Characteristics of Drought-Tolerant Perennials

Why do some perennials sail through prolonged hot, dry weather unscathed while others wilt, turn brown, and bloom poorly if at all?  To answer the question, let’s take a look at the characteristics of drought-resistant plants as well as strategies for dealing with drought conditions. But first, what do we mean by drought?

DROUGHT CONDITIONS DEFINED

Simply defined, a drought is a period of prolonged dry weather when the soil is depleted of moisture due to a lack of precipitation or irrigation.  Here, in central Virginia, a typical drought can last from several weeks to several months.  On the western half of the country, droughts can last for years.

In addition to prolonged dry weather, drought stress on plants may be exacerbated by high light intensity, drying winds, and prolonged high temperatures.  According to Lee Reich’s book entitled The Ever Curious Gardener, “High temperatures cause plants to desiccate and consume stored energy faster than it can be replenished.  Stress begins at about 86 degrees Fahrenheit, with leaves beginning to cook at about 20 degrees above that.”

IDENTIFYING DROUGHT-TOLERANT PERENNIALS

At the end of the 2023 growing season, members of the Piedmont Master Gardeners Association in the Charlottesville/Albemarle County area participated in an informal survey on drought-tolerant perennials. The goal was to share information on species that performed well over the summer and fall despite the prolonged drought conditions. The observations gleaned from the survey resulted in a list of approximately 50 drought-resistant perennials that met or exceeded expectations. The interesting thing about these plants is that they exhibited many distinguishing characteristics that make them suitable for hot, dry conditions.  Most of the species listed are native, but some non-natives are also included.  Space doesn’t allow listing all 50 plants here, but a few representative species include:

• Asclepias (milkweed)
• Coreopsis (tickseed)
• Dendranthema (hardy chrysanthemum)
• Eryngium (rattlesnake master)
• Gaillardia (blanket flower)
• Panicum virgatum (switchgrass)
• Salvia (sage)
• Sedum
• Solidago (goldenrod)
• Symphyotrichum (aster species)

This was a very informal survey with mostly predictable results.  However, there were a few surprises:

• Some sun-loving species benefitted from shade. For example: In one garden, Solidago ‘Fireworks’, a popular clump-forming goldenrod, flowered normally in full sun.  However, the bloom time seemed shorter than normal.  Meanwhile, the bloom time on a nearby ‘Fireworks’ clump that received some afternoon shade lasted several days longer.
• Some moisture-loving plants performed better than expected. In another garden, Lobelia siphilitica, a moisture-loving plant, bloomed well in a rain garden setting despite the lack of moisture.  That it performed as well as it did was surprising. More surprising, “volunteer” Lobelias that had sprouted nearby in a drier spot also performed well. As luck would have it, they sprouted near a house foundation wall, which apparently held some moisture in the soil.
• Drought tolerance varied within species. In general, asters are drought tolerant, but some varieties performed better than others.  Aromatic asters (Symphyotrichum oblongifolium), which bloom in late fall, showed no signs of drought stress.  But New York asters ( novi-belgii), which typically bloom earlier in the fall, were clearly stressed with brown leaves toward the bottom of stems, discolored leaves on upper stems, slightly smaller blossoms, and fewer blossoms.
• Performance varied from site to site. Pink muhly grass (Muhlenbergia capillaris) performed well for one master gardener in their amended red clay soil.  However, two other master gardeners reported that theirs performed poorly. One of the gardeners mused that wet conditions over the previous winter might have damaged the root system.

While more study would be useful, the following preliminary conclusions can be drawn from this informal survey:

• As growing seasons become warmer, some perennials normally grown in full sun may benefit from shade, particularly during the hottest part of the day.
• Some moisture-loving perennials may thrive in drier soil if grown near a rock wall, building foundation, or other structure capable of holding moisture in the soil.
• Drought tolerance may vary among members of a given species with some varieties showing greater drought resistance than others.
• The degree of drought tolerance may be site specific with regard to soil composition, drainage, amount of sunlight, and other variables that occur from garden to garden.

CHARACTERISTICS OF DROUGHT-TOLERANT PERENNIALS

As mentioned earlier, in exploring what makes a perennial drought tolerant, a number of specialized evolutionary adaptations or characteristics were noted. For example:

Small or narrow leaves:  The smaller the leaf, the better it can reduce foliar evaporation and shed heat.  Examples: Threadleaf Coreopsis (tickseed), Perovskia atriplicifolia (Russian sage), Achillea millefolium (common yarrow), and grasses with narrow blades.

Thick waxy cuticles:  All plant leaves have a cuticle, which is a waxy coating that protects the leaf’s cellular tissue and reduces moisture loss.  Some plant species have exceptionally thick cuticles that reduce evaporation.  This characteristic is particularly beneficial in windy sites.  Examples:  Sedum species, hellebores, and some ferns such as Christmas and holly ferns.

Deep root systems:  Long tap roots reach deep into the soil in search of moisture.  This characteristic allows plants to survive periods of surface-level drought. Examples:  Asclepias (milkweed), Baptisia australis (blue false indigo), and some native grasses such as Panicum virgatum (switchgrass).

Hairy or fuzzy-textured leaves:  Fine “hairs” or fuzz provide a little shade.  They also slow the movement of air over the leaf’s surface, trap humidity, and slow evaporation of water from the leaf. Examples: Salvia argentea (silver sage), Stachys byzantina (lamb’s ear), and Verbascum (mullein).

Reflective foliage color:  Gray-green or silver leaves reflect light and heat from the sun, which reduces evaporation and keeps the plants cooler. Examples:  Artemisia (wormwood), Santolina (lavender cotton), and Cerastium tomentosum (snow-in-summer).

Fleshy stems:  Thick, often gel-filled stems can store moisture for long periods of time.  Examples: Opuntia (prickly pear cactus), Yucca, Agave, and Echeveria species.

Slow growth rate:   Slow growing plants such as cacti and succulents have fewer leaves and stems to support during dry periods.

Age of the plant:   Root systems on newly planted perennials are generally small and inadequate for handling drought conditions.  After a year or two in the ground, the roots should be more extensive and better adapted for drought tolerance.

SIGNS OF DROUGHT AND HEAT STRESS IN PERENNIALS

Some signs of drought and heat stress to look for in perennials include the following:

• Leaves may take on a dull look initially. As stress from lack of water deepens, the leaves may curl or wilt.  These signs indicate that the roots are not taking up sufficient water.
• The plant may droop or splay open in the middle. If the problem is not detected in time, the plant may not rebound even after receiving supplemental water or rain.
• Plant growth slows and may stop altogether resulting in stunted plants.
• Flower buds may not develop resulting in fewer blossoms. If they do develop, the flower size may be smaller than normal.

STRATEGIES FOR DEALING WITH DROUGHT

Choosing suitable water-wise perennial species is just the first step in creating a drought-tolerant garden.  The following are some strategies for establishing them and keeping them as healthy and vigorous as possible so that they are better equipped to thrive in hot, dry weather:

• Know your planting site conditions. Choose perennials that are suitable for your specific growing conditions.  Among other things, this means knowing: the soil type (sand, clay, humus, or some combination), how well it retains moisture, how well it drains, the amount of sunlight it receives, and the pH.
• Avoid planting in windy sites, which tend to dry out fast. If this is not possible, then try blocking the wind with strategically placed vines, shrubs, trees, or trellises.
• Plant small specimens that will adapt to the soil conditions faster than larger specimens and ultimately require less water.
• Group plants together with similar needs for sunlight, nutrients, and moisture. Either avoid plants that require a lot of water or group them near a water source.
• Space out new plantings so that they are not crowded and amend planting holes with compost or cow manure to help hold in moisture.
• Keep new plantings watered for their first season or two to allow them to develop an extensive root system.
• Apply a layer of mulch or compost around new plantings to cool the root zone and help retain moisture. If you’re trying to avoid using a lot of mulch, plant ground covers that will fill in the spaces around new plantings and shade their roots.
• Water established plantings deeply and infrequently, preferably using drip irrigation, to encourage roots to expand deeper into the soil. Water in early morning or in the evening to curtail loss of water through evaporation.
• Keep planting sites weed free to prevent competition with perennials for the same moisture, sunlight, and nutrient resources.
• Divide plants often to stimulate new root growth and improve plant vigor. In general, most perennials benefit from being divided about every 3 to 5 years.
• Provide shade.  The combination of too much direct sun and extreme heat will stress all but the toughest, most drought-tolerant plants.  For perennials that suffer from heat and drought stress, site them near shrubs, trees, or physical structures that will provide some shade during the hottest part of the day.

THE TAKEAWAY

The updated 2023 United States Department of Agriculture’s Plant Hardiness Zone Map drives home the fact that our climate is becoming warmer. Extreme weather events such as lack of precipitation and prolonged high temperatures are making gardening more challenging than ever. To compensate, we need to be more selective in the species we grow. Our choices are made easier if we are cognizant of the characteristics that make a plant drought tolerant and if we apply a few simple gardening strategies to keep the plants healthy and well maintained.

Feature Photo of drought-tolerant Agastache ‘Black Adder’ and Heliopsis ‘Summer Sun’ by Pat Chadwick

SOURCES

A Gardener’s Guide to Botany (Zona, Scott, PhD, 2023)

How Plants Work, The Science Behind the Amazing Things Plants Do (Chalker-Scott, Linda, PhD, 2015)

The Ever Curious Gardener: Using a Little Natural Science for a Much Better Garden (Reich, Lee, 2018)

Charlottesville Weather in 2023, Extreme Weather Watch

Native Plants and Cultivars for the Northern Piedmont, Piedmont Master Gardeners Website

Perennials:  Culture, Maintenance, and Propagation, Virginia Cooperative Extension Publication 426-203

Drought Stress Impacts on Plants and Different Approaches to Alleviate Its Adverse Effects, National Institutes of Health, National Library of Medicine, National Center for Biotechnology Information Published online 2021

How to Recognize Plant Heat Stress, Utah State University Extension

Plant Hardiness Zone Map, 2023 United States Department of Agriculture

Soil Management in Home Gardens and Landscapes, Penn State Extension article