Starting seeds takes warm soil temperatures

Mar. 25, 2013 @ 11:10 AM

MONROE - Last Wednesday, March 20 marked the first day of spring and as the daytime temperatures  warm up, it is tempting to start planting our gardens. Don't be fooled by the warm sunshine! Frost is still a possibility. The average last frost date in Union County is April 12th. Whether we are planting seeds or transplants, we need to consider not just about the air temperature, but the soil temperature as well when starting our gardens indoors and out.

For every type of seed, there is an optimal soil temperature for germination. At that temperature, the maximum number of seeds will germinate, and in less time, than at any other temperature. Generally, the soil temperature is 10-15 degrees colder than the air temperature. For example, if the air temperature the last week was 65-70 degrees, the soil temp would be about 50 degrees. After two weeks of temperatures in the upper 60's, the soil temperature will be closer to 55 degrees. Moist soils are cooler than dry soils. To measure soil temperature, you can use a soil thermometer or a cooking thermometer that you have dedicated for gardening purposes only. Insert the thermometer about six inches into the garden soil. Different areas of the garden may have different soil temperatures. Raised beds warm up quicker than in-ground beds. Soil within 3 or 4 feet of south facing buildings, fences, garden walls or other garden features that can absorb heat will be warmer than soil further away from these structures. 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, grasses or perennial plants show in the spring.

Sowing seeds outdoors when the soil temperature is optimal is not always the right strategy. To get the earliest fruit or flower, we may want to start seed indoors where we can control the growing conditions or buy transplants when the soil conditions are right. A plant set in the soil when the soil conditions are right will catch up to, and in some cases, surpass a plant that was set in the garden weeks earlier and has been waiting for warmth. If you are starting your seeds indoors, avoid the common mistake of sowing the seeds too early and holding the seedlings under poor environmental conditions (light and temperature). This usually results in tall, weak, spindly plants that will not perform as well in the garden as a quality transplant purchased from a local greenhouse or nursery.

If you are sowing seed directly in the garden, the back of the seed packet will provide instructions on when to sow the seed. Instructions such as "plant as soon as the ground can be worked", "plant after all danger of frost is past" or "plant in early spring" means your garden soil needs to be dry enough for cultivation and warm enough for seed germination. If the soil is too wet soil, it can pack into hard lumps that are difficult to break up later in the year. To determine if your soil is workable, dig down a few inches in your garden, grab a handful of soil, squeeze it into a ball, then poke the soil ball with your fingers. If the ball breaks apart readily, the soil is dry enough to cultivate. If the soil stays in a ball and doesn’t give to pressure easily, wait for several warm, dry days and try the test again. Our Piedmont clay soils typically take longer to dry out than other soils.

 

 

Plant

Approximate Time to Seed Before Last Frost Date (Weeks)

Time Seeds Take to Germinate (Days)

Optimum Temperature (Degrees Fahrenheit)

Begonia

12 or more

10 to 15

70

Broccoli

8

5 to 10

70

Cabbage

8

5 to 10

70

Cauliflower

8

5 to 10

70

Celosia

8

5 to 10

70

Coleus

8

5 to 10

65

Cosmos

4 or less

5 to 10

70

Cucumber

4 or less

5 to 10

85

Dahlia

8

5 to 10

70

Dianthus

10

5 to 10

70

Eggplant

8

5 to 10

70

Geranium

12 or more

10 to 20

70

Impatiens

10

15 to 20

70

Larkspur

12 or more

5 to 10

55

Lettuce

8

5 to 10

70

Marigold

6

5 to 10

70

Muskmelon

4 or less

5 to 10

85

Pansy (Viola)

12 or more

5 to 10

65

Pepper

8

5 to 10

80

Petunia

10

5 to 10

70

Phlox

8

5 to 10

65

Snapdragon

10

5 to 10

65

Squash

4 or less

5 to 10

85

Tomato

6

5 to 10

80

Verbena

10

15 to 20

65

Watermelon

4 or less

5 to 10

85

Zinnia

6

5 to 10

70

 

 

Mary Roberts and Ray Tarlton are owners and managers of  Windcrest Farm, a USDA Certified Organic farm and greenhouse in Monroe, NC . Visit www.WindcrestOrganics.com for more information about plants, produce and classes.