Tomato plants under aqua domes

Tomato plants harden off safely in the garden under aqua domes.

Now that Mother’s Day has passed, it is time to think about vegetable seedlings — what to grow and how to plant them into the real world of the outdoor garden.

hose of us who don’t have time or space to start our own seeds may be venturing out to local plant sales, nurseries or garden stores for tomatoes, peppers, eggplants or broccoli to feed ourselves, our families and friends. What should you be looking for as you peruse the stock on hand?

It is essential to start with vigorous seedlings. Even if you manage to keep a stressed plant alive, it will never produce as well as one that got off to a good start.

Wherever you go for plants, the seedlings should look well cared-for: no yellow leaves or signs of insect or disease. The presence of damaged or neglected seedlings should tell you to shop elsewhere. A bigger seedling is not necessarily better. Look for sturdy plants with healthy leaves. Tall, spindly seedlings have probably been grown without sufficient light or have been in the pot too long. Avoid plants that are starting to bloom. These have stopped putting energy into root growth and will be hard to establish in the garden.

Once you bring your seedlings home, be ready to keep them protected for a while before planting into your garden. Many of our favorite vegetable plants cannot tolerate freezing temperatures. When they are young and tender, just a light frost may be enough to damage them, and even frost-tolerant plants such as kale are sensitive until they become established.

The process of preparing indoor-grown seedlings for the garden environment is referred to as “hardening off.” It helps the plant to become firmer and more resistant to the challenges of wind and weather. Hardening off should begin one or two weeks before setting unprotected plants out in the garden.

An easy way to harden vegetable starts is to put them outdoors on warm days in a shaded area out of the wind, and bring them in at night. The outside temperature should be at least 45 degrees Fahrenheit. As the days go by, gradually increase the amount of sunlight plants receive. Reduce the frequency of watering but do not allow the plants to wilt. You want plant growth to be slowed but not stopped.

Plants can be hardened off in a cold frame outside once nighttime temperatures stay above freezing. Vent the frame during the warmest part of the day, gradually increasing the length of time the frame is left open. If you are not able to tend to the frame during the day, try to time the onset of your hardening-off period with cloudy weather, and start by venting the frame just a crack, gradually increasing the open gap each day.

Some commercial plant protectors will allow you to plant greenhouse seedlings directly into the ground while hardening off. The most effective protectors have double walls that can be filled with water. These will warm up during sunny days and keep the plant insulated at night, and are heavy enough to avoid blowing over during windy days. It is important to remove them once the hardening off period is over, since plants can overheat once summer temperatures arrive.

Be aware of expected frost dates for your site. In Chelan County, average last frost dates can be anywhere from late April in Wenatchee to mid-June in Plain. Your particular garden spot may differ from average depending on slope and aspect. Choose varieties that you can grow successfully, so you can look forward to an abundant harvest.

A WSU Master Gardeners of Chelan County column appears weekly in The Wenatchee World. Connie Mehmel is one of four columnists featured. To learn more, visit or call 667-6540.