Every year in northern winter there are periods of weather perfect for pruning and working outdoors. Called thaws, they are characterized by daytime temperatures above freezing and nighttime in the high 20'sF. Usually skies are cloudy. In our neck of the woods (southern Michigan) we bank on and usually have a that toward the end of January, another at the end of February and more thaws than winter in early March.
A thaw is perfect for pruning. In fresh cuts, plant cells that suddenly experience frigid weather or huge drops in temperature often end up as dead cells rather than sealed-over woundwood. On the other hand, plant cells at the surface of a fresh pruning cut that have at least 24 hours of thaw to acclimate to their suddenly exposed condition can usually harden off and handle anything that winter throws at them afterward. So it's ideal to prune on the first or second day of a three day thaw.
A thaw is coming now as we write in late February, and we have on our pruning priority list one oak, two Japanese maples, four yews, three spruces, one weeping cherry and one crabapple. (Sorry, Tonya, we warned you that your tree's turn might come in February! Really, it's only the getting out that's hard. Once you're out you'll be comfortable and even have fun pruning your crabapple with us!)
Four good reasons:
Some plants, oak topping the list, are susceptible to lethal diseases like oak wilt and can only safely be pruned when the insects that carry the spores of that disease are completely dormant.
Maples, elm, beech and birch are prodigious bleeders. Inflict pruning wounds on these in spring and sap will ooze or even gush. The oozing doesn't hurt the tree but it does alarm tree owners and in some cases it can attract early flying insects that are up to no good. So we prune before the sap rises. Pruning done in a January thaw is best for this category of plants but even cutting in February when sap's beginning to flow is better than waiting until March.
Many plants that we want to prune right before they begin growing, can also be pruned in a thaw. We choose to stay ahead of the game by doing what we can in winter, since the spring rush is hectic enough. So we cut back redtwig- and yellowtwig dogwoods, blue willow, Kerria, Potentilla, dwarf spirea and others that we want to stay full of young vigorous or colorful wood.
One warning: Don't prune when it's wet. Wet tools and wet wood provide great opportunities for fungal infections. This is particularly important if the plants you're cutting have any tendency to fungus diseases of the wood (such as dogwoods, cherry, juniper, Euonymus and spruce). This is a year-round problem.
Weather reporting is a wonder now. We can see thaws coming with more accuracy than ever. Our favorite weather site is www.wunderground.com, where the 10 day forecast looks like this, and one quick look at the temperature line tells you "Thaw".