In the Garden:
A side branch has taken over after the central trunk of this spruce was damaged in a storm.
A Cut Above
The other day as I looked out the window at the rabbit tracks in the snow leading up to the chewed-off twigs of a viburnum, I tried to think of the rabbit's hungry ravaging as nature's way of pruning to encourage more shoots full of sweetly scented flowers. After all, the rabbit did choose the correct season -- winter -- to prune this shrub that flowers on the current season's wood. And the damage to the side branches won't radically change the shape of this plant.
But if Peter Rabbit had chewed the growing tip off my dwarf blue spruce, he'd be in big trouble. This is a plant that's meant to grow in a pyramid shape, and if the central growing point is damaged, side branches vie for the honor of being the dominant central trunk. Oftentimes, this results in a multi-branched top, ruining the shape of the plant.
Whether damage to a growing point is accidental (animals, wind, snow) or intentional (pruning), the plant's response is affected by a natural condition called apical dominance. I think we can be smarter about pruning if we understand how apical dominance works. Plus, it can cause some quirky situations that are just plain fascinating.
Who's The Boss?
The growing point of every stem -- the apex -- has an insatiable appetite for expansion, as well as a domineering personality. It always wants to be the first to put on new growth, the first to expand higher and wider before any of the side shoots get a chance.
Why? Because these apical tissues contain higher levels of a particular plant chemical -- called IAA auxin -- than other parts of the plant. The auxin stimulates cell division in shoot tips and suppresses the growth of side buds, especially the side buds that are closest to the apical buds. So the side buds toward the base of the stem or trunk are less influenced by the auxin and are more free to develop. That's why the first side branches that grow on a tree tend to be the lower ones.
What happens to the auxin when the apical end of a stem is damaged? The source of the auxin is gone so the side buds are no longer suppressed and they begin to develop. We make use of this effect every time we pinch the tip of a zinnia to promote more side shoots.
Plants differ in the strength of their apical dominance. For example, young conifers are strongly influenced by it, which is why they grow with a dominant, pointed top; small upper branches; and large, lower branches. As these trees mature, apical dominance declines and the tree silhouette becomes wider.
Fruiting and Flowering Plants
Apical dominance can cause a curious situation to occur on fruit trees. When the topmost branch becomes so heavily laden with fruit that it bends over, the apical bud loses its dominance. Another bud that's now higher on the branch will then try to take over the apical dominance. The resulting growth can certainly look odd, but if you remove the fruit on the topmost branch to keep the tip higher than the rest of the branch, it will make it clear who's boss.
Another effect of apical dominance has significance for fruit trees, grapes, climbing roses, and some other plants. Upright-growing branches are more vigorous and take longer to produce fruit than those growing horizontally. So some plants that are valued for their flowers or fruits can be induced to produce sooner by encouraging horizontal branching.
If you've grown climbing roses, you're probably already aware that the horizontal canes produce more flowers. Similarly, espaliered apple trees that are trained to grow horizontally on a fence or trellis can produce abundantly even though their overall size is much smaller than a upright tree form.
Apical dominance also influences the crotch angle between side branches and the main trunk of a tree. When the apical stem is healthy, the side branches meet the trunk at a wide angle, which leads to stronger branches because more wood can accumulate in a wide crotch. When the growing point of the main trunk is damaged, the side branches compete to take over as the dominant central stem and begin growing upward at narrow angles.
You may have seen this occur in trees that have lost their central trunk and subsequently have two competing trunks. These two trunks are weakened by the narrow angle between them and are more susceptible to injury. You can remedy the situation by removing one of the trunks so the other one can become the leader. Curiously, even pruning back one of them will remove its directive to grow straight up. As this trunk spreads outward, the crotch angle will actually begin to widen, and the framework of the tree will become stronger.
As you go about late-winter pruning in the next few weeks, enjoy the opportunity to get reacquainted with your trees and shrubs, and imagine how the cuts you make will alter the auxin flow and prompt new buds to grow. A few years ago the snow was so deep my dog thought a branch on my dwarf sargent crab apple was a stick on the ground begging to be chewed. The broken branch left a bare spot, and I wanted to encourage a branch to grow in its place, so in late winter I used one of my favorite techniques of outwitting apical dominance -- notching. I cut a tiny notch about a half inch above the bud I wanted to grow. This interrupted the flow of auxin to that bud so it no longer got the message to stay put. Voila, a new branch was born!
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