Epicormic buds: the sprouts that appear after damage or new sunlight

A storm takes the top out of an oak, and by August the bare trunk is covered in tufts of leaves. Trees come down along a property line, and the next summer the ones left standing sprout twigs along the side facing the new light. All of these come from the same place: buds the tree laid down years or decades ago and kept in reserve.

Buds in waiting

Every leaf on a new twig forms with a bud in its axil. A few of those buds grow into next year’s shoots. Most never do, but they do not vanish. A dormant bud stays alive, connected to the pith by a thin thread of tissue, and each year it grows just enough to keep its tip at the surface as the twig thickens into a branch and the branch into a trunk. Hormones from the growing tips keep the buds suppressed, and as long as the crown is full and the bark shaded they stay asleep.

A second kind forms later. Adventitious buds arise in the callus around a wound, in the cambium or on roots, with no thread back to the pith; they are what a stump or a root sends up when it has no dormant buds left.

Shoots from either kind of bud are epicormic shoots: water sprouts when they shoot straight up from a branch, suckers when they come from the base or the roots.

What wakes them

Anything that upsets the balance between the roots and the crown.

• Losing the top, or heavy pruning. A broken leader or a topping cut removes the shoots that were sending the suppressing signal down the stem, and the buds below wake in a rush; the harder the cut, the thicker the sprouting.
• Sudden light on the trunk. Bark shaded for decades and suddenly in full sun, after neighbors are removed or a woodlot is thinned, sprouts along the lit side. Foresters thin oak stands gently for this reason; sprouts ruin sawlogs.
• Stress. Drought, root damage, a construction trench, a girdling root, decline in the top. A trunk covered in sprouts under a thinning crown is a tree trying to replace leaf area it is losing.

Why some trees sprout and others cannot

Species differ enormously in how many dormant buds they keep and for how long. Oaks, sweetgum, elms, basswood, red maple, hickories, beech, black locust, sassafras and most other eastern hardwoods sprout freely from trunk and stump. They evolved with fire, ice, browsing and wind, and rebuilding from the bark is how they survived. Basswood is the extreme case: the clumps of basswood trunks in a northern woods are rings of sprouts on the stump of one parent. Bear oak in the pine barrens rebuilds from the root crown after every fire.

Most conifers do the opposite. Pines, spruces, firs and hemlocks carry few dormant buds in old wood and do not form new ones on a bare trunk, so a pine that loses its top or lower branches cannot replace them, and a conifer cut to a stump is dead. The exceptions are few: pitch pine and shortleaf pine sprout from trunk and base after fire, and bald cypress can sprout from a stump while young.

What the sprouts mean, and what to do with them

Epicormic sprouts are a tree fighting to rebuild its leaf area. On a tree that has lost its top or a major limb they are the recovery, and the right response for the first year or two is to leave them alone. They feed the tree, they shade bark that would otherwise scald, and the best placed of them will become the new branches and leader. After that, thin rather than strip: keep the strongest, best spaced sprouts, remove the rest over a few seasons, and cut each one cleanly at its base without leaving a stub, because a stub dies back and sends up a cluster of new sprouts. Never strip a stressed tree bare at once; the sprouts are the food it is trying to make.

On an undamaged tree sprouting under a thinning crown, treat the sprouts as a symptom and look for the cause: a buried root flare, girdling roots, compaction, a recent trench, a dry summer.

Weak attachments and safety

An ordinary branch grows from the pith outward and is woven into the trunk’s wood, layer over layer, from the start. An epicormic shoot begins at the bark surface. Its attachment strengthens as it thickens, but a fast-growing sprout on a big topped limb can reach the size of a real branch while still held by a shallow collar of wood, often on a stub decaying beneath it. That is why topped trees drop limbs, and why sprouts on a big limb over a house or a place people sit deserve an arborist’s look every few years.

Coppice: the same trick, on purpose

Cut a hardwood to the ground and the stump sprouts from dormant buds at the collar and adventitious buds in the callus, with the whole root system behind them, which is why stump sprouts grow several feet a year while a seedling beside them grows inches. Coppicing, cutting on a cycle of years to harvest the sprouts for firewood, poles and fencing, ran European woodlands for millennia. Much of the eastern oak and hickory forest regenerated the same way after the logging of a century ago; a clump of two or three big oaks joined at the base is the signature. If you cut a hardwood and want it back, leave a low stump, let it sprout, and after two or three years thin to the one or two strongest sprouts attached lowest on the stump, which root on their own and rot least.