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 equipment

Pruners 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 right

The 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.

Minimize wounds

Make the cut as small as possible. By placing your pruners as close to the stem as possible and making a flush cut, you'll inflict more damage to the tree. Instead, place your pruners farther down the stem.

Where a branch intersects the stem, there is a slight but noticeable increase in the size of the branch, called the branch collar. Pruning cuts should be made on the other side of the branch collar.

Don't overdo it

By limiting the cuts you make, you'll reduce the threat of infection. So be sure to cut only branches that need to be removed. Those include:

• Branches that are broken or diseased.

• Branches that are rubbing against each other. Rubbing can cause damage to the tree's bark, creating an entry point for diseases.

• Branches that are attached to the trunk of the tree, or to another branch, by a very narrow angle. Because these branches are weakly attached, they have a greater chance of breaking.

Jeff Gillman, an associate professor of horticulture at the University of Minnesota, is the author of several gardening books.