Horticultural forums and garden guides are full of simple one-off questions about plants: Can I root this in plain water? Are these funny-looking bulges flower buds? How do I get rid of mites? And so forth.
Then there are bits of conceptual knowledge that answer a hundred questions at once. Understanding how plants work at a biological level gives us tools to make better judgment calls, and brush off so many of the myths and bad bits of advice.
One of those powerful basics is understanding how plants use their leaves. That is: the leaves, or green tissues in general, are there to photosynthesize, making energy from sunlight. They’re the only part of the plant that makes energy, feeding all the plant’s cells, including the foliage, stems, flowers, bulbs and roots.
Of course you already knew that. Who doesn’t? And yet, taking it to its logical conclusion inverts the way we gardeners casually talk about plants, as if energy exists in the soil and is extracted, “sent to” to the canopy. Practically all of us sometimes use the inverted language: “let’s cut off the lower branches so the tree’s energy will go into growing the top,” or “chop your spent perennials down so the energy goes into making new stems.”
This is, in reality, only superficially true. If a hungry animal, unexpected frost, hailstorm or gardener removes a severe proportion of a plant’s leaves, it’s true the plant will tap into stored energy to restore the balance between leaves and roots, which means growing new leaves. If growing conditions are favorable, regrowth begins swiftly; dormant buds can launch rapid cell division within days or even hours, and will eventually erupt with visible growth. Since all a plant’s cells are relying on stored energy the moment the leaves are lost, it’s best to quickly commit a portion of that energy to replacing them.
That is, plants don’t spring back vigorously after hard pruning because they so appreciate having been deadheaded or cropped. They do it as a survival strategy, because they need to get out of energy deficit as quickly as possible. The lush, pristine new growth, free of insect bites, spent flowers or wear and tear, can make it seem as though the plant has been happily rejuvenated. But don’t be deceived: replacing lost foliage is costly. It diverts energy that would have gone towards root growth, reproduction, or chemical defenses against disease. If a plant is defoliated repeatedly, it will be severely weakened, and can eventually run out of energy and die.
How plants prune themselves
Plants survive the tumult of nature by being ruthless. When leaves or branches are no longer helping the plant, they’re sacrificed. That means a leaf or branch that isn’t a net producer—consuming more energy than it currently makes with photosynthesis—dies.
There are many reasons leaves could stop being productive. Often, a lower leaf or branch is simply shaded by other higher parts of the tree, and dies through a process called “self pruning.” Older leaves that accumulate too much wear and tear, or oxidative damage due to age, eventually stop being useful and enter senesence, a natural process when they turn yellow, break down pigments to return mineral nutrients to the rest of the plant, and fall off. If the plant faces a drought, photosynthesis slows down, meaning that a lot of leaves and branches that were net producers are now in deficit. Those too will senesce and fall off, resulting in a thinner canopy with only productive leaves left.
This also means that moving, covering or turning a houseplant forces every leaf to go through a recalculation based on its orientation towards light. Some will no longer be in a good position, fall into deficit, and senesce, stimulating the plant to grow replacements. Understanding this process helps us recognize many useful things: that moving a plant too frequently could be stressful for it, that we can expect plants to accelerate leaf turnover when conditions change, and that a few yellow leaves here and there are no major cause for concern, especially if those leaves are older and lower down in the canopy.
It also gives us clues when it comes to helping our plants through trauma. When you plant or repot something and injure some roots, should you cut off some leaves to counterbalance the loss of roots? Understanding how the plant would respond to foliage loss—by pausing root growth to prioritize foliar growth—suggests its better to pamper it with extra water for a while rather than to pare down the top. Or, if a frost or hailstorm leaves a garden in tatters, is it helpful to cut off the damaged leaves and stems? Well, the remaining foliage, unsightly as it may be, helps the plant resume growth without drawing down its reserves. If a tattered leaf is too damaged to be a net producer, we know the plant sacrifices that foliage to invest in new growth on its own, and we can assume that anything that stays green is therefore productive.
None of this weighs against good structural pruning of trees, which is intended to promote strong branches rather than stimulate fresh foliage. In fact, arborists protect the tree’s energy supply by limiting pruning to one fifth of of the canopy at a time. We can also still trim plants and leaves for aesthetic reasons—we should just know we’re doing so for our own purposes, not the plant’s, and use moderation. And if additional rounds of spring hail are possible, it might be smart to wait until the danger has passed to do a hard prune that will trigger the plant to dig deep into its energy stores and create a flush of vulnerable, lush green leaves.
As a whole, I think the knowledge helps us slow down and be a little more tolerant of how plants take care of themselves. Of course many of us garden because we find it therapeutic or fun to clip and train, and we like to think of plants as needing our constant care . There’s still a lot of room for experimentation, but in this case, the garden is better when we do it a little bit smarter and use a lighter touch.