Conifers, evergreens require little pruning
Conifers trees or evergreens usually require little pruning because each tree needs to be pruned according to their varied growth habits. One interesting characteristic of spruce, pine, fir, yews, arborvitae, junipers and hemlock is that these are self-pruning trees and shrubs. This means that if a branch does not get sunlight and it is not able to carry on photosynthesis, (the ability to absorb sunlight for the manufacture of plant sugar for maintenance, energy and growth of the plant) the tree or shrub will cut off the supply of energy to that branch and it will die. If you have wondered why a group of evergreens has all those dead branches in the middle, or an arborvitae or juniper has all those dead branches in the inner part of the tree, it is because these branch do not receive any sun and the tree has pruned them off. Likewise it is why a single evergreen standing alone, will have a nice cone shape with wonderful green branches.
Spruce, and firs do not grow continuously and are best pruned (if necessary) in late winter before the new growth begins. At this time of the year, the ends of the branches have lateral (side) buds that will sprout if the terminal (tip) bud is removed. Pruning the terminal buds will result in a denser branching of the tree. If a branch is just cut off part way back, an ugly stub will develop which will eventually die.
Pines put on only a single flush of tip growth each spring and then stop growing. Pines do not have lateral buds, so removing the terminal buds will take away any new growing points for that branch. If you need to prune the terminal buds, the best time is usually about mid to late May (depending on the year). The candles should be stiff and you should see the new growth of needles starting to expand outward. Cut the candle back two-thirds of the way, and when the needles expand all the way out, you will not see your pruning cut at all. This results in a very neat prune.
Arborvitae, junipers, yews and hemlock grow continuously throughout the growing season. They can be pruned any time through the middle of the summer, but it is probably best to be done by July 1. This will give the shrub time to heal and recover from the pruning and give a finished look. These plants will tolerate heavy shearing, however if you prune back into the brown or dead branches in the middle of the tree or shrub. It will not recover and turn green. As already mentioned this type of shrub or tree is self pruning and these branches will forever be brown.
As spring approaches trees and shrubs feel the temperature differences just as we do, according to Jeff Gillman, professor of horticulture at the University of Minnesota. Trees aren't ready to start enjoying the warmth right away. They need a certain amount of cold weather before they can spread their leaves. Although trees may look dormant in winter, certain important chemical reactions are taking place inside them. When the temperature is between 33 and 45 degrees, certain chemicals are produced in most trees. The longer the tree spends between these temperatures the more of these chemicals are produced. Only when the chemicals reach the correct level is the tree ready to respond to the warm air rushing through its branches.
People who grow fruit refer to the time a tree spends in this narrow temperature range as "chilling hours." They select trees to plant based on their requirement for chilling hours. A tree in Minnesota might require 1,200 chilling hours, while a tree in Florida might require 150 hours. It is why we select zone 3 plants in this area. Likewise our hardy zone 3 shrubs and trees if planted in Florida, do not fare so well.
To avoid late frosts, some trees also use what is called warm days. Even after they have met their chilling requirements, trees such as oaks wait until we have had plenty of warm days before they open their buds. Thus oaks almost always avoid late frost because they are not good at producing new buds for the season. Other trees, such as maples, break bud soon after their chilling requirements are satisfied. If maples lose leaves to a late frost, they will produce more.
Early blooming shrubs such as forsythia, and lilac may lose their flowers to frost, but the leaves of these shrubs are not injured. However, leaves while not as lovely as the flowers, are necessary for the plant to produce the sugars it will need to maintain itself.
Kyle Schulz is a Wadena County Master Gardener from Sebeka, and the regular gardening columnist for the Wadena Pioneer Journal.