Garden Q&A: Should I fill the hole in my tree?

Q: I have a tree that appears to be doing well but which has a couple hollowed-out branch stubs. Should I fill them in so it doesn’t cause problems?

A: No, leave them as-is unless a certified arborist inspects the tree and recommends action be taken to prevent a specific problem. While such ragged wounds are undesirable in general, once they have developed it’s usually too late to take any action, even when warranted. If storm damage breaks a branch, make a clean cut just outside the branch collar (the swollen ring of tissue joining the branch to the trunk or larger companion) so the injury seals over more quickly and completely. Refer to the Proper Pruning Cuts section of our Pruning Trees webpage for an illustration.

If an external seal can’t be formed by that dedicated callus tissue, trees have internal ways of walling-off vulnerable wood from decay organisms. This process has the long name of compartmentalization (is a Scrabble board long enough for that?), because it forms a “compartment” around injured tissues to keep that damage from spreading. It’s not foolproof, and you can’t see this from outside the tree, but it often saves a tree from extensive wood decay that begins in a ragged branch break.

These hollowed-out stubs can make valuable homes or temporary shelters for a variety of wildlife, like cavity-nesting birds, tree frogs, skinks and fence lizards, lots of different insects, flying squirrels, and snakes. (Don’t worry, snakes have no interest in bothering you and could help clean-up a nuisance rodent population).

Q: I’ve heard about doing a “Chelsea chop” on perennials but am not quite clear on what that means. Does it have benefits or is it not good for some plants?

A: Named for the time of year (May) the Chelsea Flower Show takes place in Britain, this practice involves cutting back certain perennials early in the growing season to influence their mature size and time of bloom, mainly for the benefit of the gardener.

There are pros and cons with this technique. Its focus is either to delay the bloom on summer-flowering species or to reduce their height so they bloom shorter than they typically would; you may get both results simultaneously. A flowering delay might enable you to coordinate certain plant combinations for aesthetics that might otherwise miss each other in peak bloom. It also might make floral resources available for longer periods for pollinators visiting those plants. The height adjustment can be useful for tall-growing perennials that could otherwise be many feet high by the time flowers appear, prone to lodging (tipping over or leaning) or just being past the height where you can most appreciate them.

Some specialist pollinators require plants to be in bloom at more precise times, so delaying a plant’s flowering might actually deprive them of some needed resources. Similarly, if delayed by even just a couple of weeks, some late-blooming species might not have enough time after being pollinated to ripen seed before the days get too short or the nights too cold, so whether you intend to supply birds or other animals with a food source or just want to have seed to keep for yourself (or to reseed in the garden), this reduction in seed set might not be desirable.

How far back you trim plants being chopped is up to you, but the convention is to prune off about the top third of the current height of the plant. An alternative is to cut shorter only some of the stems, leaving others at full height. If you view the planting mainly from one side, cut back the stems at the front of the clump (those closest to the viewer) and leave those in the back their normal size. This can allow the untouched stems to flower on time while the others may flower later, or at least at a lower height which should remain sturdier.

Which plants to try this on can be an experiment, but candidates include perennials in the aster and mint families, such as New York ironweed (Vernonia noveboracensis), New England aster (Symphyotrichum novae-angliae), beebalms (Monarda), Joe-Pye weeds (Eupatorium/Eutrochium), mountain-mints (Pycnanthemum), goldenrods (Solidago/Euthamia), Helen’s flower (Helenium), upright-growing sedums (Sedum), summer phlox (Phlox paniculata), and Culver’s root (Veronicastrum). Plants with less of an upright-linear stem with leaf buds along its length, such as blazing star (Liatris) and ornamental grasses, won’t benefit from this.

University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at extension.umd.edu/hgic. Click “Ask Extension” to send questions and photos.