Vegetable seed germination is the process of transformation of a small, wrinkled vegetable seed that begins to absorb water. With the activation of plant enzymes, the plant embryo grows and breaks through the seed coat. Roots emerge and anchor the seed, then the seedling plant unfolds. In vegetable seeds, the proper temperature of the soil and the moisture level are key elements necessary to successfully germinate vegetable seeds. If seeds are not planted at the proper soil germination temperature the seeds may rot or be susceptible to slow germination, diseases and growth problems.
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The Colorado State University Extension experts suggest that the soil should be measured daily at 8 a.m., at a depth of 4 inches, using a soil thermometer for vegetables. If measuring for beans, however, the soil should be tested at a depth of 6 inches. In addition, for vegetables that will be transplanted, measure at a depth between 4 and 6 inches.
After inserting the thermometer to the proper depth, allow the thermometer sufficient time to stabilise the reading. This will be a minimum of three to five minutes. Check the soil temperature daily until the desired temperature reading has been met for at least three consecutive mornings.
It is important to take soil temperatures near or ideally where you will be planting your vegetables. If you are unable to take the soil temperatures in the primary planting location, choose a similar site in respect to soil cover, sun exposure levels and soil moisture including soil texture and drainage aspects.
Cool weather crops can be hardy or semi-hardy. Hardy vegetables will likely survive a frost whereas semi-hardy vegetables are less tolerant of frost. According to the Arizona Cooperative Extension, the accepted minimum germination soil temperature for semi-hardy cool weather crops is 4.44 degrees Celsius. The accepted minimum germination temperature for hardy crops is 1.67C. Hardy vegetables include broccoli, cabbage, kohlrabi, onions, lettuce, peas, radishes, spinach and turnips. Semi-hardy cool weather vegetables include beets, spinach, carrots, cauliflower, parsley, parsnips, potatoes and Swiss chard.
Warm weather crops do not tolerate frost or cooler weather, and the minimum soil germination temperature required is between 12.8 and 15.6 degrees Celsius. Warm weather crops include beans, celery, corn, cucumbers, New Zealand spinach, summer squash, lima beans, cantaloupe, eggplant, pepper, pumpkin, winter squash, pumpkin, tomato and watermelon.
Improving Soil Temperature Conditions
Carol Savonen from Oregon State Extension suggests that, to help ensure a successful germination of warm weather crops, the soil can be warmed by using a cloche, plastic mulch, floating row covers or a cold frame. Another options include waiting until the soil reaches the proper temperature for the vegetables you are trying to grow or planting short season varieties.
Likewise, germination may be inhibited if cool weather crops are exposed to too warm temperatures. Therefore, cooling the soil may be desirable. Horticultural specialist Erv Evans from NC State University suggests to cover the area with burlap, newspapers or boards or to open a furrow, put in the seeds and cover the seeds with potting soil or vermiculite.
Soil thermometers often are dual thermometers and may have both Fahrenheit and Celsius readings. Also, soil thermometers can be digital or manual read types and can be purchased individually or as part of a soil testing kit.
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- Colorado State University Extension: Vegetable Planting Guide: 2006
- The University of Arizona Cooperative Extension: Vegetable Garden: Seed for the Garden: 1998
- Oregon State University Extension: Garden Hints: Carol Savonen
- NC State University: Growing a Fall Vegetable Garden: Erv Evans
- University of Idaho: The Gardening Climate of Bannock County