How Plants Work (2015)
8 Garden Care, Not Control
EVER WONDER WHY the Mary of nursery rhyme fame was so contrary, in spite of her obvious success in growing flowers? Maybe it was the trees and shrubs in her landscape that didn’t quite act the way she expected them to. Gardeners are often puzzled, if not downright frustrated, when their newly pruned privets or trimmed trees grow back in ways they didn’t expect—or maybe not at all! Or they wonder why a gargantuan branch with equally enormous needles suddenly splits the tidy crown of their dwarf Alberta spruce. How about that explosion of bamboo shoots mysteriously appearing in the middle of your lawn? Or those green-leaved shoots appearing on your red-leaved crabapple?
Never fear, this chapter will help you understand why plants grow the way they do. When you’re able to think like a plant, you’ll be able to predict their moves and won’t get checkmated in the garden maintenance game. Let’s start with pruning.
Growing Pains: Why Pruning Can Destroy a Plant’s Natural Form
We have a lovely noble fir in our front yard. It was barely 6 feet tall when we moved into our house, and now it’s close to 20 feet. For the most part, it’s been trouble-free: no fertilizers or pesticides needed, just a little additional water in the summer, but otherwise low maintenance. The one glitch we’ve had is that every other year or so the leader dies back, probably the result of some insect larva chewing away at this young tissue. And every time this happens, we have to do a little corrective pruning so we don’t end up with a coniferous candelabrum.
A coniferous candelabrum? Why would the tree put out four or five replacement leaders, rather than just one, like a lizard regrowing its tail? It’s one of those intuitions we have about how plants grow that proves to be exactly wrong.
Candelabra trees are prone to breaking.
Whenever an actively growing shoot on a tree is cut, whether by an insect, a browser, or you, it dramatically changes auxin activity. Rapidly growing shoots, either the tree’s leader or the tips of branches, garner most of the plant’s resources, so the plant can expand its height and width as quickly as possible. To make sure these shoots get the lion’s share of the goodies, auxins in the shoot chemically induce dormancy in buds below the growing shoot. When that growing shoot disappears with a clip of the teeth or pruners, the hormonal suppression of nearby buds disappears as well. In a matter of days, you can see these buds swelling and developing into new shoots. However, it’s not just one shoot, but several.
All of a sudden, where one shoot was reaching up or out, now four or five new vigorous shoots have taken its place. This can be a desirable characteristic if you’re shearing a hedge of boxwood or creating topiary, where bushiness is a blessing. But what about those stately conifers, whose pyramidal Christmas-tree shapes are both natural and aesthetically pleasing? Removing the tops of these trees—not surprisingly called topping (or heading back)—creates a hideous hydra of a tree. Ironically, the reason many conifers are improperly topped is to improve views. Instead, homeowners are left with a flat-top fir with an unruly cowlick that just keeps growing. This means that someone has to continue pruning these new shoots off, every year, to keep the tree unnaturally short and boxy, hardly an aesthetically pleasing view and certainly a long-term maintenance problem.
A Time to Prune
There are times when you should prune your woody plants, so keep those shears sharp! You’ll need to refer to specific books on pruning, some of which are suggested at the end of this book.
TRAINING YOUNG TREES
Fruit trees need to be trained into a form that allows for vigorous fruit production and ease of harvesting. Likewise, trees and shrubs used for topiary, pleaching, pollarding, and other formal training will need some help in their early years. Be forewarned: trees and shrubs pruned into these artificial forms need to be constantly maintained or they’ll try to revert to their normal shape and size.
Formal pruning includes pleaching (LEFT), pollarding and espalier (TOP), and topiary (RIGHT).
MAINTAINING A SINGLE LEADER
When the top of your tree has been damaged or removed, you’ll need to keep an eye on the new shoots that spring up to take its place. Select one of these and prune away the competitors within the first year. If the tree is too tall, hire a certified arborist to take care of this for you.
REMOVING THE THREE Ds
Branches that are diseased, damaged, or dead should be pruned from trees and shrubs as soon as they are discovered. With diseased branches, especially, be sure to clean your pruners with a disinfectant such as alcohol or household cleansers. Don’t use bleach, because it can corrode your tools (and isn’t so great on your plants, either).
RESURRECTING MORTALLY WOUNDED PLANTS
Sometimes there seems to be no way to salvage a damaged tree or shrub. During one winter we got a lot of heavy snow, causing part of our arborvitae to collapse right on top of a nice rhododendron. Most of the rhodie’s trunk was broken, and the only thing I could do was cut it off right at ground level. This cure-or-kill approach should only be used if there is no other alternative. It works best with species that sucker easily or naturally develop multiple trunks, and should only be used on well-established plants that have sufficient root resources to help with regrowth. My own rhododendron has grown back quickly and within a few years will reach its original size.
And a Time to Wait
An old adage is to “prune when the blade is sharp,” meaning any time you think pruning is needed. For the most part, routine minimal pruning causes little harm. But there are some times when you shouldn’t prune at all. A good generic example is pruning conifers back to the wood. Unlike most flowering trees and shrubs, conifers have few dormant buds hanging out under the bark of the trunk and branches. How many juniper hedges have you seen that have been sheared back to the wood? Looks horrible, doesn’t it? And it never, ever fills in. This characteristic of conifers is one of the best arguments I can give for using a flowering shrub species for your hedge: it will tolerate the shearing and come back with a flush of growth from its numerous latent buds.
Although pollarding can help maintain trees in a smaller than normal form, it also creates unusual rounded structures full of dormant buds. In the spring, the buds break and stems grow in dense profusion from each of the pollard heads. Being a cat person, I always manage to see cat faces in these knobby structures. To maintain this appearance, the branches must be cut back to the heads every autumn. The heads should never be removed, as it not only destroys the pruning form but will lead to very ugly branch development. Pollarding should only be attempted if you know what you are doing and you’re willing to do it for the lifespan of the tree.
have knobs of densely concentrated buds.
Rhododendron flower buds remain dormant through autumn and winter.
Speaking of flowering shrubs and trees: don’t prune their flowers off! That may seem like silly advice, but it happens every year when gardeners prune their rhododendrons and lilacs and other spring bloomers in autumn. These species and many others set their flower buds in the summer, and the buds overwinter until spring. To avoid losing the gorgeous floral show, prune these species immediately after flowering, and then leave them alone.
Likewise, don’t be tempted to help deciduous trees and shrubs with their autumn leaf drop by pruning their branches at the same time. Leaves that turn red in autumn are busy transporting sugars and other resources to the branches, trunk, and roots. Lopping off limbs while resources are being relocated is a sure way to decrease your plant’s winter larder.
Another common mistake gardeners make is crown pruning trees and shrubs after transplanting them. It appeals to our sense of symmetry that if we’ve cut or damaged roots in the process of planting, then we should reduce the crown in the same manner. Conventional wisdom says that fewer leaves means less water needed for the crown, so less stress is imposed on the root system.
That’s the way we would expect it to work, anyway. But plants have their own way of responding to crown damage, which is to immediately send out more shoots and leaves to replace those that we’ve cut off. Anyone who’s ever cut back a rose or sheared a boxwood knows this, so it’s amazing that we can ignore that experience and assume new transplants that have also been pruned will patiently wait for their roots to grow before they push out new leaves. By crown-pruning new transplants, you remove part of the photosynthetic machinery needed to provide food for establishing roots, and the existing resources are now directed to new leaf growth instead. New leaves mean more water needed from the roots. It’s the worst possible scenario.
This is where your new-found knowledge on how plants work will allow you to trust in what you can’t see. When plants are putting resources into root growth, they may not grow at all on top. This is especially true of trees that have been root pruned to correct root flaws. You can gently wiggle your new transplant once a week by grasping its trunk and feeling how much it moves. As time goes on, it will move less and less as roots become more numerous and hold the tree more firmly in place. This will happen quickly with bare-rooted plants. The crown will stay dormant until enough roots have established to take up water needed for supporting new leaf growth. At that time, the buds will burst open, birds will sing, and you’ll know that your tree is alive.
Color changes indicate that programmed cell death is occurring in these damaged salal leaves.
Nature’s Little Pruners
We’ve talked about gardeners as pruners, but what about all the creatures to whom our gardens and landscape are a giant smorgasbord? Insects and other grazers nibble, notch, chew, suck, drill, and mine nutrient-rich leaves, and a plant’s response to biological pruning is often different than that to simple mechanical shearing. When a leaf is attacked by an herbivore, there’s a flurry of biochemical activity in the wounded tissue, called the hypersensitive response. A combination of factors, including the physical destruction of leaf tissue, the presence of proteins or hitchhiking viruses in the nibbler’s saliva, and exposure to the environment, trigger this response, which has the end goal of isolating the injured parts of the leaf from the rest of the plant. Destroyed tissues are walled off in this process, which has the oddly robotic moniker of programmed cell death. We see these cellular changes as red, yellow, brown, or blackened areas on the leaf, due both to anthocyanin pigments and to reactions between biochemicals that once were separated and now are all mixed together.
Acrylic, latex, or wax materials that are sprayed onto leaves to form a barrier over the surface. There are also antitranspirants that cause leaf stomata to close.
THE SUPPOSED BENEFITS
By sealing the surface of the leaf or forcing stomata to close, evaporation is reduced.
HOW PLANTS RESPOND
While antitranspirants do reduce water loss from leaves, they interfere with normal plant physiology. Reducing water loss from the leaves means that water transport and nutrient uptake are reduced as well, because it’s evaporation from the leaf that pulls water and any dissolved substances through the plant. The lack of water flowing through the plant also eliminates evaporative cooling of the leaves, making them susceptible to solar heat damage. Finally, closed stomata don’t take up carbon dioxide, so photosynthesis is reduced.
Antitranspirants are best used for reducing water loss in Christmas trees and cut flowers. They should not be used on living plants.
By clogging stomata, antitranspirants interfere with a plant’s ability to take up water.
Besides the interesting color changes you might see, leaves can be inadvertently infected with viruses or microbes carried by hungry herbivores. Aphids and other insects with piercing mouthparts can inject viruses that create white or yellow mottling patterns on leaves. While viruses are rarely harmful enough to kill plants outright, they are a nuisance and can reduce the vigor as well as the aesthetic quality of the affected plant.
Since we’re discussing the proper care of trees and shrubs, let’s consider another error some gardeners make when planting trees: staking. Like a stake through the heart will kill a vampire, a misplaced stake will just as surely kill the tree it was meant to support. This mistake, like many others gardeners tend to make, results from not understanding how plants work. Let’s look at what happens in nature as a predictor of bad staking.
We’re in the middle of a forest. The trees here are tall, skinny, and have few lower branches. All the growth is happening in the canopy as trees compete for sunlight. As we walk to the edge of the forest, we notice that the trees get shorter and stockier; their trunks are decidedly thicker than similarly aged trees in the middle. These edge trees have developed thicker trunks and are shorter in stature as a result of buffeting by the wind. Those in the middle are untouched by the wind and their resources have gone into growing taller, not wider.
Overstaked trees won’t develop normal growth forms.
When we stake a tree too tightly, we are literally providing a crutch for the tree by eliminating the effects of wind. Like its wild cousins in the middle of a forest, the tree can put its resources into growing taller, and these trees characteristically look like lollipops with their skinny trunks and lush crowns. There’s little trunk girth development, because the stake makes it unnecessary. What do you suppose will happen when (and if) that stake is finally removed?
It’s not too difficult to predict that these tightly staked trees will no longer be such upright citizens once they’re removed from bondage. The heavy crowns perched on top of tiny trunks cause the entire tree to bend over, if not break outright. There is no way to easily fix this problem, other than by restaking the tree properly and allowing it to develop adequate girth.
Instead, let nature help prepare your trees for life on their own. If they need to be staked (and many do not), stake them low and loose. Be sure to remove the stakes after a year, because anything that can’t stand on its own after a year in the ground will never develop that ability. It’s better to dig it up and try again, rather than keep the poor thing propped up in a botanical life support system.
Don’t Add Insult to Injury
One of the biggest misperceptions people have about plants is that they heal when they are wounded. Yes, plants do recover from damage inflicted by insects, browsing deer, pruner-crazed gardeners, and errant automobiles, but they don’t heal like humans do. So treating wounds like you might treat your own scratches and scrapes can be a very bad idea.
We’re used to seeing cuts disappear as scabs and then new skin slowly cover and replace them. But this doesn’t happen with plants. Once their tissue is gone, it’s gone. It can’t be regenerated. Instead, the plant creates a wound tissue that both protects the cut area and often has antibiotic activity of its own. In trees, you can see wound wood quite clearly. It looks different than the surrounding bark—it’s smooth and often lighter in color—and it swells along the perimeter of the injury. This donut-like structure continues to expand over the cut surface of the wood and ideally will join in the center to create a physical barrier.
If you’ve spent any time around older trees, you’ll have seen tumor-like protrusions on their trunks. Sometime these growths are closer to eye level, but often they’re at the base of the tree, providing the tree with a substantial derriere. These odd deformities are burls, woody growths on the trunks and branches that develop after environmental damage from pests, disease, and even people. This is a different response than the formation of wound wood. Sometimes injury will turn on phytohormones, which results in uncontrolled growth. If the underlying wood also includes meristematic tissue (points of potential growth), then the developing burl can contain numerous dormant buds.
Many trees, both coniferous and broadleaf, can create large burls, and these in turn are prized by woodworkers. However, this should not be taken as an invitation to remove them from the tree. Burls are thought to be benign and removing them will both injure the tree and remove a substantial supply of stored carbohydrate. Instead, think of these odd growths as badges of life experience. And while some burls develop naturally, gardeners can prevent those caused by mechanical injury. So, be careful with cars, wheelbarrows, weed whackers, and anything else that could physically damage the bark and underlying tissues of trunks and branches.
Burls are unusual but harmless growths on tree trunks.
Wound wood seals and protects cut surfaces.
These suckers originated from roots of one poplar tree.
Let’s consider the bark of my young styrax tree that’s perpetually scraped up by squirrels running up and down on the way to the nearby birdfeeder. When the squirrels’ claws tear through the bark, nearby cells are called up like paramedics to come to the rescue. These cells begin to form all kinds of defensive compounds to keep the injury and possibly disease organisms from spreading to healthy tissues. In trees, we call this wound wood, and it’s quite different than the original tissue. We can see this wound wood on some of the older scratches on the trunk; the texture is different and it will never look like the original bark. These new tissues seal off the wound from the sides and from below, isolating it and any unwelcome hitchhikers from the rest of the tree.
When well-meaning gardeners paint wounds with sealer or cover the wound with so-called natural tree healing products, they interfere with the tree’s own ability to seal itself. In fact, oxygen is an important component of the biochemical reactions that are taking place, and these tree wound products effectively keep oxygen away from the very tissues where it’s needed. Worse, some of these products are desirable food sources for microbes, which means you’ve just issued an invitation to invaders. The only thing gardeners should do in treating tree wounds is to carefully trim away any ragged or torn bark edges using sterilized pruning tools.
The only time wound dressings are warranted is if your area is threatened by specific diseases (such as oak wilt) or insect infestations (like emerald ash borer). In such cases, the cuts should be treated with the appropriate fungicide or insecticide. Hire a friendly certified arborist to do this for you rather than dealing with it yourself.
The Amazing Green Machine Replicates Itself as If by Magic!
Back to that bamboo-in-the-lawn puzzle at the beginning of this chapter. Did it come from seed? Not very likely, as most ornamental bamboos don’t grow easily from seed, especially in an established lawn. Let’s take a peek over the fence at the neighbor’s yard. Sure enough, there’s a nice stand of bamboo just waiting for the local panda to drop by for a snack. What you can’t see is the system of vigorous underground rhizomes that can run for many feet before snorkeling up through the soil surface. If there’s enough light in this spot, a new bamboo clump will form. And if you don’t take care of it, soon you too will have a panda paradise in your yard.
Many weedy plants spread this way. It’s a characteristic that can be annoying, but gardeners can take advantage of it. When you’re planting your perennials, groundcovers, and other permanent plants, do a little research to find out how quickly they spread. You might be able to get away with purchasing far fewer plants and just letting them do their thing. I planted five little native strawberry plants as a groundcover about ten years ago. Today my strawberries cover about 400 square feet of the low-maintenance landscape along the front of our house.
But be sure to consider the other side of the coin, too, before you commit to buying aggressively spreading landscape plants. Will you still be happy with that ivy groundcover when it spreads to infinity and beyond? Or are you prepared to care for a thicket of vine maples or willows that has replaced the single specimen you planted a few years ago? Any time you see a nursery tag that says “spreads easily,” “naturalizes,” or “engulfs anything that doesn’t move,” you should consider the long-term effect of those deceptively small specimens.
Sprouts, Suckers, and Stress
Have you ever seen what look like wild hairs growing in and at the base of landscape trees? They’re ramrod straight, vigorous, and look more like saplings than anything else. These are suckers. The ones in the crown of the tree are called watersprouts, and you can frequently find them in dogwoods, cherries, and other common ornamental species. In general, watersprouts and suckers are structures that trees produce to reinvigorate themselves. While some species naturally sucker as a way to self-propagate, in others suckering can be a signal of stress. Understanding why suckers and watersprouts appear can help gardeners decide how, and whether, to treat the phenomenon.
Watersprouts usually appear when something has happened to the crown of the tree. Perhaps it’s been pruned back improperly or a branch has broken out. Regardless of the reason, the plant’s response is to create a new crown. Watersprouts can be a great diagnostic tool, though by the time you see them it may be too late to solve the problem. Here’s a dramatic example.
Uphill from my place in Poverty Flats is an expensive view home (big house, small yard) that has a single landscape tree. It’s an ornamental cherry, quite mature. A few years ago we noticed large watersprouts on the lower two limbs. The rest of the tree was normal—or so we initially thought. It turns out that the upper crown was slowly being girdled by neglected staking wire, which of course meant less food for the roots. It’s a shorter distance for water to travel from the roots to lower branches than to higher ones, so the tree was essentially bonsai-ing itself. An impaired root system can service a smaller, lower-crowned tree better than a full-sized one. And, in fact, a few years later this tree did fail, although inexplicably the homeowners have left the butchered remains for all to see. It’s a great teaching tool, but not a very good landscape design element.
There was nothing to be done in the Case of the Choking Cherry. Had these watersprouts been observed and their cause diagnosed early on, the tree probably could have been saved. Sometimes trees will send up watersprouts for no apparent reason—we have a dogwood that seems to sprout if I look at it sideways—but generally something has happened in the past that the tree is responding to.
The causes behind suckering are a little more complicated, but still easily diagnosed once you know the root causes. Let’s consider that red-leaved crabapple I mentioned at the beginning of the chapter. Many landscape plants have been bred for red leaves, but they often don’t have cold hardy rootstocks. So the red-leaved cultivars are grafted onto native rootstocks that are adapted to colder temperatures. In a successful graft union, the vascular tissues of the scion (the top part of the grafted plant) and those of the rootstock fuse and create a functional plant.
Now we have to look at the ultimate flaw in this practice. The native rootstock is adapted to the conditions where this tree is planted; the scion, probably not. Under the best circumstances, the rootstock is kept under control, servicing the scion but pretty much limited to life downstairs, as it were. But sometimes rootstocks revolt and send up their own shoots. The native species are green-leaved and vigorous, usually able to outgrow the red-leafed scion. It’s crucial for gardeners to remove these suckers as quickly as they appear or the rootstock will literally drain the nutrients away from the scion and eventually subsume it. The same is true for any grafted specimen, where the scion has been chosen for an unusual leaf color, crown architecture, or some other horticultural curiosity that renders it less vigorous than the rootstock. Working classes arise, indeed!
Watersprouts are vigorous upright branches.
Sprouts on Steroids
Many of the plants we cultivate for our gardens are mutants, with their arthritically twisted limbs, leaves splattered with white blotches, and forms of every shape and size. In the real world, these botanical oddities wouldn’t survive long because their mutations make them less likely to reproduce successfully. Yet even in our protective care, you’ll sometimes see an odd branch appear in the middle of a cultivated tree or shrub that is definitely not like the others. In dwarf species, like the Alberta spruce I mentioned earlier, this branch may be enormous in comparison. In variegated species, you might have branches with all-green leaves. These are reversions to the wild type, and they are always more vigorous than the cultivar. For this reason, you’ll want to cut these sports out as soon as you see them, because they’ll suck the lion’s share of resources from the rest of your tree. Nature always finds a way to overcome whatever constraints we put on it.
One Big Happy Family
The easiest way to kill a tree, slowly but surely, is to girdle its trunk. Girdling can happen accidentally, as when string trimmers and lawn mowers get a little too close for comfort. Or it can be a deliberate and often illegal act by disgruntled homeowners seeking to improve their views of nature by removing all vestiges of anything green. In either case, the complete removal of a band of bark and the thin layer of live tissue underneath effectively shuts down the transport of sugars from the leaves to the roots. The roots are alive, but they literally starve to death, because they can’t make their own food. Through this extended dying process, the roots continue to take up water to support the crown, but as their energy reserves dwindle they sputter to a stop. We finally see the results of this when the leaves wilt from lack of water. Mercifully, the end comes pretty quickly after this point.
Neglected staking wire means a slow but certain death for trees.
The wild hairs growing out of this dwarf conifer are sports.
On occasion, though, girdled trees appear to defy death and continue to grow happily, even without a continuous band of phloem running through the trunk to the roots. It’s most common to see this in groves of alders, willows, or poplars in wetlands near beaver dams. The beavers girdle trees in part to feed on the nutritious tissues underneath and, in the case of smaller trees, to create lumber for their building projects. So how do these lucky trees appear to transcend mortality?
It’s a combination of root physiology and genetic compatibility that saves these trees from becoming part of Chez Beaver. Underground, plant roots grow in all directions, frequently crossing over one another. If the roots belong to trees of the same or a closely related species, they will fuse or graft together as they thicken, effectively linking the two trees in a permanent, intimate relationship. They share water, nutrients, hormones, and anything else that can be carried through the xylem or the phloem. The partnership can be extended through many individuals, so that an entire forest of the same type of tree is joined by one vast, interconnected root system. When one member of the cooperative is damaged by girdling or some other environmental injury, its roots essentially become dependent on the neighbors for food. The aboveground portion of the tree remains alive and functional, and the entire grove benefits because competing species can’t grab a foothold, an obvious risk if a tree were to die, fall, and create an empty space waiting to be filled.
Root grafting is also important for gardeners to understand when they apply translocatable herbicides. These are herbicides, like Roundup, that move through the plant courtesy of the phloem and kill the entire plant, roots and all. Plants that are connected by roots, whether they are grafted individuals or clones along a stolon, will all be affected by translocatable herbicide applied to just one of their members. This can be a boon to gardeners hoping to take out a stubborn cluster of hedge bindweed, for example: you only need to spray a few of them to kill the entire group. On the other hand, there are unhappy, unintended consequences. Many a gardener has innovatively sprayed the annoying sprouts arising from a lilac or some other suckering species, only to find that the cherished original tree is also deleted from the landscape.
Finally, we need to remember the role our mycorrhizal fungal partners play in the underground railroad. Though it’s clear that water and nutrients can be shared between the fungi and plants using mycorrhizal connections, it’s not clear whether herbicides would move through the pathways. In fact, some mycorrhizal fungi are able to degrade herbicides and use them as a food source. This fascinating science continues to evolve, and for now gardeners can only marvel at the interconnectedness of life underground.
Root fusion may occur underground or aboveground.
Plants are masters of the ground game, with solitary individuals able to colonize huge swaths of land through cloning. The clone war might be a good strategy in such cases, but it creates a monoculture that can be wiped out by disease. It’s much better to do a little genetic rearrangement with another plant partner. But how to find that special someone when you’re literally a homebody?