Question: What are the steps for successful seed planting outdoors?

Martha: By the time the last frosts are passing, and the soil has thawed, every gardener is impatient to be outdoors, especially me! That’s when we can finally focus on the seed sowing you can do in the garden, employing the same techniques for planting directly into flower beds and vegetable patches that have proven successful in my garden and in other gardens in every region of the country.

Long before there were fluorescent lights or windowsills in heated rooms, there were gardens, and the vegetables, herbs, and flowers in those ancestral plots were all “direct sown”. That is, the seeds were planted directly into the ground. This worked centuries ago, and it works just as well today. Poppies, peas, beans, and nasturtiums are only a few of the plants whose roots are easily damaged during transplanting, and which usually grow better if directly sown. In addition, there is a host of other plants whose seeds may be started equally well outdoors. You can find this information on the back of seed packets as well as online as you research the seeds you want to plant.

WHEN TO SOW OUTDOORS GUIDELINES

Check the zone map. As a guide to which plants can be grown successfully in a particular region, and when to direct sow their seeds, gardeners often refer to the United States Department of Agriculture Plant Hardiness Zone Map.

Timing your seed planting can be a bit tricky when you direct sow. Besides scheduling around the average dates of their last and first frosts, the gardener who direct sows has to consult the weather as well. If spring is slow to arrive, and the weather stays frosty after it would normally have begun to warm, you should delay sowing—even if the calendar shows the correct number of weeks left before the last frost. Likewise, a rainy spell may leave the soil too soggy to be dug, forcing you to wait for the damp ground to drain.

AS SOON AS THE SOIL CAN BE WORKED

This phrase is commonly used to describe an early spring sowing. In very early spring, your garden soil may still be frozen and therefore impossible to dig or cultivate (to “work”). And right after it has thawed, the soil is likely to be so wet that any cultivation will compact it into muck. But once the soil has thawed and dried—if you squeeze a ball of it in your hand, it should crumble when you poke it with a fingertip—then it is ready to be worked and ready for early sowings.

AS SOON AS THE SOIL HAS WARMED

Some seeds won’t germinate unless the soil is warm; if planted into cold soils, they are likely to rot. Squash seeds, for example, need a soil temperature of approximately 65°, and their seed packets routinely advise delaying sowing “until the soil has warmed.” Other seeds that require warm soils include castor beans, sweet corn, cucumbers, and melons. A temperature of 60–65° is adequate for all of these. An inexpensive soil thermometer, sold by garden centers and greenhouse-supply companies, is the best tool for measuring soil temperature. For the most accurate reading, test in the morning when the soil is coolest, and push the thermometer probe two inches below the surface.

WHEN ALL DANGER OF FROST IS PAST

Because the last frost date (see Seed Packet Translator, below) is an average date, there is always some danger of frost in the days immediately following it. For this reason, the instructions on seed packets often recommend waiting to plant out particularly cold-sensitive varieties until “all danger of frost is past.” To ascertain that date, consult your Cooperative Extension office, local nurseries, or experienced gardeners in your area.

TEST YOUR SOIL & AMEND AS NEEDED

Bring a sample of your soil to the local Cooperative Extension office and have a complete soil test carried out by its laboratory technicians. Though such a test costs only a few dollars, its results furnish an exact prescription for the fertilizer, lime, organic matter, or other amendments your garden beds need to nurture healthy seed and plant growth. 

Next, drop the seeds along the bottom of the furrow, again observing the recommended spacing. Always sow the seeds more closely than you want the mature plants to stand.  The germination rate of direct-sown seeds is relatively low, so you’ll need the extra seedlings to avoid gaps in the row when the seedlings emerge.  Where seedlings are crowded, you can thin.

With a sifting of soil, cover seeds to the depth specified by seed packets. Or use an old rule-of-thumb: Cover a seed to a depth equal to three times its diameter. After sowing each packet of seed, write the plant name and sowing date with an indelible marker on two wood or metal labels, and place one at each end of the row before moving on to your next sowing.

SOWING IN HILLS

A few vining fruit and vegetable crops, such as melons and squashes, are usually planted in circles of seeds, called hills (PHOTO ABOVE). You’ll find the proper spacing between hills on the seed packet. After germination, remove all but the two sturdiest seedlings from each hill.

PROTECTING SOWN SEEDS

Water the bed with a fine spray of water from a hose or watering can, enough to thoroughly moisten the top couple of inches of soil. Then cover the row or block with a “floating row cover.” Made from light, porous translucent, plastic fabric, a row cover filters the sun and blocks the wind, keeping the soil underneath moist and slightly warmer than uncovered beds, and thus aiding germination. 

Row covers are good for both vegetables and flowers. They also keep birds and flying insects from feeding on seeds and seedlings.

Drape the row-cover material loosely over the planted area, leaving plenty of slack so that the emerging seedlings can push the fabric upward (that’s how the row cover “floats”). Then dig a trench a few inches deep around the perimeter of the planted area, tuck the edges of the row cover into the trench, and bury them with the loosened soil to anchor it. In the flower garden, a row cover should be taken off as soon as the seedlings have produced a couple of true leaves. In the vegetable garden, a cover may stay in place until the onset of hot weather, or until fruiting plants such as squashes start to flower, at which time the cover must be removed to let pollinators reach the blossoms.

THINNING SEEDLINGS 

If all goes well, most of the seeds you have sown will germinate. The result will be a row or block that is overcrowded. Thin by pulling out excess seedlings, so that the remainder stand at the intervals recommended on the seed packet. It is important to thin while the seedlings are still small, before they develop extensive root systems; that way, in pulling one you won’t disrupt its neighbors. I use thinnings from beets to flavor stocks and soups, and to add a delicate crunch to salads. I also add pulled seedlings on the compost heap or feeds them to my chickens.

USING STARTER PLANTS OR PLUGS

If you find that you’ve missed the appropriate seed planting window and still want to grow outdoors, starter plants, sometimes known as plugs, are an option. These are plants grown from seed or from cuttings in commercial greenhouses. They are more expensive than growing from seed, but less time is required before planting them outside. Purchase them close to your outdoor planting window and care for them the same as you would your seedlings grown from seed.

Article by Martha Stewart, as part of the Growing with Martha Stewart partnership.