Some landscaping sources, like the Warnell School of Forest Resources of the University of Georgia, say the best time of year to plant trees or shrubs is in the fall. In general, though, trees or shrubs can be planted anytime of the year as long as the soil is not frozen. But how do you know when the soil is frozen?
Too late range:
November to January depending on where you live
Most of the continental U.S. experiences frozen soil during the winter. The National Snow and Ice Data Center (NSIDA) provides a chart of the U.S. depicting seasonally frozen ground. The NSIDA says that we can expect different rates of ground freeze because sandy soil freezes easier than clay soil.
I overlaid the NSIDA map with the USDA plant hardiness zone map. The U.S. Department of Agriculture map divides the U.S. into planting zones based on average annual minimum temperatures. By finding your USDA zone, you can get a close estimate of how deep the soil freezes. The approximate freeze depth for affected USDA planting zones are: zones 6 to 8, soil freezes down to 6 inches; zones 4 to 5, soil freezes as much as 36 inches below ground level; and in USDA zones 1 to 3, soil freezes to a depth as much as 72 inches. It takes several repeated days (and in some locations, weeks) of below freezing daytime air temperature to cause the moisture in the soil to freeze to the indicated depths.
Too early range:
March to May depending on where you live
In late winter to early spring when nighttime lows remain above freezing, the moisture content of soil begins to thaw. To find the when the last expected freeze is in your area by zip code, check Dave's Garden. It takes several repeated days (and in some locations, weeks) of above freezing nighttime air temperature to cause the moisture in the soil to melt.
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