Bare ground is warmer
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
Spring frosts are a worry for 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. 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. Cover crops insulate the soil surface from the sun reducing heat storage. Taller unmowed cover crops also raise the effective ground level, increasing the height of the cold air layer so higher flower buds may be injured where there is a tall stand of grass or weeds.
Growers can use cultivation to increase soil temperatures during radiation frosts. Cover crops serve many valuable functions in fruit plantings such as reducing or preventing soil erosion and reducing soil compaction from 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.
Cultivation should be done as early as possible to allow the cultivated soils to settle. The condition of the soil is important in determining the amount of heat it is able to store during the day and release at night. Wet or moist soils have a higher heat capacity than dry soils. 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 warmer than unmowed cover crops during radiation frosts. It is unlikely that such high increases in temperature are 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 or cold air out of the planting is not a major factor in the orchard.