Hilling-up, a practice of covering the graft union using soil, is one of the most effective methods of preventing winter damage. Soil hilled-up around the vine acts both as a thermal mass that holds heat, and as an insulator that slows heat loss during extreme cold temperature events. By using the soil available near the trunk base, growers can improve long-term survival of the vines. If the mound is big enough to cover 3 to 4 inches above the graft union, the temperature experienced at the graft union is typically close to subsurface soil temperatures. Studies show that temperatures under the hilled soil can be significantly warmer than air temperatures, protecting the graft union from damage 

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