Heat Tolerant Plants for Iowa
Globally this summer is about to go on record as the hottest in recorded history, and Iowa has been no exception this year. Heat and humidity can be as stressful on plants as below zero temps, and if you struggled with plants that could not hold up to the stress, there are choices that thrive better than others. You may want to add some of these to your shopping list next year to make sure your planting mix includes genera that can tough out hot, humid Midwest summers.
Many of our native grasses and perennials are heat tolerant due to their evolution and adaptation to the occasional scorching heat often experienced in prairie states. Familiar plants include species of Rudbeckia (orange coneflowers), Echinacea (purple coneflower), Salvia (sage), Nepeta (catmint), Veronica (speedwell), Achillea (yarrow), Helianthus and Heliopsis (false sunflowers), Silphium (rosinweed, prairie dock and compass and cup plants), Liatris (gayfeather) and Solidago (goldenrods). For textural interest, mix in grasses such as Panicum (switchgrass), Andropogon (big bluestem) and Schizachyrium (little bluestem). In the shade garden Epimedium (barrenwort), Asarum canadense (Canada ginger), Polygonatum odoratum cultivars (Solomon’s Seal) and the Drypteris (wood fern) ferns handle more heat than most other shade lovers.
When I wrote Growing the Midwest Garden, the publishers loved lists of plants that would thrive under particular cultural conditions. If I had to pick out a dozen top performers for a summer like we have just experienced, they would include:
Geranium maculatum and related species (wild geranium)
Wild geraniums that naturalize readily in woodlands provide hardy genetic material for hybridizing many free-flowering cultivars durable in sun or shade and adaptive to less-than-ideal soils. Even though evenly moist soils are ideal for this species, it becomes extremely heat and drought tolerant once established. White, pink, rose, violet, and multi-colored flowers light up the woodland floor framed by dark green, deeply dissected palmate foliage that may assume rusty-red fall tinges. Some variegated forms exist; size varies by cultivar. ‘Rozanne’ is an outstanding lilac-flowering cultivar garnering much attention. I am particularly attracted to densely mounding, florific hybrids such as G. ´cantabrigiense ‘Karmina’. Selections are wide and appealing.
Epimedium species (barrenwort, epimedium, bishop’s hat)
When you see the shallow, rhizomatous root systems of barrenwort you may wonder how it can possibly be drought tolerant. During a recent severe drought when other perennials were drooping or dying, the foliage of this plant looked untouched by stress. I love this genus for tiny, delicate spurred spring flowers that “dance” over the foliage like ethereal tiny orchids in orange, yellow, red, purple, white, and mixed colors. Lanceolate foliage can display spots, toothed edges, or red to purple coloration and is evergreen in mild climates. It establishes as an excellent groundcover with medium growth rate. Favorites you will find in my garden include hybrids ‘Lilafee’, ‘Amber Queen’, ‘Pink Champagne’, ‘Spine Tingler’ (serrated leaves), ‘Dark Beauty’, ‘Pink Elf’, ‘Yokihi’, ‘Rubrum’ and ‘Princess Susan’ as well as the orange flowered ), E. x warleyense ‘Ellen Willmott’ and ‘Orange Queen’.
Polygonatum odoratum (Solomon’s seal)
Woodland Solomon’s seal is so robust it can become a thug. However, these tough attributes have made less aggressive cultivar ‘Variegata’ a desirable perennial. It is easy to grow in average soils with adequate moisture in sun or shade, but once established it becomes amazingly heat and drought tolerant. I have seen it effectively bordering driveways, difficult sites for most plants. The upright arching stems multiply moderately by thick rhizomes and are easy to divide and share. The white bell-shaped flowers are lightly scented although you may have to lean closer to arouse the nose fully. The flowers turn into autumn blue-black berries and the foliage turns a pleasing soft tan-yellow.
Sesleria autumnalis; S. caerulea (autumn moor grass and blue moor grass)
This is possibly the most underutilized of the ornamental grasses. It thrives in full sun but tolerates a great deal of shade and drought. I had a mass planting massed under a dense white ash that robs the area of moisture and sun and others perched above a stone retention wall in glaring sun and heat. It is cool season early emergent and makes a lovely clump of narrow, fine foliage no more than a foot high. It is charming massed or as border specimen. Autumn moor grass blooms with shiny white small flowers in late summer and blue moor grass has glaucous foliage flushing greenish flowers in spring. Not to be confused with Molinia moor grasses.
Asclepias tuberosa (butterfly weed)
A great selection with the current interest in saving monarch butterfly populations, once established it forms a deep, deep taproot that allows it to survive intense sun, heat and drought. It is a native perennial with flat-topped orange or yellow flower clusters at terminals of stems or in its leaf axils. It produces late season clusters of brightly colored flowers that attract insects including many butterflies followed by fruit and long dry seed pods typical of milkweeds. The fact that it is often found along railroad beds, in dry fields and prairies and along roadsides indicates how tough it is. ‘Hello Yellow’ is a bright yellow cultivar.
Baptisia australis and related species (blue false indigo)
You might swear blue false indigo is a shrub. This member of the legume family develops great mass in mounds of trifoliate glaucous foliage and roots so deep you will never enjoy dividing and transplanting it. It tolerates extremely challenging conditions and rewards gardeners with stiff, long racemes of indigo-purple flowers. Recent breeding efforts have produced cultivars with chocolate, cream, apricot, lemon, bi-color, blue, and violet flowers such as the Prairieblues series from Chicagoland Grows and the Decadence series from Walters Gardens. B. sphaerocarpa (yellow false indigo) flourishes bright yellow flowers. B. leucantha (white false indigo) and B. alba var. macrophylla (largeleaf false indigo) are smaller white-flowering forms native to prairies.
Calamintha nepeta subsp. nepeta (calamint)
Tough. Massively florific for a long period of summer. Aromatic. Issue free. Covered with every insect imaginable, especially bees. The attributes of this durable plant are many and I continue to use it heavily. The small bright white flowers are mounded on bushy plants giving it a baby’s breath impression under conditions Gypsophila would never survive. It is a superb performer above zone 7 range, below that it languishes.
Callirhoe involucrata (poppy mallow)
Poppy mallow is a native wildflower that sprawls with summer-long display of bright neon magenta-dark pink flowers, long branches spread out across the ground to create colorful mats of flowers and foliage well into fall. Some people feel it is aggressive but I love the way it winds through and around other plants providing bright splashes of color. Its long taproot makes it difficult to transplant once established but gives the plant excellent heat and drought tolerance. It may self-seed and can spread. C. alcaeoides ‘Logan Calhoun’ is a spreading relative with pure-white flowers.
Coreopsis species and hybrids (coreopsis, tickseed)
A large number of coreopsis are available but use caution and research hardiness to your locality. Many recent hybrids of C. ´grandiflora hybrids have not been as durable as their hardiness zone designation. Most tickseeds are native to various parts of the United States but they are not equally hardy to cold climates. Darryl Probst has been crossing more durable hybrids including his Big Bang and Permathread series. If you are in zones 5 and colder, I highly recommend starting with this series, I have been having excellent success with them. C. verticillata (threadleaf coreopsis) and C. auriculata (mouse-ear tickseed) cultivars are much hardier. C. tinctoria (plains coreopsis) is an annual that has been used in hybridization of many of the garden cultivars so you can see the need to research true hardiness. Tickseed are bright sunny flowers in the landscape and non-hardy cultivars do make excellent container plants.
Echinops ritro; E. sphaerocephalus (globe thistle and great globe thistle) and Eryngium species (giant sea holly; sea holly, small globe thistle)
Globe thistle may not appeal to everyone but I love its unique strongly architectural structure and steel-blue color. A tall upright perennial with metallic purple-blue, spherical flowers and spiny leaves attract bees; they work well fresh cut and dried. E. ritro is a compact species with cobweb-like, woolly foliage; stainless blue maturing to late summer bright blue fading to everlasting brown. E. sphaerocephalus forms a bushy, upright mound of gray-green leaves with large, rounded silver-white flowers that contrast beautifully with sturdy, reddish stems. Eryngium are, in my view, the smaller cousins. They appear as reduced versions of globe thistle featuring open umbels of prickly steel-blue flowers that attract butterflies. It is a very hardy genus, small flowered, and spreading into small patches. They are tolerant of hot dry sites and soils high in salts. E. giganteum (giant sea holly) boasts two of the most spectacular forms: ethereally silver ‘Miss Willmott’s Ghost’ and ‘Silver Ghost’. Cultivars of E. planum (sea holly) include ‘Jade Frost’ and ‘Sunny Jackpot’, both with variegated foliage; extra blue ‘Blue Glitter’; silver-flowered ‘Silver Salentino’; and dwarf ‘Blue Hobbit’. E. ritro (small globe thistle) stands tall at 4′ with golf ball-sized steel blue flower heads on rigid stalks with deeply lobed, dark green, thistle-like foliage. ‘Veitch’s Blue’ is the most common cultivar. All make excellent cut and dried flowers.
Gaillardia aristata (blanket flower)
There has been a large influx of blanket flower cultivars on the market; unfortunately many are hybrids of hardy G. aristata and G. pulchella (Indian blanket) which is native but an annual which doesn’t survive winters and heavy soils. Blanket flower detests heavy soils but will thrive in sandy, well-drained media. Described as “sunshine on the western prairie,” it blooms in golden beauty with red-tinted centers; some cultivars have a red ring around the center of the outer petals. The subdued silvery green leaves vary in shape and stay low growing. If you want blanket flowers as perennials, find aristata forms, such as ‘Amber Wheels’, ‘Oranges and Lemons’, ‘Sunburst Burgundy Silk’ and ‘Sunburst Tangerine’, otherwise treat most cultivars as annuals or short-lived perennials at best.
Verbascum hybrids (mullein)
This large genus of more than three hundred species has a multitude of common names including beggar’s blanket and old man’s flannel. In England the poor put thick leaves into shoes for warmth. Some are dwarf; others are towering giants but nearly all have fuzzy stamens, lamb’s ear-like foliage and prefer very well drained sandy or rocky soil. Most garden forms are hybrids of V. chaixii although large, woolly 8-foot forms such as V. olympicum (candlewick) are gaining popularity as statement plants. Hybrid mullein produces tall flower spikes in colors including white, yellow, gold, tan, purple, pink, and red.
prepared by Ed Lyon, Reiman Gardens’ Director | photo caption: Baptisia australis