Generally, trees are good, and we don't want to kill them.
But stewards of prairies, savannas, and oak ecosystems know their biggest problem is the dark. Too many trees. Girdling is a way to kill a tree without herbicide.
The basic principle behind girdling is simple: remove the phloem and leave the xylem. The inside of a tree is the wood, the xylem, the stuff a xylophone is made of. The inner wood is important to a tree, but dead. The phloem is not the bark. It is a half-inch or so of living tissue that surrounds the xylem, and it has a crucial purpose. It brings the products of photosynthesis down from the leaves to nourish the roots.
With a saw or axe, make two cuts around the trunk about six inches apart. 
Then with a hammer or the back of an axe, knock out the phloem, which may even pop off in one piece if the cambium is “slippy” enough. Do not try to shave off the phloem, even though it might seem easier, because it’s then difficult not to leave strips of connecting phloem or cambium, in which case the trunk will recover, and you’ll have wasted your time.
Most trees, if you cut them, will put up lots of shoots, from what’s stored in the roots. (The roots send water and minerals up to the branches and leaves through the sapwood, the outer wood, just beneath the phloem.) In time, those shoots will grow into new trunks. But if you girdle most trees, the roots will sense that the top is still doing fine and doesn’t need more shoots. Yet the roots will starve.The dead tree will then stand for many years as a bounty for woodpeckers, other hole nesters, beetles, and mushrooms. Or, at that point, you can cut it down to be rid of it.
Girdling aspen at Shaw Prairie in Lake County.
The next photo shows the heart of Shaw Prairie, about 100 feet west of these aspens. The major problem here is encroachment by little aspen. With lack of fire, this one tree species can destroy a rare high-quality prairie as the network of tree roots launches an army of thousands of new trunks. Great effort for years had gone into driving the them back, but the roots of the large aspens around the periphery sent reinforcement energy to the invaders, and they continued to advance.
When cut and herbicided, these death-dealing little aspens have survived thanks in part to resupply through root connections with the big aspens behind them.
Stewards and staff spend much of our winters cutting trees down and burning them in bonfires. This too is important and good. But look at the photo below and compute how much work was saved by girdling. Biodiversity loss is a crisis for the planet. Do we have time to waste?
Here you can see thirty girdled trees. Compare that number to three trees we saved (bur and white oaks, marked with blue-and-white flagging). An oak savanna thrives best with fewer, scattered trees. The rare plants and animals here needed more light. Reproduction of the oaks requires more light.
One more dramatic fact about the photo above. We had marked those few young oaks years ago. By the time our work reached here, the one in the foreground was already dead. Shade kills. Let there be light.
Let’s review in more detail why girdling may be a good approach:
- Herbiciding may be needed for many purposes, but we prefer to minimize it in the natural ecosystem. Girdling uses the trees' natural processes to replace herbicide.
- Especially when there are thousands of invading stems close together, there is a risk that enough herbicide to kill the invaders will kill many of the surrounding grasses and wildflowers also. We worry about that risk most in the finest, rarest areas.
- In many preserves, old standing dead trees are missing. They're an important part of a natural ecosystem. To thrive for wildlife, woodlands and savannas often need more standing dead trees than they have today.
- There is urgency. Most nature preserves don’t have either the budget or staff that they need. They never will. The needs are too great. Bigger budgets, more staff, and more volunteers are needed – and more efficient methods. It’s not good enough to argue that “we’ll just keep plugging away.” More and more priceless biodiversity is being lost day after day, year after year, decade after decade. Girdling speeds the recovery.
- It’s safer. You girdle and leave before the limbs and trees fall. In most cases, the tree never does fall. Instead it disassembles: little branches drop first, then fungus rots, and finally, dried out, standing or fallen trunks burn up during a controlled fire.
While “the old snag” is standing, the ecosystem appreciates the diversity of fungi, bacteria, lichens, grubs that eat wood, and woodpeckers drilling holes. Animals that raise families in those holes include flying squirrels, great crested flycatchers, eastern bluebirds, wood ducks, screech owls, chickadees, titmice, wrens, white-footed mice, all the woodpeckers of course, and much more.
When to girdle
It is by far easiest and most successful to girdle during the “period of bark slip” – late spring and early summer. At this time, the cambium is growing fastest, the phloem will pop off with a little help. (The cambium is the thin, weak, creative layer that divides the xylem from the phloem – and which generates both.)
Special cases
Aspen is an especially good tree for girdling because any other approach is likely to result in monstrous re-sprouting from far-flung roots, no matter how much herbicide you apply. In my experience, isolated stands of large aspens were entirely killed with no re-sprouting by simple girdling. If the aspen already have a lot of root sprouts, this approach does not kill them. But it does prevent the large trees from over-riding the herbicide. Someone might also ask: “Why kill aspen at all? It’s a beautiful and native tree.” That’s true. In large preserves it may play an important role in the fire dynamic. But for some remnant savannas and prairies it has become a lethal invader and needs control.
Black locust, silver poplar, and tree-of-heaven don’t respond well to girdling. Unlike most trees, for unknown reasons, they put up massive root-sprouts. These species can be better controlled by regular cutting and stump treating, or by frilling, in which you make deep cuts into the tree and apply herbicide. Even then, you may get lots of resprouts, which then will be best controlled by foliar spray.
Buckthorn can be successfully girdled, but it’s rarely worth it. Girdled buckthorns often put up a circle of sprouts under the girdle, which can be knocked off easily, and then the tree dies. But buckthorns are often so dense that all the dead and falling trees make such a mess that it’s hard to facilitate ecosystem recovery, so we cut and burn them.
For more girdling details, check out this blog post.
For more on how to control woody pests generally, check out this index blog post.
Acknowledgements
Girdling diagram by Paul Nelson from the Tallgrass Restoration Handbook (Packard and Mutel, eds.)
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