When summers are long and hot, with little precipitation, how do plants survive? Learn about some of their evolved strategies which enable them to cope with inhospitable weather so you can make the best plant choices for dry soil beds and container gardens during hot, dry conditions. Please note that all new introductions to the garden, drought-resistant or not, need supplemental watering for at least the first year, until well-established. For a list of plant care tips during a heat wave, go to https://dbg.org/plant-care-tips-during-heat-wave/ .
Summer dormancy
While some plants avoid mid-summer drought by flowering, setting seed, and completing their life cycle before the onslaught of hot, dry, weather (annuals like Shirley Poppies, for example), others leaf out early in the spring garden, flower by mid-June, then go summer dormant, returning sometimes vegetatively in the cooler, wetter, fall or in the following spring.
Some examples of the latter are Oriental Poppies (Papaver orientale), native ephemerals like the stately Mayapple (Podophyllum peltatum) and Virginia Bluebells (Mertensia virginica), and several non-native species of ephemeral Fumewort (Corydalis spp.). For shade or part shade, the Fumewort I particularly like is hardy Corydalis solida, pictured below. The pretty bluish-green foliage appears in early spring, followed by pink flowers. The plant disappears by June, but returns the following year, slowly spreading with the help of ants. It’s an unusual plant for early colour amongst later appearing perennials.
Some of our native perennials also go summer-dormant or partially dormant in full sun during periods of drought, like Canada Columbine (Aquilegia canadensis), Hairy Beardtongue (Penstemon hirsutus) and Common Yarrow (Achillea millefolium).
Enhanced root systems for water collection and storage
Some plants, called geophytes, have underground storage systems (bulbs, tubers, corms, rhizomes) which enable them to survive drought, fires and cold winters. Colchicums, native to Europe, Asia, and Africa, are excellent examples. Most species leaf out from the corm in spring, then die back until fall. At that time, they flower profusely to the delight of native bees. Often misnamed Autumn Crocus, Colchicums have much larger flowers and 6 stamens. Easy to grow, they must be sited carefully to hide the dying leaves in June, and to allow full to part sun on the emerging flowers in fall.
Below, Colchicums in my garden, and a few recommended varieties. Purchase them in late summer and plant in groups.
Another cormous plant worth trying, if you’re in Zone 6 or more, is the Saffron Crocus, Crocus sativus, grown for its large stigma which is the source of saffron. Flowering in fall, this plant prefers hot, dry summers and naturalizes well in dry gardens as pictured below. It is propagated by division only.
Some of our native perennials are geophytes as well, flowering well in hot dry summers. These include Ontario natives Liatris spp. (Gayfeather), Allium cernuum (Nodding Onion), and Lilium philadelphicum (Wood Lily, pictured above). Gayfeathers and Nodding Onion are pollinator magnets in a sunny, native meadow or prairie garden.
Roots that go deep
Extensive and/or deep root systems are critical in most drought-resistant plants. Some have developed taproots like native Silphium species. These plants establish more slowly, while initially forming their 3+m long taproots. Silphium are tall plants, forming striking architectural features in the garden. Taproots are also found on smaller plants like Callirhoe involucrata (Purple Poppy Mallow), native to the Great Plains and adjacent states in the United States It is a low, scrambling plant, forming a long-blooming ground cover in full sun. Note that in general plants with deep taproots are difficult to move once established in the garden.
Below, left, native Silphium mohrii (Mohr’s Rosinweed), growing in a drought-tolerant meadow covered with 10 cm. of crushed concrete in Sussex, UK., and right, Callirhoe invulucrata (Purple Poppy Mallow).
A native shrub gaining in popularity for its long bloom time, ecological benefits and drought resistance, is St. John’s Wort (Hypericum spp.). A pollinator and, in particular, bumble bee favorite, it is a short shrub (45-90 cm), with bright yellow flowers and a deep taproot. Kalm’s St. John’s Wort (Hypericum kalmianum) is native to the Great Lakes’ region, growing in dry, sandy and rocky soils. H. kalmianum ‘Gemo’ (pictured below, right), is a hybrid available locally.
Native ‘prairie’ grasses are renowned for their extensive, deep root systems. Switchgrass (Panicum virgatum), a tall (1.5-2m) grass, and the much smaller Purple Love Grass (Eragrostis spectabilis), use their extensive root systems to enhance water collection and infiltration. The latter (pictured below) is a much underused warm-season grass that has striking purple seed heads in August. A small plant (20 cm), it shows well in smaller gardens at the front of a border or intermingling amongst shorter forbs.
A few additional recommended drought-resistant plants with taproots or extensive horizontal root systems include:
- Wild Lupine (Lupinus perennis)
- Coneflowers (Echinacea spp.)
- Cup Plant (Silphium perfoliatum)
- Thimbleweed (Anemone cylindrica)
- New Jersey Tea (Ceanothus americanus) (shrub)
- Shrubby Cinquefoil (Potentilla fruticosa) )shrub)
- Little Bluestem (Schizachyrium scoparium) (grass)
- Canada wild rye (Elymus canadensis) (grass)
- Sand Dropseed (Sporobolus cryptandrus) (grass)
- Grey Goldenrod- Solidago nemoralis
Adaptations of leaves and stems
Some drought tolerant plants utilize adaptations of their leaves and/or stems to minimize water loss and lower internal temperatures.
Succulent leaves and stems: Many of the best-known succulent plants come from the world’s driest habitats. Succulent tissues in stems and leaves store water, and often are covered by a waxy cuticle (surface) with fewer stomata which reduces transpiration. Succulents also have an adapted photosynthesis process (Crassulacean Acid Metabolism or CAM) whereby they fix carbon dioxide (CO2) at night. Stomata, the microscopic pores on the stem and leaf surfaces, open for most plants during the day to absorb CO2 and discharge O2. With succulents, the stomata open at night instead when temperatures are cooler, significantly reducing water loss. For this trait, look for plants in the Hylotelephium and Sedum genera with common names Stonecrop, Sedum, and Hens and Chicks. Below left, , the popular ‘Autumn Joy’ Stonecrop (Hylotelephium ‘Autumn Joy’), and, right, Cobweb Hens and Chicks (Sempervivem ‘Arachnoideum’).
Reducing water loss with fewer and/or smaller (narrow) leaves, sometimes modified into spines: Many of our sunny natives for drier, poor soils have narrow leaves, like Butterfly Milkweed (Asclepias tuberosa), Heath Aster (Symphyotrichum ericoides), and Narrowleaf Vervain (Verbena simplex).
As for spines, Ontario is home to the Eastern Prickly Pear Cactus (Opuntia humifusa). While rare in the province, it has been introduced in many gardens because of its ability to thrive in harsh, dry, conditions. The small cactus (30cm tall) has edible fruit, and beautiful, yellow, summer blooms.
Pubescence (surface hairs): Hairs reduce direct sun exposure on leaves, lowering leaf temperature, trapping moisture on surfaces, increasing humidity and reducing transpiration. Hairs also protect surfaces from cool nights. Pictured below is the drought-resistant native Pale Purple Coneflower (Echinacea pallida) which combines pubescence with an extensive taproot system.
There are many natives with hairy stems and leaves. A few to try: Hairy Beardtongue (Penstemon hirsutus) and Sky Blue Aster (Symphyotrichum oolentangiense) for sun, and White Wood Aster (Eurybia divaricata) for shade.
Pale or silvery stems and leaves which reflect sunlight and heat: Two unusual natives with silvery coloration for the dry, sun/ part sun garden are, below left, Pearly Everlasting (Anaphalis margaritacea) and Field Pussy-toes (Antennaria neglecta).
Popular drought-resistant non-natives I recommend for full sun are Russian Sage (Salvia yangii) and several species of Eryngium including Rattlesnake Master (Eryngium yuccifolium), below left, and ‘Miss Willmott’s Ghost’ Sea Holly (E. giganteum “Miss Willmott’s Ghost’) below right. The latter are pollinator magnets, attracting a wide variety of bees and beneficial wasps to the garden.
Growing in sand and gravel never looked so good!
For those who despair over dry, infertile soil, be encouraged by a lot of work around the world with gardens planted in a substrate of sand, gravel, and /or rubble. They demonstrate the importance of plant choice and plant communities. Here are just a few examples you may want to read about:
Peter Korn (renowned horticulturalist and author) and Julia Andersson have a beautiful, diverse and resilient garden in Sweden (pictured below) planted in sand.
- Peter Korn’s pioneering nursery and garden near Malmö | Gardens Illustrated
- Enter Sandman: The High Diversity Plantings of Peter Korn – The New Perennialist
Chanticleer’s Gravel Garden – “tough, resilient plants set in gravel mulch”
Virens, a Canadian west coast-based studio that offers eco landscape and planting design, inspired by Peter Korn, created their own sand garden. Their website outlines the process and result for the first year.