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.
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.