For as long as we've grown trees, we've pruned them to shape them, to make them conform to the way we want them to grow. But removing a limb from a tree is as dangerous for the tree as getting a deep cut is dangerous for us -- and for the same reasons. Wherever there's a cut, an infection can occur.
To try to prevent infections in trees, people have tried all kinds of wound coverings.
Back in the 1700s, a gardener named William Forsyth came up with a concoction to spread over tree wounds. Not-so-creatively called Forsyth's composition, it was made of cow dung, river sand, lime rubbish and ashes. As you may have guessed, it was smelly and not particularly useful. It's no longer used to dress tree wounds, but other paints and tars still are.
As early as the 1900s, horticulturists started to question the value of covering wounds on trees. Liberty Hyde Bailey, the father of modern horticulture, wrote that covering pruning wounds hadn't proven effective, even though the practice was widespread.
Modern research has not only supported his position, but taken it a step farther. Covering the wounds with tar or paint doesn't protect a tree from infection. It may even slow down the normal healing process by preventing "woundwood" (the tissue that grows over a wound) from developing quickly.
We're still pruning trees, but we've learned more about how and when to do it and how to protect trees from infection. Here are the best pruning practices:
Sterilize your equipmentPruners can transmit diseases from one tree to another. So, before you make a cut, sterilize your pruners, loppers or shears. The easiest way to sterilize is to dip your pruners into a can of ethanol or isopropyl alcohol. Household disinfectants, such as Lysol, also work.
Time it rightThe best time to prune is when diseases aren't actively growing. In Minnesota, that means wintertime. You can prune anytime from December to early March, but the snow and cold can make pruning in the depths of winter challenging work. That's why November and March are considered prime pruning time.