Moist, weed-free soil retains more heat

Modifying the soil to capture and retain more heat is a way growers can reduce spring freeze injury. Weed-free soil retains more heat than freshly cultivated or unmowed sites and a few degrees may make a difference this spring.

Spring frosts are a worry for all fruit growers. Radiation frosts occur when clear, calm conditions during the night allow the ground to cool by radiation to the sky. The cool soil chills the air above it, lowering the air temperature. Many growers cultivate the soil in the spring to try and reduce the weeds and expose the soil to the sun. This practice should be done as early as practicable. Cultivating just before a freeze is a bad idea. Freshly cultivated soil is cooler than mowed sod.

Cover crops serve many valuable functions in fruit plantings such as reducing or preventing soil erosion, reducing soil compaction, and allowing vehicle traffic over wet soils. Cover crops also shade the soil resulting in cooler soils during radiation frosts. Keeping the soil surface clean of vegetation allows it to absorb more heat during the day. Soils have a large heat capacity, so they can capture and store considerable heat during sunny days. This heat can maintain warmer air temperatures during cold nights. Weeds and sod insulate the soil surface from the sun. In addition, tall, unmowed cover crops raise the effective ground level, so even higher flower buds may be injured where there is a tall stand of grass or weeds.

Also important is the fact that wet or moist soils have a higher heat capacity than dry soils, and packed soils are able to absorb more heat than recently cultivated soils. This means that clean, moist, and packed soil surfaces will absorb more radiant energy during the day, and protect from frost by releasing this heat during the night. In general, unmowed cover crops are cooler than mowed covers, which are cooler than loose cultivated soils. Packed bare soils are warmer than loose soils, and wet soils are the warmest of all.

Moist, packed soils can be as much as five degrees F warmer than unmowed cover crops during radiation frosts. Such high increases in temperature are not common, but I have seen noticeable differences in fruit set between orchards that were cultivated as opposed to those nearby where nothing was done to the cover crop. Cultivation is not for everyone, especially where the fruit planting is on uneven ground where soil erosion is a concern. Cultivation is more suited to flatter plantings where drainage of cold air out of the planting is not a major factor in the orchard.

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