature. Buy one at a garden or hardware store for less than $20. Use it to measure the soil temperature at a depth of 2 to 4 inches, depending at which depth you will be planting. The soil temperature should be taken in late morning and needs to be consistent for several days before you can consider that the soil is adequately warm.

Below 60 to 55 degrees is too cool for warm-season crops like tomatoes, peppers, cucumbers, melons, squash and beans, but 40 to 45 degrees is suitable for cool-season crops like peas, lettuce and spinach.

Before you even think about planting crops that need warm soil, you should work at getting that temperature up a bit. Here are a few tips and tricks from other gardeners:

1. Mulch insulates the soil from cold and warm temperatures. If you have a layer of mulch over your beds, rake it back to allow potential sunshine do to its work warming the soil.

2. About three weeks before you are ready to plant, work the soil and till in any planned organic amendments. Next, install a temporary layer of clear plastic mulch, being sure to secure the edges with a layer of soil to keep it from sailing away on a windy day. Plastic mulch will help the soil warm more quickly. Thicker plastic, or double layers of plastic, retain heat longer at night. Remove the plastic mulch just before planting, and reuse it again next year.

You will note that clear plastic mulch will also encourage the germination of weed seeds. Before planting, use a shuffle or hula hoe to easily remove the seedling weeds.

3. Keep your seeds or transplants more comfortable after planting by covering the beds with a floating row cover, also known as garden fabric. The heavyweight types of this white spun-bonded polyester, or polypropylene fabric, can provide young plants with 4 to 10 degrees of frost protection. The lighter, thinner fabrics provide 2 to 6 degrees of frost protection. Floating row covers also modestly increase the temperature and humidity underneath, and provide protection from wind.

4. If using cylinder water-wall devices, such as Wall ’O Waters, around tomato and other transplants, fill and place them in the garden for a week or two before planting.

Be sure to check out AgWeatherNet at http://weather.wsu.edu. They gather their data from 177 automated weather stations primarily in the irrigated regions of Eastern Washington, making the site a useful tool for farmers and gardeners in our area.

This story was originally published March 19, 2017 at 7:04 AM with the headline "Garden Tips: Check soil temperature before planting."