Get Those Seeds Sprouting

When should you plant different vegetables from seed? That depends on a lot of factors.

What time of year is it? What’s the soil temperature? What’s the weather forecast? Do you have time to devote to the garden? All of these are important considerations.

A few weeks ago my column included spring planting dates recommended by Texas A&M University. These dates are based on average weather conditions for this area of Texas. But the weather can change from year to year. We might get a late frost. Or we might waste a few valuable weeks of growing time waiting for a late frost that never comes.

Seeds don’t really care what time of year it is when they decide to germinate. For the most part, they germinate when the soil reaches an optimal temperature for their species. Seeds need moisture to germinate, of course.

Seed germination of some species depends on other factors besides temperature. For example, some large seeds with a hard coat such as okra and certain beans germinate better if you scratch or nick the seed coat. This is called “scarification,” and it mimics weathering that occurs in nature. This can also be accomplished by soaking in warm water.

Other seeds, including those of many common herbs, need exposure to cold temperatures for a certain amount of time before they germinate. In nature, these seeds are dormant when they fall from the plant. They will not sprout until they experience a period of cold temperature. This characteristic prevents the seeds from sprouting in the winter, when cold temperatures will kill them. We can mimic this natural process by placing the seeds in a freezer for the required amount of time. This is called “cold stratification.” This mostly applies to herbs and certain ornamental plants. Most vegetable seeds do not require cold stratification.

For the vegetable gardener, soil temperature is the most important factor when it comes to seed germination.

The two charts printed with this week’s column (One of C1, other at right) show soil temperature ranges for vegetable seed germination and the number of days for seeds to emerge at different soil temperatures. The data comes from the University of California Division of Agriculture and Natural Resources.

When you purchase seeds, the package will often list the number of days to germination. This number is based on ideal soil temperatures. Seeds don’t simply wait until the number of days printed on the package before they germinate. Germination is a process. It takes time. Typically, seeds germinate faster with warm soil temperatures and more slowly under cold soil temperatures. If it’s too hot or too cold, they may never germinate.

The chart at right illustrates the point. Tomato seeds will germinated at temperatures down to 50ºF, but it will take them more than 40 days to emerge from the soil. At 7786º, however, they can germinate in less than six days.

Number of Days for Vegetable Seeds to Emerge at Different Temperatures

Crops 32ºF 41ºF 50ºF 59ºF 68ºF 77ºF 86ºF 95ºF 104ºF

Asparagus 0.0 0.0 52.8 24.0 14.6 10.3 11.5 19.3 28.4 Beans, Lima --- --- 0.0 30.5 17.6 6.5 6.7 0.0 -Beans, Snap 0.0 0.0 0.0 16.1 11.4 8.1 6.4 6.2 0.0 Beets --- 42.0 16.7 9.7 6.2 5.0 4.5 4.6 -Cabbage --- --- 14.6 8.7 5.8 4.5 3.5 --- -Carrots 0.0 50.6 17.3 10.1 6.9 6.2 6.0 8.6 0.0 Cauliflower --- --- 19.5 9.9 6.2 5.2 4.7 --- -Celery 0.0 41.0 16.0 12.0 7.0 0.0 0.0 0.0 -Corn, Sweet 0.0 0.0 21.6 12.4 6.9 4.0 3.7 3.4 0.0 Cucumbers 0.0 0.0 0.0 13.0 6.2 4.0 3.1 3.0 -Eggplant 0.0 --- --- --- 13.1 8.1 5.3 --- -Lettuce 49.0 14.9 7.0 3.9 2.6 2.2 2.6 0.0 0.0 Muskmelons --- --- --- --- 8.4 4.0 3.1 --- -Okra 0.0 0.0 0.0 27.2 17.4 12.5 6.8 6.4 6.7 Onions 135.8 30.6 13.4 7.1 4.6 3.6 3.9 12.5 0.0 Parsley --- --- 29.0 17.0 14.0 13.0 12.3 --- -Parsnips 171.7 56.7 26.6 19.3 13.6 14.9 31.6 0.0 0.0 Peas --- 36.0 13.5 9.4 7.5 6.2 5.9 --- -Peppers 0.0 0.0 0.0 25.0 12.5 8.4 7.6 8.8 0.0 Radishes 0.0 29.0 11.2 6.3 4.2 3.5 3.0 --- -Spinach 62.6 22.5 11.7 6.9 5.7 5.1 6.4 0.0 0.0 Tomatoes 0.0 0.0 42.9 13.6 8.2 5.9 5.9 9.2 0.0 Turnips 0.0 0.0 5.2 3.0 1.9 1.4 1.1 1.2 2.5

Notes: 0.00 means little to no germination. Entries marked with “---” were not tested at that temperature.