Garden Advice: Don’t Prune that Crown!

Quercus_Oak_COS-CO_LAH_1854It’s a common question. You’ve just planted a new tree. In the process, the plant has lost a significant portion of its roots—sometimes up to 95 per cent! Should you prune back the crown to compensate?

The intuitive answer would be yes. We assume that with fewer roots, there’s no way the plant will be able to sustain all that foliage on top—and that’s the advice I see on website after website. But if you do decide to prune, you’ll be doing the tree a disservice. You might even kill it! How can this be?

The primary reason we think we need to prune transplanted trees is to reduce transpiration. All those leaves are losing water, and the roots needed to replace that water are missing. It’s true—initially. Removing all those thirsty leaves will reduce the tree’s need for water.

But what does pruning do to a tree? It encourages new growth! When the ends of branches are removed, the dormant buds that remain will burst into life. Think—we nip off the tips of young flowering annuals and perennials to create a bushier plant. It’s the same principle at work—the tree will end up with more leaves than it had to start with. All that new growth will put more stress on the tree than if you’d simply left it alone.

It’s far better to provide adequate irrigation, making sure the soil is wet so that what roots remain have plenty of water available. Then let the tree decide how many leaves it wants to keep. Yes, some may wilt and fall off, but they’ll be replaced in due time.

Now consider—where does the tree get the energy to grow new roots? All plants get energy from the sun through the process of photosynthesis. And where does photosynthesis occur? In the leaves. Removing foliage reduces the carbohydrates available for growth, right at the time the tree needs more resources. It’s like starving a patient after surgery. How can the tree grow when it’s simply trying to survive?

It seems logical that adding fertilizer at this point would compensate for the reduced carbohydrates available. Doesn’t fertilizer “feed” plants? Actually, no. It simply provides some elements—mainly nitrogen, phosphorus, and/or potassium—that might be deficient in the soil. Fertilizer is more appropriately compared to vitamin pills, not food. Just as we can’t substitute vitamins for dinner, fertilizer doesn’t replace photosynthesis. Worse, it will stimulate top growth, something we don’t want to do.

There’s more. Normally, the size of a tree’s crown will be balanced with the mass of roots below ground. When part of the tree is removed, there is a transfer of hormone-like chemicals that encourage the plant to replace what was lost. For example, if significant top growth is removed, perhaps due to a late freeze, the roots produce chemicals that “tell” the tree to produce new twigs and leaves, restoring that balance.

Our transplanted tree has more top growth than roots. Therefore, the foliage sends out a signal for the tree to concentrate its resources on root growth—just what we want. If we remove the crown, “correcting” the imbalance, those chemicals won’t be produced, and the roots won’t be stimulated to grow.

We can’t see the roots growing and we may be worried that nothing is happening, but eventually the roots will catch up and top growth will resume. In fact, it’s the opposite—top growth occurring immediately after transplanting—that should concern us.

What about the various root stimulators and growth hormones on the market? Would those help? It depends. Although promoted heavily, products based on vitamin B1 (thiamine) are useless—there is no research indicating vitamin B1 stimulates root growth. There are several other chemicals, however, which are effective: indole butyric acid, paclobutrazol, and naphthylacetic acid all suppress crown growth, redirecting resources to the roots.* They may be difficult to find at your local garden shop, however.

Finally, pruning in this way can also destroy the tree’s branch structure, ruining its appearance. Pruning should never be used to reduce transplant shock. And if you do need to prune—such as in the case of broken or dead branches, disease, or to correct structural issues—cut the branches off at the trunk where they originate, or at least back to a side branch, and not randomly in the middle. I cringe when I see mutilated trees.

Give your new tree the best start possible. Ideally, transplant while it’s dormant. But if you have to move it while it’s leafed out, don’t prune that crown!
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