The Complete Guide to Landscape Design, Renovation, and Maintenance
by Cass Turnbull

HOW TO PRUNE A TREE LIMB
Thank God for Dr. Shigo! He is the world-renowned tree expert who has scientifically proven what many gardeners suspected all along. He has shown that what a lot of other experts have been doing is wrong. Dr. Alex Shigo has degrees in biology, and a Ph.D. in plant pathology from West Virginia University. Between 1959 and 1985 he was employed by the U.S. Forest Service as Chief Scientist and Project Leader of a pioneering project on discoloration and decay in forest trees. He has dissected over 15,000 trees with a chainsaw. He has studied trees in many countries. His research has yielded 270 publications and he has received many honors and awards. Shigo is Who's Who in the world of arboriculture. He is famous for his amazing energy, inquisitiveness, and for his fearlessness in challenging old truths. His respect for trees and the people who actually work with trees is inspirational. He has shown why topping (also called heading, stubbing, or dehorning) is very bad.

Dr. Shigo Debunks Myths And Dispels Misconceptions
Shigo has proved that the common practice of flush-cutting limbs off trees is bad, and that tree wound paints and seals don't do any good whatsoever (except perhaps on rose canes, or with some special sprays that may aid the reduction of suckers on some trees). He and his cohorts have done this by scientifically testing these products and practices and their effects on trees. He cuts and drills and wounds and saws trees by the scores and then, later, dissects them to see what's really happening. He refers to himself as an "inside man" because he likes to look inside the trees. (I call myself an "inside woman" because I like nothing better than to crawl inside a tangled mass of overgrown shrubbery and prune it back to good order.)

Tree roots extend well beyond the drip line and are found mostly in the top 18 inches of soil.

Dr. Shigo's findings are very bad news for many professional pruners who learned to do it wrong at their daddy's knees. Experts have been passing on misinformation for decades, sometimes centuries.

Dr. Shigo has also pointed out the damage done by the commonly re-printed picture of a tree's root system that looks like a giant underground carrot, or inverse tree. It's misrepresentative of the group of plants we call trees, which tend to have a flatter, broader root system.

This misconception about tree roots becomes more germane when the bulldozer starts pushing dirt near them, when trenches are dug, cutting off one-third of the roots, or when herbicides put on the lawn or driveway wind up killing the tree.

Dr. Shigo also can tell you exactly why digging and carving out that old rotten hollow in your tree back into nice live wood is precisely not the thing to do, and in fact, does great and brand new harm to your tree. What some "professionals" do to trees borders on the criminal.

Although Dr. Shigo is highly respected and famous and his evidence is practically indisputable, people often refuse to change. Those who do change, tend to do so very, very slowly. Even new books on pruning, written by authors who know of Shigo's work, often repeat misinformation, although with a little less certainty.

I intend to give you the straight skinny here, somewhat simplified, but I trust not oversimplified. If you couldn't give a hoot about tree biology or the whys of the matter, you may wish to skip to the section on "How to Prune a Tree Limb."

Some Tree Biology
The world can be roughly divided into objects, plants, and animals. Animals often avoid injury and death by moving away from the wound source. Pain, caused by, say, fire, is the signal to move away. Trees cannot move rapidly away from the source of the injury. They wall it off internally and then outgrow it. If a limb gets injured, it will get rotted out by microbes, and ultimately it dies. The reason the injury doesn't usually kill the tree is that the tree sets up an interior barrier or wall around the rot, which stops it. That's why that pocket in your tree where a tree limb rotted out doesn't get bigger, it just makes a cute home for a squirrel. When some of that rotten branch gets trapped inside the growing tree, it is totally stopped and surrounded by a wall. It's the knothole that drops out of a piece of lumber. It's easiest for a tree to wall off a dead or dying limb. It's somewhat harder to wall off the gash you left when you backed into the trunk with your car last Christmas party. It's hardest for it to wall off all the rot that comes charging down the trunk when you top it. It's sort of the difference between bruising your knee, cutting off your hand, and cutting off your head.

Click here for a graphic view of some tree limbing wisdom.

Whether your particular tree dies back totally or partially when wounded depends a lot on how well it walls off wounds generally. This varies according to its genetic make-up or species, and even among members of the same species there is genetic variation. There are several types (species) of trees that do not compartmentalize well; in the trade they're called "rotters." They include bigleaf maples, alders, willows, poplars, tulip poplars, elms, and madronas.

Rotters often make up for their lack of toughness and short life spans by reproducing like crazy. You want to help these plants by prompt removal of injured limbs and by avoiding wounding or cutting (pruning) healthy green wood. Cut out the dead wood and little else. Leaving dead wood rotting on a tree acts, as Dr. Shigo says, as a "big stick of sugar" drawing in the rotting bugs. Besides, it's UGLY.

Dr. Shigo tells us that branch wood is different from trunk wood (see illustration). When you go to remove a limb for whatever reason you should be careful to cut off only the branch wood and to avoid cutting or wounding the trunk wood, which would doubly injure the tree. You will know where to make your cut on many trees because you will see a bulge or collar of trunk wood at the base of the branch.

The right place to cut is almost like a dotted line. It's also the most logical, the easiest place to cut; lazy gardeners have always cut there, and also the ones to whom natural things look right. The chainsaw enabled a lot of damage to occur with little effort. For literally decades arborists have been doing things wrong. They have recommended "flush-cutting" that wounds well into the collar, which is trunk wood, not branch wood, thus causing a second and worse type of injury. How did this happen?

Great Mistakes in Tree History
Dr. Shigo and his associates can trace the way the mistake has been passed on from generation to generation back to an old arborist's convention. Probably Dr. Shigo even knows the arborists' names! They were sitting around talking shop talk and they all had noticed that the branch collar or callous "rolls over" the cut area and covers it up faster when it's sliced into than when you cut to just outside it. They declared that this was the wound healing and just assumed that the wound healed faster with a flush cut. Actually, trees are not healing with the "callous roll" as it is called. The real work is going on inside where things are being walled off. Cutting the collar opens the trunk to rot and is responsible for many serious problems that show up years later.

If you can't see where the collar is on a tree, you may wish to refer to a somewhat geometrical method of determining the dotted line in the accompanying illustration.

Inside a tree. Remember, the most living, most vulnerable part of a tree is the cambium, just under the bark.	Don't wound or girdle trees with string trimmers, lawn mowers, or tree stake ties. Doing so can and will shorten the lives of even old trees.

More Oversimplified Tree Biology
Let's divide a tree up into four sections. The outside section is the protective bark. Just under the bark is where all the action takes place; it's made up of the cambium, the phloem, and the xylem. The cambium is where all the growth happens. It pushes out the cells that turn into everything else, including the phloem and the xylem. These cells conduct nutrients and sugars up and down the plant. Phloem and xylem are both great scrabble words. They are also the major plumbing area. Just inside this high activity area is the sapwood, which is just the major regular old wood part of the tree. It's actually old xylem. It conducts nutrients up the plant, too, but less energetically. Very old, old xylem at the central core of the tree is called heartwood and is often darker in color. Scientific types argue about whether it is alive or dead.

When you nick the bark to see if a branch is alive or dead, you're checking out the green cambium. When you saw down a big tree and paint the edges with a deadly chemical to keep it killed, you are painting the cambium. Cambium is important stuff.

It is essential that people understand that the most living, most vulnerable part of a tree is like a sheath just under the bark. When you strip the bark and cambium all the way around the base of your tree, in the fashion of a ring, you have girdled the tree. It cannot wall off the wound or send nutrients up to the part above the ring. It will die down to the stripped section, or perhaps it will die altogether. If it's a good compartmentalizer, it will wall off just below and send up new shoots from the roots or trunk just below. Don't girdle your young tree by scraping a lawn mower around the base, or by weedeating that soft young bark and cambium off from it. You will get lots of suckers at the base where you wounded it, and you'll risk its total death when someday you finally strip it all the way around. You can girdle the top of your tree by leaving something tied around it; an old tree stake tie is the favored thing. Once we managed to top five little Douglas firs at Discovery Park where I worked by forgetting to untie harmless-looking burlap tree stake ties. In two years we had topped all five by simply ignoring them.

The two most common errors in limb removal. Forgetting to make an undercut will cause the limb to rip down the trunk. A narrow branch angle will cause the pruner to saw off the bottom of the collar inadvertently.

Trees Die from a Series of Wounds And Stresses
People mistakenly assume that since your tree didn't die right after you drove nails or your car, into it, therefore it's all right. Wrong! Trees usually die from a series of blows over a long time and when one dies, it is just the proverbial straw that broke the camel's back. You may have a giant column of dead rotten wood walled off inside your tree from some old wound and when the drought hits, well, WHAMMO! you've got a dead tree. Or, as Dr. Shigo explains, wounds and injuries, including flush cuts, will often "cock the gun" and a freeze or bright sun will "pull the trigger," so that frost cracks or sun scald will appear a year or years later. The sun or the frost gets the blame. Your trees can wall off a lot of abuse, but that doesn't mean you should abuse them.

The proper way to remove a limb uses the three-cut system. On narrow, angled branch crotches you may hoave to saw entirely from the bottom upward.

How to Prune a Big Limb
The major objective in taking off a big limb is: (1) to remove all the limb so that a big stub is not left to look ugly, sucker back, and/or die and rot, and (2) not to injure the branch collar, which is trunk wood. If it's a big branch, this may not be as simple as it sounds. Most people (1) start at the dotted line, and (2) cut down. This will cause two bad things to happen: (1) the weight of your big limb will cause it to break and rip down the trunk wood when you get about three quarters through. This is obviously bad; or (2) if your branch collar crotch is too skinny, and you saw down from the top, you will saw off the bottom of the collar. This is also bad.

To avoid these hazards, use the three-cut system. Somewhat farther out on the branch than you plan to make the final cut, you make a bottom cut and then a top cut that doesn't quite match up; this will act as a hinge so that your limb will gently fold down to the ground instead of dropping on your leg. Or, you can just saw it off some distance out. This relieves the weight of the branch and makes an accurate pruning cut easier and safer. When you have a skinny trunk branch crotch, you will have to use the somewhat awkward method of sawing up from the bottom to avoid cutting into the collar. If you ignore this and cut from top down, you cannot appropriately angle your saw and will wind up sawing through the collar. Even when the angle is wide enough to cut from the top down, always, always make a brief undercut to keep the branch from ripping the bark down the trunk.

A narrow branch crotch will sometimes have "included" bark. Bark disappears inside the trunk, where it is trapped, causing limbs to break out even on large, old trees.

Look for Narrow Branch Crotches
Speaking of narrow branch crotches, you should be looking for these on your tree. It is the narrow crotches that break in storms, causing a big limb to fall on your car. Bark gets trapped between limbs (called included bark) in narrow crotches and prevents them from holding together: as the limb gets bigger and heavier, it is more likely to fall apart. Check out the branch bark ridge between the trunk and the limb--does the bark disappear into the tree? Bad news! If the bark is being pushed up, you're okay. Irresponsible pruning by growers early in a sapling's life can cause narrow crotches that fifteen years later turn into dangerous limbs. So, look for narrow crotches or crowded branches and remove one if to do so will not ruin your tree. Some species, such as plums, just grow that way. As with all pruning, you use logic and good sense. You just do the best you can in the less than perfect plant world.

Correct Double Leaders
Another situation to correct in a tree is a double leader, which often is a narrow branch crotch as well. You don't want double leaders on your tree. Double leaders are often caused by topping. If it's a young tree (under fifteen years), you can cut out one of these leaders. If it's an old tree and it would ruin its good looks to take out one leader, you could hire a qualified arborist to cable and brace them together, if they pose a threat to life or property. When arborists cable and brace, they put bolts through the branch and the trunk. This causes less injury than tying something around a branch, which would girdle it. Sometimes, as in the case of my young katsura, I'm letting two leaders duke it out fort awhile to see if one will naturally win. Gardeners have such an odd sense of time. They're always watching their plants whose lives are action-packed in a slow-motion sort of way.

Early pruning of a tree to force branching will sometimes cause crowded branches and narrow crotches to develop. Corrective pruning is shown.	Prune to prevent you tree from growing up with a double leader.

SUMMARY
Trees don't need much pruning. Taking off a few lower limbs, even big ones, is okay, as long as the limbs are only a small portion of the whole tree, no more than one-eighth of the total. Don't top trees.
Here's your list of do's and don't's for proper tree care:

Don't's:
- Don't leave stubs
- Don't paint cut or wounded places on limbs
- Don't flush cut
- Don't girdle
- Don't top--unless it's a hedge
- Don't leave things tied around your tree
- Don't wound carelessly.
- Don't drill holes to drain water from cavities.

Do's:
- Do take out dead wood.
- Do prune off, to the collar, any limbs you don't want, especially lower limbs.
- Do remove narrow crotches and double leaders, or have qualified arborists cable and brace them if they are a potential hazard.
- Do remove tree stake ties as soon as trees are established--within a year or two at most.
- Do hire a qualified arborist (one who does not advertise topping) to prune large trees or to give you a hazard tree evaluation.
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A New Tree Biology, Shigo and Trees, Associates, 1986.
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