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What seeds to sow when the soil temperature is above ...
Planted July 27, 2001
Last tended to on October 16, 2024
Reading time: 4 minutes
Why should you care about the soil temperature?
The temperature of your garden soil is one of the primary mechanisms that initiate germination of seeds, as well as to directly affect the growth of your plants. Soil temperatures affect the health of microbes in the soil, the rate of nitrification[7], which in conjunction with soil moisture and aeration, promotes healthy plant growth.
Since the food crops that we commonly grow in our gardens originated from all corners of the globe, they have varying requirements that we must observe. For example, since watermelons originated in the warm temperatures of Africa, we can’t pop seeds in 40ºF soil and expect them to grow. The following table can be used to better understand when to so various species.
Soil Temperatures Associated with Germination and Growth
| Minimum Soil Temperature | Type of Seed / Effects on Growth |
|---|---|
| 40ºF | Arugula, Broad or Fava beans, Endive, Kale, Lettuce, Parsnips, Garden or English Peas, Radish |
| 50ºF | Chinese Cabbage, Cress, Kohl Rabi, Leeks, Onion, Rutabaga (Swedes), Swiss Chard, Turnips |
| 60ºF | Bush Beans[1], Kidney Beans[1], Pole Beans[1], Runner Beans[1], Beets, Brussels Sprouts, Cabbage, Carrots, Cauliflower, Celery, Parsley, Spinach |
| 70ºF | Asparagus[2], Tomato[3], Eggplant[3], Pepper[3], Cucumber, Corn (Maize)[4], Muskmelon, New Zealand Spinach[5], Okra, Pumpkin, Rhubarb[6], Sorghum, Summer Squash, Winter Squash, Watermelon. |
| 65ºF to 86º | The optimal temperature range for nitrification and 100% of soil moisture is available for plant growth. |
| 90ºF | Plant growth slows. |
| 100ºF | 15% moisture used for growth, 85% moisture lost through evaporation and transpiration. |
| 113ºF | Some bacteria species start dying. |
| 130ºF | 100% moisture is lost through evaporation or transpiration. |
| 140ºF | Soil bacteria dies. |
How to measure your soil’s temperature.
You cannot rely on looking at your local weather report, or even the thermometer you have hanging on the side of your house. Air temperature obviously has an affect of the temperature of the soil, but so does your garden’s slope, soil composition, exposure to sunshine and airflow, etc. The only real way of knowing its temperature in a specific spot is to measure it! Soil thermometers are relatively inexpensive, a wise investment, and easy to use. They will come with instructions that you should follow, but in general terms, measure the temperature between noon and 2:00 p.m., at a seed sowing depth. As the season progresses, measuring at the same time of day, but at about a depth of four inches, will help you monitor plant health. If you also bought a moisture tester, you can use both bits of data to adjust your watering and mulching.
Here is an example of an analog soil thermometer available for under $20. Click on the image for more information. If you prefer a digital-type device, here is a link to an inexpensive model by Luster Leaf.
Notes:
- Beans will not tolerate frost so don’t take risks, or plan on replanting if frost occurs.
- Start Asparagus seeds indoors or in a greenhouse 60 to 90 days prior to your last expected frost date. Direct seeding into the garden is not recommended.
- Tomatoes, peppers, tobacco, and eggplants need to be sown indoors 6 to 8 weeks prior to your last expected spring frost date.
- Soil must be at least 65ºF for corn seeds to germinate. Be patient or you will waste a lot of seed! Click here for more information.
- New Zealand Spinach should be direct sown after all danger of frost has passed or start transplants indoors 3 to 4 weeks prior to your last frost date.
- Sow Rhubarb seeds ¼-inch deep, indoors, six to eight weeks prior to last frost.
- Nitrification is a microbial process by which reduced nitrogen compounds (primarily ammonia) are sequentially oxidized to nitrite and nitrate.
Tips:
- For more information about germination temperature, check out the article entitled, “Days Required for Vegetable Seedling Emergence at Various Soil Temperatures”
- A basic understanding of biological science is critical to a successful partnership with nature and to produce non-native plant species in our gardens.
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