Determine soil temperatures before planting vegetables for improved results

Make use of a vegetable’s seed germination requirements to get started in the garden.

Photo credit: Jasmine&Roses,
Photo credit: Jasmine&Roses,

Imagine ripe tomatoes, fresh spinach and greens, baby red potatoes, and fragrant cilantro. The harvest sounds wonderful, but getting there requires planning and identifying the needs of your vegetables while keeping an eye on natural conditions in the garden. Since the winter was extended, many Michigan locations are behind on their growing degree days. (Check out Michigan State University’s Enviro-Weather website to see how you can track your area’s growing degree days.) This impacts the soil temperature required for seeds to germinate and whether or not gardeners can begin to plant. The smart plants initiative informs gardeners to “not fight the site.” Identifying what your vegetable garden site’s conditions are throughout the growing season are very important observations for a smart gardener to make. The key to successful gardening is to grow specific vegetables that will grow best under current conditions.

Observations of a vegetable garden site include current soil temperatures, light conditions and moisture levels. Throughout the growing season, these will change and may even vary between locations within a site based on shading provided by trees, differing soil types and proximity to a building or shelter that creates a microclimate of its own. Currently, smart gardeners need to be aware of their garden’s soil temperature. Soil temperature thermometers are used to measure a soil’s temperature. Smart gardeners also plan to plant vegetables that are either cool or warm season and are best suited to current soil conditions.

Vegetables are categorized into either cool or warm season crops. Cool season crops, such as spinach, leaf lettuce, peas, onions and root crops, can germinate at minimum temperatures of 35-40 degrees. The optimum germination rate is 80 degrees, yet they do not prefer to grow in 80 degree temperatures. So, starting some of these crops indoors and heating soil to 80 degrees will speed germination, allow you to transplant outside once temperatures are above 40 degrees and will provide an earlier harvest. If you’d like to direct seed the cool crops outside, wait until your soil temperature is at least 40 degrees and follow seeding directions as to depth and spacing. Plant root crops directly in the ground as they do not transplant well. Check out Colorado State University Extension’s Vegetable Planting Guide for identification of cool season and warm season vegetables and their germination and specific planting requirements.

Please read “Smart vegetable gardening with season extenders” if you’d like to learn ways to increase soil temperature or use structures that help protect vegetables during this early part of the growing season.

For more information on a wide variety of Smart Gardening topics, visit the Gardening in Michigan website at or contact MSU’s toll-free garden hotline at 1-888-678-3464.

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