High Soil Temperature Effects on Stands in Seeded Vegetables

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Many vegetables for fall harvest are direct seeded in July and August. This is also the time of year with the highest soil temperatures. This summer, temperatures were in the 90s and even 100°F for days on end and this past week the heat was back again.

High soil temperatures can cause problems with direct seeded vegetables, even where irrigation has been applied. Small seeded, shallow planted crops are at highest risk of uneven stands due to high soil temperatures because soils are hottest nearest the surface (try walking on dry sand at the beach in the middle of the day with bare feet – ouch!)

There are also large differences between vegetable species on ability to germinate in high soil temperatures. Spinach and lettuce are notorious for having poor germination in high temperature soils. Some large seeded vegetables such as snap beans can also be affected.

Stand problems due to high soil temperatures are most likely in conventionally tilled ground and very sandy soils with low organic matter. Delays in germination may occur and germinating seeds may be killed. Roots of shallow seedlings may also be heat damaged in high temperature soils.

To prevent losses due to high temperature soils, irrigate prior to planting to moisten the soil and then again soon after planting to keep the soil moist. The more moisture in the soil, the more energy (from the sun) is needed to raise soil temperatures. For larger seeds, consider planting deeper when soil temperatures are high.

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