Pruning shrubs and ornamental trees in the dormant season is ideal due to the fact that the tree is – well – dormant.
Pruning ornamental trees and shrubs during the winter helps encourage healthy new growth in the spring.
Shrubs pruned during the dormant season are less susceptible to disease and insects. Freshly cut pruning sores and bruises typically heal faster during the dormant season. Remember, things are dormant, but not dead. Just like your body heals when it's sleeping, the same is true for shrubs. In all likelihood, properly pruned, healthy shrubs will not get any disease or insects during the spring and summer either, but by pruning them during the dormant season, that risk is even smaller.
Dormant pruning allows you to see the structure of your tree or shrub better. Sometimes during the summer, if you're looking at a shrub, you may not see that a branch is crossing or rubbing.
Dormant pruning also allows you to get a better look at the overall health of the tree. You can get a better look at disease or insect damage.
“Pruning truly is an art”, said Ron Picco, horticulture specialist at The Morton Arboretum in Chicago. “It takes a few years to learn. Each time you prune, observe how the plant reacts”.
Start your pruning by removing any obviously diseased, damaged, or dead stems back to healthy wood. Use sharpened pruners to make a clean, angled cut just above the bud or collar – the slight flare where the stem meets another stem. You want to make a 45-degree cut. Take a look at this diagram from “Friends of the Urban Forest”. Make sure it's a clean cut because a jagged cut or flush cut (a cut that's flush with the branch) will invite disease and hinder healing. If a branch is too big for you to cut with a nice pair of bypass pruners, a hand saw is a great tool to have. If you're using a hand saw on larger branches, be sure to start from the bottom and saw in an upward direction or the weight of the branch could cause the branch to break and tear when you're almost through. Be sure to use ta 45-degree angle just as you would if you were using your bypass pruners.
Once you've cut out any dead and/or diseased wood, it's time to remove any rubbing or crossing branches. When branches rub against one another, it can cause a wound to the tree or shrub and is an invitation for insects or disease.
Once you've taken care of the business end of things, it's time to think about aesthetics. You'll want to shape your shrub or tree. Take a step back and look at the shrub you're pruning. Does it fit the space? If not, it's time to prune it back hard. This is called renewal pruning and is a good way to control the growth of a shrub. Don't prune too much as you don't want to shock the shrub. It still has its root system to support. Generally, a good rule of thumb is never more than 1/3. If you're objective is to control, rather than remove, an old overgrown shrub, consider renewal pruning. Renewal pruning helps old, overgrown shrubs create new growth from the base. Each year for 3 years, remove 1/3 of the stems to ground level. When I do this for clients, I randomly select 1/3 of the shrub throughout the growth and try to space it out so it's not obvious.
Winter is an excellent time to prune shrubs who depend on the sun for their color. Burgundy barberries and gold colored evergreens such as gold thread cypress are two such shrubs that get their color from the sun. Once you prune theses shrubs they will be green underneath and take a while to color up with the sun again. Now is a good time to prune those shrubs so that they will have their desired color over the summer.