Turning point: When is it safe to plant garden?

Spring in the vegetable or flower garden is a carefully orchestrated series of events. The goal is to keep things moving forward with the warming season, taking into account weather predictions and, because those are just predictions, average spring warming trends where you garden.

Many gardeners use Memorial Day as a turning point in their gardening calendar. But Memorial Day weather in San Diego is very different from Memorial Day weather in Concord, N.H.

Last killing frost

A better measure is “the average date of the last killing frost in spring” where you live. Note that this is just what it says, an average. In any year, that date might be earlier or later.

You can figure out that average date of the last killing frost online at a variety of sites where you just type in your ZIP code. Or contact your local County Cooperative Extension office for that information.

Some like it hot

Vegetables and annual flowers can be grouped into the cold-tender and the cold-hardy. Cold-hardy vegetables include onions, lettuce, arugula, parsley, spinach, radish, carrot, parsnip, and cabbage and its kin, such as kale, collards, Brussels sprouts and broccoli. Snapdragon, pansy, petunia, pinks and verbena are some cold-hardy, annual flowers.

Corn, winter and summer squash, tomato, pepper, eggplant, okra, melon, cucumber and bean are some popular frost-tender vegetables. Marigold, zinnia, impatiens and heliotrope are frost-tender annuals that should not be planted too early.

Vegetables and flowers can also be grouped according to whether their seeds are sown directly in the ground or, for an earlier harvest, started indoors or in a greenhouse, to be moved into the garden eventually as transplants. Some vegetables are grown both ways, as transplants for an early harvest and also seeded outdoors for harvest later in the season.

Putting it all together

How to relate all this to that magical “average date of the last killing frost in spring”?

First to go into the ground are seeds of cold-hardy vegetables and flowers. Generally, they might be planted as soon as the ground has thawed. Even cold-hardy seeds won’t germinate well until the temperature of the top few inches is above 40 degrees Fahrenheit. Figure on planting these seeds four to six weeks before the average last frost date.

Next in the lineup are transplants of cold-hardy vegetables and flowers. Set them in the ground three weeks before that average date of the last spring frost.

Follow this group with seeds of cold-tender vegetables and flowers, which can go into the ground about a week before the last frost date. The ground at that point retains heat to smooth out any swings in temperature, and the tender new shoots won’t peek above ground for a few days, after temperatures have further settled.

Finally, time to move cold-tender transplants into the ground. Timing for this is about a week after the last spring frost date. Remember that the magical date is an average, so frosts can sometimes occur after that.

Frost is most threatening on cool, clear, windless nights. Be ready, when frost is predicted, to cover cold-tender plants with blankets, upturned flowerpots or anything else that can hold in heat.