March 11, 2010 in Washington Voices

Take garden’s soil temperature before planting

Pat Munts
 
Pat Munts Special to photo

Soil temperature is important to know when planting early crops. Special to
(Full-size photo)

With the coming of spring, seed packets are flying off the shelves of the garden stores. Now that you have your little packets of gold and all the things to make them grow, you have to interpret the directions.

Take the seed packet phrase “plant as soon as the ground can be worked.” This means two things. First, is the soil dry enough in the spring to stand cultivation? Wet soil can pack into brick hard lumps that are difficult to break up later in the year and impede the flow of water to your plants.

Determining if your soil is workable involves getting a little dirty. Dig down a few inches in your garden and pick up a handful of soil and squeeze it into a ball. Now poke it with your fingers or a small tool. If it breaks apart readily, the soil is dry enough to dig in. If it stays in a ball or doesn’t give to pressure easily, wait a couple of weeks and try again. Clay soils characteristically will take longer to dry out than other soils.

This phrase also indicates the soil temperature that is needed for the seeds to germinate outdoors. Another common seed packet phrase used to indicate this is “plant in the early spring.” Crops like cabbage, kale, onion, peas, lettuce, spinach and potato will germinate when soil temperatures are in the 32 to high 40 degree range and are referred to as cool season crops. Warm season crops like tomatoes, peppers, squash and cucumbers will require temperatures closer to 60 and above to do well even as transplants.

To measure soil temperature, insert a soil thermometer about six inches into a garden bed. This is the root depth of the plant; surface temperatures will always be a few degrees warmer. A regular rectangular outdoor thermometer can be used.

Different areas of the garden will have different soil temperatures. Raised beds will always warm up quicker than regular in-ground beds. In an experiment in my own garden last week, the regular soil measured 35 degrees while my raised beds were 42 degrees. This is a big difference to a seed.

Soil within 3 or 4 feet of south facing buildings, fences, garden walls or other garden features that soak up heat will be warmer than soil further away from the heat sink. South facing slopes in full sun will always be warmer because the angle of the sun hits them more directly. A good way to determine where the warm spots are in your yard is to watch where the first new green weeds and grass show up in the next few weeks. Finally, weeds have a useful purpose in the garden.

One last warning: As warm as it has been this winter, don’t get too ambitious too quickly. Winter isn’t over yet and we could still get that last arctic blast. Keep some mulch handy to cover early planting if the temperature is going to drop well below freezing and wait to prune the roses.

Pat Munts is a Master Gardener who has gardened the same acre in Spokane Valley for 30 years. She can be reached by e-mail at pat@inland nwgardening.com


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