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Do trees die of old age?

Everything in nature dies. Human beings can live close to a century, but few do, and no one has lived past 122. Dogs live on average 8 to 14 years. Some tortoises can live to be 200. In vertebrates, changes on the cellular level give animals an upper limit on lifespan. Cells can only divide so many times before they’re compromised, making older individuals much more frail and vulnerable than youthful individuals.

Plants are fundamentally different: the larger and more established plants are, the higher their survival odds become. Out of 1,000 seedlings, only one may live to become a tree. But once a specimen is mature, it can be extremely resilient. Many common trees have potential lifespans in the centuries, towering over rivals and growing stronger every year.

Despite this trajectory, it’s uncommon to see single-stemmed trees grow past a thousand years old. So what limits them?

This is one of those questions I love, because there are so many angles to understand it from.

A tree is a system of redundancies

Unlike vertebrates, plant cells do not age. A rooted cutting from an ancient tree is identical, on the cellular level, to a young plant.

In fact, even in old trees, the live cells—leaves, roots and cambium—are young. Old leaves fall off, and new leaves grow. Feeder roots—the thin, water-absorbing tips and branches of the robust structural roots—also die and regrow cyclically, the same way leaves do.

The xylem—the tubes that carry water and minerals up a tree—are already dead. These cells are the most important type of tissue in determining what makes a tree a tree. Xylem cells form cylinders and die as soon as they mature, becoming conduits for fluid. Because they’re dead, xylem tubes cannot self-repair. Within a few years they develop tears and air pockets that stop them from working, so new cells replace them. At their final stage, defunct xylem cells are the wood. They are the old growth rings on the inside of the tree, existing only to hold it up. Their replacements, new xylem tubes, continually form on the outside of the trunk’s circumference, as new growth rings.

Phloem cells—which transport hormones and sugars from the foliage to the roots—also die annually. Unlike xylem, they are soft and rubbery, not rigid. They are also still living when they do their work. When spent, they die and collapse to a fraction of their former size, and are ultimately pushed outward as bark.

Between the phloem and xylem is a thin ring of tissue—the secondary meristem—which originates all the structural cells. It is rapidly dividing, and cells there migrate in to become xylem and eventually wood, or outward to become phloem and eventually bark.

Thus, all tissues in a tree are constantly turning over. The tree is less like a body, and more of a system. A tree’s young living layer clings to the skeleton of its own dead xylem, like a modern city on ancient ruins, or the live surface of a coral reef. All the while, it is growing in size. With time the system progresses through a structural evolution that defines the stages in its life cycle.

One mechanism that gives trees more resilience is redundancy. Remove a branch, and the loss of its hormone signal will resonate through the rest of the tree. Other branches grow more vigorously to correct for the loss, restoring the ratio between roots and shoots. If a fire or animal damages the vascular cambium on one side of a tree trunk, the cambium on the other side of the tree will thicken to compensate. Meanwhile, cells around the edges of the wound will grow inward to hopefully seal it off.

The hormone signals plant parts send throughout the plant are the only way a twig has any awareness, so to speak, as to whether it is a lower branch on a big tree, or a seedling on a forest floor. Aside from environmental stimuli—light, water, temperature and nutrients—hormones control how different sections of a tree cooperate. For example, if you remove a twig and root it, the hormone signals it receives will change. It is now much closer to cytokinin-producing roots and not getting suppressing auxins from higher foliage. In response, it will begin growing more vigorously, taking on characteristics of a seedling.

Now with that background info out of the way I can get more directly to the question: if the tissue in the tree does not age, and it continuously resets its behavior based on ongoing hormonal cues, why don’t trees live forever?

The problem is the accumulation of defects, which, over time, can overwhelm repair processes. The tree eventually reaches a point where it is much more difficult to keep things working properly.

Plants do not have cellular aging, but old trees suffer structural defects

• First: bigger trees usually have a higher ratio of unproductive (non-photosynthesizing) tissue compared to productive tissue (foliage). A seedling has leaves, a short stem, and roots. In a big tree, the leaves and branches are much higher and farther from the root tips. That means there is a long span of vascular tissue that must be kept alive. Trees expand their canopy as they grow, but the ratio between the massive cambium covering the trunk and branches, and the finite horizontal surface area determining the canopy’s access to sunlight, increases. The vascular cambium needs to maintain complete coverage over the heartwood to protect it from decay. Over time, there’s less energy to invest in new growth, so bigger, older trees grow more slowly. They’re also less able to correct structural defects like wounds.

• As a tree grows, the redundant parts communicate using hormone signals to work as a cohesive whole. Genetics, size and environmental stressors determine the size and circumstances in which the tree transitions to its mature stage. Gradually it uses resources to produce flowers, fruit, and seeds, investing less in growth. Some long-lived species may also invest more in energy storage when they get big, slowing growth further.

• The most important limit on the lifespan of old trees is decay in the heartwood. Even healthy middle-aged trees have a few pockets of decay here and there, but the tree’s ability to add new wood each year keeps pace so the tree remains strong. In very old, very massive trees, it is more difficult to defend a vast surface area, especially when decay pockets begin to coalesce inside the tree.

(One of the limits on the size and age of douglas firs, a very big and long-lived tree, is the fact that virtually all wild trees live with a decay fungus called dyer’s polypore. The fungi grows slowy, and the trees can be ancient before they eventually fall, so it’s not harmful to the species as a whole. But theoretically, if the polypore were not there, doug firs might be able to outgrow redwoods).

• In natural settings, competition limits the lifespan of old trees. Size provides incredible advantages reaching light and absorbing water, but there’s a point when the benefits max out. A big tree doesn’t have the opportunity to shrink its tissues to a more manageable size in a drought or disaster without exposing its dead heartwood to the environment and decay. Any loss of canopy requires a corresponding loss of roots, allowing other plants and trees to colonize soil and challenge the old tree’s dominance. Ancient trees tend to develop large sections of exposed, decaying heartwood, which means big portions will eventually break off. This means they often lose their tops, resprouting foliage from lower down. That costs them the advantage of height, while they still face the burdens of high tissue mass and very large sections of exposed heartwood that lead to continued breakage. An ancient tree may go through multiple cycles of breakage or dieback and regrowth. Each time, it accumulates a greater burden of decay, since the cambium is less and less of an intact cylinder covering the structure. Eventually, trees cannot compete against their less burdened neighbors, and die off while middle-aged trees assume dominance.

One other thing, more speculative on my part: plants get viruses, and viruses are not curable in plants. Usually, the plant continues to live, but less vigorously. Many plant viruses are asymptomatic, and their only effect is a metabolic burden leading to more stress and less growth. Luckily, most plant viruses do not get passed on to seeds. So there may be a point when the viral burden is high enough that the old tree is struggling too much and a seedling tree would be much better off. However, there would have to be more study into old trees and the presence of asymptomatic viruses for me to decide whether this is a realistic factor.

Why are there so many dying trees? What emerald ash borer damage looks like and what we learn from it

If you follow news about trees and gardening, you’ve probably been hearing for many years the ominous news of a devastating invasive insect called the emerald ash borer.

The emerald ash borer, a shiny green beetle destroying ash trees across North America, emerges fron a D-shaped burrow in ash tree trunks in early spring.

History of the emerald ash borer in North America

Ash trees are—or were—one of the most common tree types in North America. Our native ash species include green ash, black ash, white ash and a few others, with extensive natural ranges as one of the most dominant tree caregories in the eastern U.S. They’re also popular in yards for their dense shade and tolerance against late cold snaps, summer heat, periodic drought, and soil compaction found in developed areas or under pavement.

Ash trees, a genus with many species, grow wild across an extensive range spanning North America, particularly in the eastern part of the continent. Additionally, ash trees are one of the most common trees planted in urban areas.

In addition to the North American ash species, there are other ash tree species found around the world, including northwestern Asia where ash trees have contended with the emerald ash borer for millennia. The trees co-evolved with the beetles, making the trees resistant to severe infestation just as the beetles became completely dependent on the ash trees to complete their life cycle.

The pill-shaped, iridescent green flying beetle lays its eggs on bark. The larvae burrow beneath to feed on the carbohydrate-rich cambium layer, the green living tissue just under the bark that lays down new growth rings each year and carries sugars from leaves to roots.

After growing beneath the bark until fall, emerald ash borer grubs pupate through winter and emerge as beetles to infest other ash trees. When a beetle finds a fitting ash tree host, it produces pheromones to attract many others to come in, overwhelming the tree’s ability to drown them out with sticky sap.

In ash trees with no resistance to the emerald ash borer, the larvae burrow so extensively under the bark that they completely cut off the flow of carbohydrates through the vascular system to the roots.

Asian ash trees have long adapted to the presence of the borer; North American trees have their own species of slower-growing borers that can damage but rarely kill the trees. Asian ash trees survive infestation from the more aggressive borer species by producing chemicals that slow the insects’ digestion and thus slow their growth, among other adaptations. That way, they can create new tissue faster than the borers eat it, and achieve a healthy equilibrium with the beetles.

North American ash trees have no resistance to the Asian beetles, so when the first infestation was discovered in Michigan in 2002, it set off sirens for ecologists concerned about the future of several vitally important tree species.

The borer population exploded. Within years, ash tree forests across the Midwest and eastern Canada were wiped out. Entire pure stands of ash were converted into shrubby meadows, with ominous dead gray trunks standing over them, as if a wildfire had burned through. Shortly after, mixed forests lost their ash tree stocks. Traveling a few miles per year, the borers spread throughout the region, and occasionally hitchhiked on firewood or other human transport to arrive in other portions of the continent.

Ash borers invading the continent have resulted in vast swaths of dead forests in the Eastern U.S. and Canada.

Infestations in Boulder County, Colorado began several years ago, initiating a government quarantine against ash wood products being moved from Boulder to Denver or beyond. But when the borers were identified in the northern Denver suburbs in 2019, the quarantine was retired since there was now no way to stop them from flying tree to tree and infiltrating the entire metro area.

The documented extent of emerald ash borers in North America as of summer 2020.

In 2021, this is the first summer we’ve seen extensive damage to virtually all of the untreated ash trees in the region. While it’s uncommon to see adult beetles, the signs are impossible to miss.

How ash borers kill ash trees, and how to identify the damage

Emerald ash borers destroy the part of the tree that carries sugar from the leaves down to the roots (the outer layer of cambium or phloem), but they do not stop water from rising from the roots to the leaves, which travels through nonliving tissue (xylem) in the inner cambium. Because the xylem carries only water and minerals, it doesn’t have the carbohydrates borers need to grow and the grubs won’t eat it. That means that the tree can produce leaves for a while even after the infestation is advanced.

But without phloem to carry sugars made in the leaves down to the roots, the entire root system begins starving. It is unable to produce an adequate number of annual feeder roots that sprout from the larger woody roots. Unlike other tree diseases that may kill a section of the canopy at a time, the entire tree’s system for staying alive begins to fail. The appearance is similar to a tree that has a girdling root, or is being girdled by a grate or has experienced other extensive damage to the roots: the canopy thins from the top down and starts retreating.

An ash tree with an advanced ash borer infestation has already suffered extensive damage to roots, starved because the insects have cut their supply of energy from leaves. The canopy, in turn, dies back from the top to try to achieve a more manageable size. It happens on all branches at the same time rather than in sections, which would be a sign of a different kind of pathogen.

During the first season, an ash borer infestation can go completely unnoticed. The first subtle sign is either woodpeckers showing an unusual interest in the tree or small, D-shaped holes in the bark where the first generation of beetles hatched out to mate and lay more eggs.

Trees that are already catastrophically damaged by ash borer can continue to produce dwindling leaves for a year or two because the part of the circulatory system that carries water—located deeper in non-living wood—is still intact. But there’s no hormone communication between the roots and leaves, and no sugar making its way down to feed the roots. Soon, the roots die back and reduce their ability to absorb enough water for the whole tree at its current size.

A tree with inadequate roots, no matter the cause, thins out at the top (which is the the most distant point for water and hormones produced by roots to reach). Trees cope with severe root loss by trying to restructure themselves: they sacrifice leaves and branches that are no longer getting enough water and nutrients to photosynthesize, which starts at the top. Ash trees will produce new green shoots from large branches or trunk, and the new growths can be very vigorous and dense. However, their vigor is deceptive since the total number of leaves is much lower. This would effectively make a smaller tree, which in some scenarios would return to balance with the weakened root system and allow the canopy to regenerate, albeit with a very poor branch structure.

Unfortunately, in the case of an ash borer infestation, these attempts at restructuring the canopy are futile—the root system is not going to rebound since it has been separated from the canopy by ash borer tunnels. The root system is already dying by the time the top level of branches is completely bare.

Usually, the severe damage becomes obvious in late spring when higher portions of an ash tree will fail to leaf out or the tender new leaves die off shortly thereafter. You don’t see much progression in the middle of the growing season since the dying roots can continue to provide a limited water supply to lower leaves. Meanwhile, chunks of bark may begin to curl back or fall off since there’s nothing attaching them to the underlying wood.

You also don’t see much severe wilting and browning in the canopy from ash borer. Browning and singing of the leaves, which occurs, for example, in apple and pear trees affected by diseases like fire blight, suggests a cause that is affecting the ability for water to reach the leaves through xylem. Xylem exist below the upper cambium in channels made of columns of cells that have died and become like hollow straws (the wood’s grain). Bacteria or fungal diseases exist as single cells or filaments that are one cell wide, so they can grow right into the water-carrying xylem channels and shut them off. Infected twigs and branches wilt and turn brown in days.

But because the xylem on infested ash trees are still intact, leaves stay hydrated and don’t yellow or wilt dramatically. Yet, the tree, sensing a nonproductive branch, eventually cuts off the water supply to limbs and retreats further down toward its base.

Later stages of ash borer infestation in a tree

Even green shoots from the base of a mature ash tree infected by emerald ash borer will eventually die as the root system dies, but younger trees, or trees that produce shoots through the soil itself, can form permanent shrubby base growth as they die. The ash borers eventually leave and move on, since they only eat living trunk tissue, and only lay eggs on trunks and branches that are more than a few inches thick.

In trees with basal shoot growth that outlives the dying tree, it’s due to new root systems emerging from the base of each green shoot where they contact moist soil. These bushy, limited growths are vulnerable to drought until the roots grow larger, and they won’t be useful as shade trees. They will only be infested again if they reach a substantial size.

In the last phase of an ash borer infestation, the borers may move on since the part of the trunk they eat is now dead. The tree will produce new shoots from the base, but even these will continue to decline since the root system is so severely starved.

Mature, valuable trees can be protected with systemic insecticides applied before or early in the infestation. But trees that are completely girdled by borers are too far gone even if they still have living portions of canopy, since the roots are dying. Arborists can make a judgment call as to whether the tree is salvageable, usually deeming trees to be too far into the process if the top of the canopy has lost 50 percent or more of a healthy density of leaves.

Asian ash tree species, or hybrids between North American and Asian species, may have enough resistance to ash borers to survive after the first big wave of infestation has passed through a region. Horticulturalists and ecologists are already working hard to develop new ash tree stocks that can resist the borers and potentially restore some of the ash tree options for urban trees. They may even be able to develop strains of native trees to reseed American forests.

Lessons from the emerald ash borer

We as gardeners and tree lovers can take a few valuable lessons from this slow-moving disaster. First and foremost, we need to be conscious of the threat of invasive species. This is not the first time a wave of disease or pest insects threatens to wipe out a major class of North American trees, and since other types of tree epidemics are still making their way around the continent we know it won’t be the last.

Better inspections of plants coming into the U.S. can help, and by carefully observing the environment around us, we may be able to notice, slow down, or eradicate ecological threats before they become unstoppable. County extension offices, which are local branches of state agricultural universities that exist to communicate with the public about landscape health, can be a resource to funnel information to the attention of trained scientists.

A second lesson is to encourage a broader diversity of trees and plants in our urban landscapes. In the recent past, Denver’s urban canopy was dominated by a handful of tree species or families. A loss of one species means a loss of a huge portion of the mature urban canopy, leaving tremendous gaps that take decades to fill with newly-planted trees. By recruiting new species into cultivation (particularly natives in the region), and making sure that each park, yard or city block has several types of trees in it, we can hedge against the chance that a future tree epidemic leaves huge portions of the city bare.

Finally, we can be conscious of the benefits of inviting a diverse array of wildlife into our spaces. Although pests like the ash borer are sometimes unstoppable, they do have natural predators here: woodpeckers, with many native species in this area, eagerly eat emerald ash borer larvae and have actually grown in population in places where the ash borers provide a generous food supply for raising chicks. Other microbes, insects and birds can help compete against current and future destructive pests as long as they have a large enough base population to respond quickly to a major change.

By planting diverse seed-producing gardens, tolerating a moderate amount of native insect pests providing steady food sources, converting rooftops, pavement and gravel strips to open landscapes with lots of region-appropriate plant species, and limiting the use of chemical controls to occasional spot-treatments only, we can create a balanced, abundant urban ecosystem that provides shade and beauty to humans and animals alike, for generations to come.

Helpful Concepts In Gardening: How Plants Decide When To Recycle Leaves

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.

Growing your garden’s resilience to drought

A drought isn’t merely an absence of rain or snow. In fact, many ecosystems around the world—and gardens designed to mimic them—are adapted to thrive despite long gaps without precipitation. If a dry period is part of a region’s natural climate cycle and doesn’t threaten local farms or the native ecosystem, it won’t be called a drought. On the other hand, a rainy region that experiences a heatwave or a partial drop in precipitation that stresses plants can be in drought even if it continues to receive some moisture.

Plants survive regular dry periods through a variety of adaptations. They store water (cacti and succulents), reach deep long-term reserves in the soil (tap-rooted plants), or gain the ability to slip into dormancy and regrow leaves when rains return (grasses are particularly good at this). Beyond all that, plants will grow at the density and size that the climate and soil allows. A reliably-wet region will grow into dense forests, tall prairie or lush marshland. A dry or variable climate will have clumps of short bunchgrass or scrub surrounded by gaps of bare earth.

Any of these ecosystems can be healthy and well-balanced. A stable, old-growth desert or chaparral landscape can be captivating and beautiful. In a garden, we seek to replicate that equilibrium by planting the right kinds of plants at the right density to match the amount of water we plan to give it.

Plants in vs. wet landscapes — In most natural ecosystems on Earth, the limiting factor on the size and density of plants is water. Plants will naturally fill in to the level that the climate and soil moisture can support before reaching an equilibrium. Although deserts carry fewer or smaller plants than the redwood forest at right, they still form attractive, captivating landscapes and gardens when they are balanced and healthy.

Yet even if you garden with well-adapted native plants, or water frequently, droughts happen. Healthy plants grow and multiply, bigger plants take up more water, and at some point they start to deplete the soil moisture and reach their limit. Eventually, your garden will max out its resources and become susceptible to drought stress. This can happen even if you are still watering! Your goal, as a gardener, is to help your plants approach the level in which they max out the carrying capacity of the garden and soil as a gradual plateau, rather than growing overly lush until a summer heatwave drops it off a cliff.

In Denver this year, we’re definitely feeling that cliff. This is a semiarid climate with an average of 14 inches of precipitation per year. But it anything between 8 and 20 inches is fairly normal, and most gardens here are irrigated. In mid-August 2020, we’ve had 6 inches of precipitation so far this year (about a third below normal) and are experiencing successive 95+ degree days that increase evaporation from plants and soil. Even gardens designed to be “drought tolerant” require supplemental irrigation right now, and many are looking stressed despite the help.

In the last post I went over some of the signs that plants are being stressed by hot summer weather. But no one wants to be dumping excessive amounts of water on the garden to try to revive traumatized plants, only to see them wither again the next day. So here’s how you prevent that from happening in the first place.

Encouraging a resilient garden

Mulch

Arbor mulch—a grinded mixture of sticks, bark, leaf fragments and blocky chunks of trunks and branches—is the best mulch for cooling soil and reducing water loss. It’s the best mulch for perennial beds, superior to stone or bark. But any organic mulch, including grass clippings or straw (which I prefer in vegetable gardens), will help cool and insulate soil.

Pea gravel mulch is an option in desert-themed gardens. Gravel heats up in the sun and doesn’t retain soil moisture as well as organic media, so it should be used where plants are well-adapted to dry heat. Some gardeners will choose it because it’s easier for reseeding plants to germinate in a thin layer of gravel than wood chip mulch, and gravel is more visually harmonious with cacti and succulents. Additionally, there are rare cases where certain plants like cold-hardy agave and other xerophytes (drought-loving plants) are vulnerable to crown rot in woody mulch.

Group plants by similar water needs

A garden needs as much water as its thirstiest plant. That’s what determines how much you need to water the garden to keep that plant alive. Spot-watering with a drip system can help account for some variability, but water spreads horizontally through soil, so you end up watering a larger area than one plant needs. The most efficient strategy is to section of larger plots of 10 by 10 feet or more according to the plants’ water needs.

Water deeply and infrequently

The concept of watering deeply and infrequently is confusing, or even counter-intuitive sometimes. Why would watering a lot, all at once, save water? Or why letting the ground dry out sometimes help plants stay hydrated? The answer to these questions will reveal a lot about the way plants grow.

The concept is basically this: plants’ roots will penetrate wherever the soil contains enough water and oxygen. Often, in gardens irrigated in short bursts every one or two days, that’s the top four inches of soil. Watering deeply—keeping the irrigation on for a long time so that it can penetrate more than a foot deep—makes sure that roots find an abundant water supply if they keep growing downward.

At the same time, watering infrequently serves three purposes: it allows the upper levels of soil to dry out, which limits surface root development and causes plants to direct their energy to the deeper roots; it allows soil pores to drain so oxygen can reach the lower layers of soil and enable deep root growth; and it conditions plants to toughen their tissues and moderate their growth so that they won’t be traumatized by intense summer heat.

A deep-rooted plant is more resilient to drought because there is a bigger, longer-lasting water supply in the deeper layers of soil. Deep soil is also safe from temperature swings, particularly heat, that can injure roots.

But the strategy takes consistency. It’s not enough to begin a deep watering regime in August when the heat wave is at its peak; if deep roots aren’t already there, you’ll be stuck watering daily to keep your plants alive.

Summary

Water is usually the biggest factor in how big and lush plants can grow on a site. When vegetation grows dense enough to use most of the available water, new growth will slow and stabilize.

Drought happens when soil moisture drops below normal, meaning there is now more vegetation than the soil can support. Plants will begin to show signs of stress. Gardens may become less attractive and more vulnerable to pests and disease.

Gardeners can limit drought stress by recognizing the amount of natural precipitation they get and the amount of irrigation they plan to provide in a specific spot, and planting accordingly.

Arbor mulch—wood chip mulch that comes from the disposal of whole trees and branches—has been shown to be better at preserving soil moisture than gravel.

A layer of straw or grass clippings can help preserve soil moisture and are better suited for vegetable gardens, where the soil is disturbed more often.

Group plants by water needs to make them easier to care for, and plant at a density that the site can support.

To encourage deep, healthy root systems, water deeply (long enough for water to soak deep into the soil) and wait a longer time before watering again, rather than applying small amounts of water on a daily basis.