Planting By Degrees
When To Plant Hardwood Trees, Avant Gardener, October 2014
Except where cold comes early and is accompanied by high winds, fall is the best season for planting trees and shrubs. A long autumn, plus fall rains spur rapid and abundant production of new roots because the plant does not need to put energy into making and maintaining new, above-ground growth. Most references say its safe to plant woody plants until the ground starts to freeze.
A more precise safe-date determination was made by researchers from Cornell University. Plantings were made on the 21st of each month from August through November. Evaluation of the plants during spring and summer of the following two-years showed that those planted as late as October 21 had only slightly more winter injury than earlier plantings, but those planted on November 21 suffered significant to considerable winter injury, with reduced size and quality, still evident two-years later. The November plantings were found to have developed virtually no new roots, due to the soil temperature a 6-inches depth having fallen below 40-degrees at the time of planting. Thus the last safe planting date the researchers concluded, is about 4-weeks before soil temperatures in the root zone falls to 40-degrees, at which time root formation is halted.
Maryland Soil Temperatures
The Beltsville Agricultural Research Center has a 24-hour/7-days per week soil temperature reader off of Powder Mill Road with 13-years worth of data. The soil temperature reader covers 2, 4, 8, 20 and 40-inches in depth. I looked at the mean monthly temperatures for 4 and 8-inches, during September, October, November and December and then extrapolated when we hit the 40-degree mark on average, and then backtrack 30-days. I had to pull out my iPhone metric conversion app to convert Celsius to Fahrenheit. I found that soil temperatures hit the 40-degree mark right around December 1. Backtracking 30-days for central Maryland, would give us a optimal planting window of September 1 through November 1, with the month of October being “prime time.”