drought Archives » Botanish

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.

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.

It’s hot! It’s barely mid July and we’ve already had some 100-degree days in the Denver area. The hottest time of year here is the last week of July and first week of August, so we’ll be struggling with these temperatures for some time. Some parts of the world are, of course, even hotter. Heat stress forces plants against the upper limit of their adaptations, and they may require intervention.

Signs of heat stress in plants

When the weather is hot, water evaporates much more quickly from leaves and from the soil. This is especially true in low humidity, which is even harder on plants. Plant tissues that lose too much moisture will wilt, scorch or die back.

Symptoms of heat stress overlap with symptoms of drought stress, but there are some differences. Because of the way heat peaks in the middle of the day, plants suffer from repeated assaults rather than a gradual scarcity of water.

Wilting

A brugmansia (angel’s trumpet) wilts in summer heat despite growing in moist soil.

The most immediate sign of heat stress in plants is wilting. When wilting is related to heat, it is notable that plants can wilt even when the soil is moist. The roots are still taking up water, but they can’t keep up with the rate of evaporation from foliage. The short-term solution is to wet the soil even more thoroughly to increase the absorption rate. You can further ease the strain on roots by mulching with wood chips, straw or leaves to cool the soil.

Leaf curl

This sunflower has curled leaves caused by damage from heat and drought. The deformation resembles damage from pesticides or certain infections, but in this case it began during a heat wave and didn’t affect nearby plants with a better water supply, suggesting hot weather was the culprit.

When dehydration occurs repeatedly or lasts for days, it can change the way young leaves develop. Cells on the margins of a developing leaf can’t get enough water to expand, while leaf centers develop normally. The uneven expansion forces leaves to crinkle or curl as they mature, and sometimes the deformation is permanent. This type of drought stress is especially common on fast-growing plants like sunflowers, tomatoes and peppers. (Note: viruses and other diseases can also show up as leaf curl. But if the problem began in abnormally hot weather and leaves are not discolored or dying, the likely cause is heat stress). The typical solution to leaf curl is to water plants more regularly.

Scorched leaf margins

Ohio buckeye trees are particularly prone to scorching during hot days, in which the margins of the leaves dry out and die. Although the damage is unsightly, this young tree will survive and produce healthy foliage in the spring.

Severe or repeated heat stress can scorch mature leaves by dehydrating and killing the outer edges of the leaves. Scorched leaves will not improve and the plant won’t look better until it grows new shoots. Mulch, deeper watering, and shade protection on atypically hot days will reduce the incidence of scorch. Resist the temptation to cut off damaged leaves; they don’t look good, but they still photosynthesize and that helps the plant to grow new, undamaged leaves. Meanwhile, the dead, scorched tissue helps to shade and protect lower foliage from further damage.

Sunscald

Unlike wilting and scorch, which are primarily caused by dehydration, sunscald appears when plant tissues are killed by bright sunlight. In the same way that ultraviolet light burns human skin, high-energy photons from the sun damage the chlorophyll and DNA in plant cells beyond the plant’s ability to repair them. Scalded tissue is permanently bleached white or tan. It’s especially common on plants that have recently been moved or brought home from the nursery. To prevent sunscald, move plants to a brighter setting by acclimating them gradually with shade, part shade, light shade and finally full sun. That allows leaves to produce compounds that absorb excess light or free radicals and will prevent sunscald from occurring.

Thermal burn

Leaves can burn on exceptionally hot days, especially when air temperatures are above 100 degrees. Foliage in direct sunlight, or plant tissues in contact with rocks, gravel or metal reach lethal temperatures. Plants are literally cooked to death: the absolute maximum temperature plant cells of any species can tolerate is 140 degrees, while 100-degree temps can kill cells if they go on for hours and the plant is not acclimated to heat. Thermal burn on leaves resembles frost damage. Stems through solar-heated rocks or gravel can be killed from the base, cutting the plant off from the roots. On extremely hot days—the kind of heatwave that happens once every few years—only shading can protect foliage and it’s impossible to prevent some damage. Water and mulch soil with organic material to prevent thermal burns on stems. To protect plants from further damage, leave burnt leaves in place to provide shade until the heat wave is passed.

Yellowing leaves

This Brugmansia (angel’s trumpet) is showing signs of moderate heat stress, triggering the plant to drop its older leaves. Yellowing is a natural part senescence (natural aging and death) and the fact that the plant has time to selectively drop certain leaves means that the stress is not as damaging, but is occurring repeatedly or over a long period of time. Although the stress is mild, it does mean that growth is being slowed significantly and the plant may even decrease the amount of live tissue so that the roots can better support the canopy.

It’s normal and healthy for plants to drop older leaves and grow new ones, and plants do so regularly. Older leaves don’t produce as much energy for the plant, and plants regularly recycle their canopies. When leaves turn yellow before they fall, they are dying in a controlled process that usually doesn’t harm the plant. However, during times of severe stress, yellowing can accelerate and many or most leaves may drop.

Spider mites

Spider mites are barely visible to the naked eye, clinging to the sides or undersides of leaves and sucking the fluids out of plant cells. Mite damage appears as small, yellow dots that can expand and fuse together as the mites proliferate. Plants sensing this damage can drop all their leaves in an attempt to starve the mites out or make them go away. Mite-damaged perennials usually survive, but the damage can set them back for a season or severely stunt their growth. Smaller plants or annuals may die. Mites increase their activity in hot weather, and can overwhelm and kill plants in a severely drought-stressed garden. Water and humidity lower spider might activity. Insecticides, which kill beneficial predatory insects, can exacerbate the problem in outdoor gardens. The long-term solution is to water, mulch, and make sure gardens are full of heat-tolerant species planted at a density that the water supply can support.

Shock or early dormancy

Extreme heat stress can force a plant into dormancy as a last-ditch effort to survive. Trees will occasionally do this if they are transplanted in summer. Many cool-season grasses and lawns can go into and out of dormancy easily. Other plants will be left weakened or traumatized when they finally begin to regrow. Some plants are unable to go dormant at all, and just die. The solution is to water, and adapt your garden to drought so that it doesn’t happen again. If plants go dormant easily, it may be better to withhold water and let them stay dormant until cooler weather comes.

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Growing your garden’s resilience to drought