There are so many wonderful reasons to plant vegetable seeds directly outside into soil this spring. More plant varieties are available as seeds, as compared to buying potted plants. Direct sowing of seeds is more economical and can be very satisfying.

Unfortunately, one of the biggest mistakes made is to sow seeds too early. All too often, the excitement of a few sunny days in early spring prompts sowing the seeds in the soil and thinking they will grow. However, too many seeds either rot from damping off fungus or germinate very slowly.

So, when is the right time to plant vegetable seeds directly into soil? Some gardeners consider frost dates; however, those dates relate more to when to plant seedlings than to sow seeds.

Also, even after the last frost date, the soil will continue to remain cooler than the air temperature. Frost dates don’t do a good job of accounting for the diverse micro-climates that exist on the Olympic Peninsula.

Soil temperature is not the same as air temperature. The air can change fast with a late winter storm or sunny day. Soil temperatures change slower because of the mass of the soil and moisture content.

While many factors affect germination, often the least understood and most overlooked is soil temperature. Proper soil temperature will go a long way toward seed starting success and vigorous plant growth.

Warmer soil temperatures speed up chemical reactions in the seed and cooler temperatures slow them down. Those chemical reactions help break down the protective seed coat and tell the seed that it is time to wake up and start growing.

For cool-season vegetables like carrots, onions, lettuce, peas, radishes, and spinach, the ideal temperature for germination can be as low as 60 degrees Fahrenheit.

For warm-season vegetables like eggplant, melons, pumpkins and tomatoes, the minimum soil temperature is 70 degrees.

A basic soil thermometer looks like a meat thermometer and costs under $20 online or at your local garden center. Digital thermometers are another option; however, any one will do, as long as it measures down to near 32 degrees.

Temperature readings should be taken on three consecutive mornings at the same time, usually mid-day. Insert the probe to a depth of 2 inches for seeds.

Make sure the thermometer is firmly touching the soil and allow a few minutes for the temperature to register. Take temperatures from a few different locations and get the average.

There is an amazing website at the public’s disposal — the WSU AgWeatherNet, a gardener’s best friend! (see weather.wsu.edu)

It tracks several indicators, such as rainfall, growing degree-days, and the soil temperatures of most towns in Washington on a weekly basis. This website is particularly useful for gardeners who don’t want to take the exact temperature of the soil but want to find out the approximate local soil temperature.

Interested in raising the soil temperature in the garden? Fortunately, there are several ways to do just that. Location is one vital way. A south-facing garden in full sun does a smashing job at warming up soil after a cold winter.

Raised beds are another great way. Gardeners have long loved how raised beds warm up faster in the spring than soil on the ground. Also, raised beds are often more productive than beds in the ground because the soil is less compacted, has better drainage, and generally has less weed pressure.

Don’t forget about container gardening! Containers can easily be moved around throughout the day and protected at night to achieve the ideal soil temperature. Almost any vegetable can be grown in a container.

Fabric grow bags have become a popular method of container gardening. The small holes in grow bags allow excellent circulation of air. This allows roots to efficiently absorb water and nutrients from the soil. It is almost impossible to over-water plants using grow bags; be more concerned about under watering.

Likewise, heat up the ground with plastic sheet mulch. Clear or black plastic mulch will take advantage of solar energy to raise the temperature of soil by several degrees and will hold that heat overnight.

To increase both the soil and air temperature, use a cloche, hoop house, or a cold frame. All of them trap heat like mini-greenhouses and are excellent for extending the garden season.

Using soil temperature to help decide when to direct sow seeds outside can be a real game changer.

For more information about home vegetable gardening, go to tinyurl.com/SEQveggies.

The WSU soil temperature website is at weather.wsu.edu/?p=89750.

Dave Eberle is a Clallam County Master Gardener trainee.