The Effects of Soil Temperature in Gardening
Prepared by Richard J. Brzozowski, Extension Educator, University of Maine Cooperative Extension

The long cold winter will probably influence the garden and other plants in the yard for weeks to come. The frost seems to have been extraordinarily deep in some areas. Cold soil temperatures influence seed germination, seed vigor, seedling growth, and plant growth.

Soil Temperature and Seed Germination

The seed is a wonderfully designed package of energy. However in order for germination to take place, conditions must be right. Moisture, temperature and even light or lack of it may influence seed germination. If one plants seeds in cold soils:

1. The seed may not germinate at all. You run the risk of seed rot through excessive moisture or seed death via disease agents in the soil. Some soil borne disease agents are active in the soil even at cold temperatures.
2. The seed that does germinate may be weak, lacking vigor for good growth and development.

Soil Temperature Ranges for Vegetable Seed Germination in Degrees Fahrenheit*
Bean	59 to 95	86 best
Beets	48 to >95	95 best
Broccoli	46 to 86	77 best
Cabbage	50 to 95	86 best
Carrots	41 to >95	77 best
Cucumber	63 to 95	86 best
Lettuce	40 to 84	75 best
Muskmelon	68 to >95	90 best
Onion	45 to 95	77-86 best
Peas	40 to 86	77 best
Peppers	68 to 95	86 best
Radish	50 to >95	83 best
Spinach	48 to 86	72 best
Sweet Corn	55 to >95	86 best
Tomato	59 to 90	86 best
Winter Squash	61 to >95	90 best
*Source: Johnny’s Selected Seeds 2008 Catalog

Note: Soil temperature influences annual flower seed germination too. Cool season flowers such as calendula and alyssum can be planted in cool soils while warm season flowers such as marigold and zinnia should be planted in warm soils.

Phosphorous Availability

Phosphorus (P) is less available to plants when the soil is cold. Even though the soil test results may show adequate P, the element is not available to the plants in the early spring when the soil is cold. It is only when biological processes “kick in” does P become available to plants. Phosphorus promotes rooting and fruiting of plants. As a gardener you can use a “starter solution” -- a special fertilizer with a relatively high amount of phosphorus. Phosphorus is the middle number of the analysis on a fertilizer bag N-P-K. For example: a bag of fertilizer with a 15-30-15 analysis means that 15% of the bag is Nitrogen, 30% is Phosphorus and 15% is Potassium.

What is a Gardener to do? 

The following options might be considered to solve or avoid a low soil temperature problem.

• Monitor the soil temperature with a soil thermometer within the zone of seed placement. Keep records of your data and use the information for future plantings. Note where the soil warms more quickly in your garden.
• Sow seed on a layer of fine horticultural grade vermiculite in your garden. Cover the seed and vermiculite with a suitable layer garden soil at the proper depth. The vermiculite is warmer, well drained and makes for suitable environment for seed germination. 
• Build a temporary structure such as a hoop house or high tunnels over a specific area of your garden. These structures can be fabricated with plastic pipe, wood and a clear plastic covering. The soil inside the hoop house will warm more quickly than that area outside.
• Use raise beds for some of your garden space. The soil in raised beds drain more quickly and typically warms up sooner than soil at the natural ground level.
• Lay clear plastic on the ground to warm the soil in certain parts of your garden a few weeks before planting. Remove the plastic after germination. Be aware that the environment under the plastic is also good for weed seed germination.
• Use a starter solution fertilizer at the recommended rate on seedlings at planting to promote root growth
• Some seeds can be started indoors, where you can more easily control the temperature.
• In some cases, you can select seeds that germinate at cooler temperatures. Supersweet corn, for example, requires warmer soils for germination than standard yellow corn.
• If your soil has a high percentage of clay and therefore is poorly drained, you can work toward better soil structure by adding organic matter every year. This organic matter will, over time, increase the soil particle's aggregation, which will improve drainage, which will produce soils that warm more quickly in spring.
• Be patient -- the soil will warm up over time.