Multistem or single stem? Sometimes the trunk is hiding the answer

A clump of stems that touch will grow into one trunk with bark trapped in the seam, a joint that splits in wind and ice because no wood crosses it. Decide while every stem is thinner than your wrist: keep one leader for a single-stem tree, or three to five stems that never touch for a shrub.

Cut through any young stem and there is a small soft core at the center: the pith. It is the oldest part of the stem, laid down by the growing tip in the shoot’s first season, and every later ring of wood is wrapped around it. A trunk that grew from one bud has one pith, from the root collar to the top. That makes the pith a birth certificate, and it is why some big trees that look like one turn out, on the day they split, to have been two or three all along.

How clumps start

Trees grow in clumps more often than people expect, usually for one of three reasons.

• A seed cache. Squirrels, chipmunks and jays bury acorns, hickory nuts and walnuts, and a forgotten cache with more than one nut in it sends up two or three seedlings an inch apart. Several seeds from one fallen persimmon do the same.
• Stump sprouts. When a hardwood is cut, burned or blown over, dormant buds around the base of the stump wake up and throw a ring of sprouts. Oaks, hickories, maples, sweetgum, black gum and tuliptree all do this; many oaks in second-growth eastern woods began as sprout clumps after the logging of the 1800s.
• Browsing or mowing. A seedling nipped by deer, rabbits or a mower loses its leader and regrows two or three shoots from below the cut. If none takes over, the tree grows up as a fork from ground level.

Some species simply grow this way. Serviceberry, redbud, hazelnut, witch hazel and river birch are naturally multistemmed, and a clump is their normal shape rather than a defect.

How two stems become one trunk

Stems that start a few inches apart cannot stay apart. Each adds a ring of wood a year, the trunks widen until they touch, and then they press together. Arborists call two such stems of nearly equal size codominant stems. The bark caught between them cannot become wood; it is squeezed into a seam, called included bark, that runs down the inside of the union. On the outside, bark eventually grows over the seam, and from a few steps away the result looks like a single trunk, often an impressive one.

Inside, nothing has fused. A cross section shows two or three piths, each with its own ring pattern, separated by a dark line of trapped bark. Where the stems touched for long enough, the outer wood may have grafted across, but the seam itself has no fibers running from one stem to the other. It is a joint held together by pressure and a shell of shared bark, and a joint is not a trunk.

Why it matters

A branch growing out of a trunk is anchored by wood that interlocks across the union. Two stems pressed together have none of that, and the included-bark seam fails in predictable ways:

• The stems lever against each other in wind, ice or heavy snow, and one splits away, usually taking a strip of the other with it.
• Water runs into the seam and sits there, and decay follows it into the heart of the tree, where it is hard to see and harder to assess. A shelf fungus on old dead wood is one thing; rot inside a seam that is holding two stems together is another.
• The load keeps growing. A fused clump at eighty years carries far more weight on the same weak joint than it did at twenty.

This is why some of the biggest old “single” trees in a park or a pasture come apart in a storm and reveal two centers of rings and a rotten seam. They were never one tree. It is also one reason size is a poor guide to age: a fused pair is far thicker than a single stem of the same age. Our growth estimator shows how far apart two sites pull the same species even without a hidden second stem.

“The presence of included bark significantly weakens the junction of co-dominant stems.” (University of Florida IFAS Extension, Is My Tree Safe? Recognizing Conditions that Increase the Likelihood of Tree Failure)

How to tell in the field

You do not need to cut the tree. Look at where the stems meet.

• A seam or fold. A vertical crease running down the trunk from the fork, sometimes with bark folded inward on both sides, is included bark. A trunk that grew from one pith has no such line.
• V or U. A tight V with bark pinched between the stems is a sign of fusion. A rounded, U-shaped union with a smooth saddle of wood across the bottom shows the stems are connected by wood.
• The bark ridge. Where a branch meets a trunk, the bark is pushed up into a raised ridge along the top of the union. Where two stems have pressed together it is pushed inward instead, and the ridge is missing or replaced by that crease.
• Bulges and weeping. A swelling on either side of the seam, or a dark wet stain running from it, means the seam is under strain or starting to decay.

“U-shaped branch unions have a stronger attachment and are preferred over those with a tight V-shaped branch union.” (NC State Extension, General Pruning Techniques)

What to do with a young clump

The time to decide is early, while every stem is thinner than your wrist.

If you want a single-stem tree, choose the straightest and strongest leader and cut the others off at ground level. Do it in the first few years; a cut that small closes quickly, and the tree puts its growth into the survivor. The base will try to sprout again, so rub off new shoots for a season or two. Every clump that leaves Blue Whale Trees has had this decision made for it in the first year or two, while the cut is the width of a pencil and closes over in a season. On a tree too big for that, an arborist can shorten one stem to subordinate it, or cable the stems if the tree is worth the expense.

If you want a shrub or a small multistemmed tree, keep the clump but thin it to three to five stems that leave the base at clear angles and do not touch. A serviceberry, a redbud or a hazelnut is at its best this way. The rule is the same either way: stems that grow apart stay separate and sound; stems that grow into each other become one trunk with a fault line down the middle.

Common questions

Is a tree with two trunks dangerous?

It depends on whether the stems are joined by wood or only pressed together. A seam of trapped bark has no fibers crossing it, and that is what splits in wind, ice or heavy snow.

What is included bark?

Bark caught between two stems as they widened against each other and squeezed into a seam down the inside of the union, where it cannot become wood and gives the joint no strength across it.

Should I cut one trunk off?

If you want a single-stem tree, yes, and do it while every stem is thinner than your wrist, then rub off the new sprouts for a season or two.