Some perennial flowers can take all the heat a central-Pennsylvania summer can dish out and grow on with no ill effects.
Others, not so much.
High heat has the ability to wilt leaves, shut down flowering, stunt growth, and maybe even kill some species — especially recently planted ones.
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Read George’s tips on how to keep a young plant alive in a hot summer
Knowing the difference between heat-tough perennials and heat-wimpy ones is becoming more and more of an issue as our summers are getting hotter for longer.
Plants native to cooler climates generally struggle more than plants from areas where 90-degree summer heat is the norm (Africa, Mediterranean regions, the southwestern U.S., etc.)
That’s because plants from hot areas have developed coping features that prepare them for our Julys and Augusts (not to mention June heat waves). These include deep root systems, spines or leaf hairs that reflect heat, and succulent or waxy leaves that store moisture and slow evaporation loss.
Perennials from cooler climates don’t have those protections and are more at risk of summer heat injury. Examples of ones that struggle in our hot summers include delphinium, lady’s mantle, lupine, and verbascum.
If you need some help shoring up the yard for heat-tolerance, here are 12 blooming perennials to put at the top of the list. All can tolerate the heat even into the deep South and are cold-hardy in Harrisburg-area winters as well.
‘Black Adder’ is a heat-tough agastache that blooms purple for weeks in summer.
George Weigel
Agastache
Nicknamed hyssop, this under-used summer-long bloomer sends up licorice-scented spikes of bluish-purple and is a favorite of pollinators.
‘Blue Fortune’ is a popular variety, ‘Black Adder’ is a purple-blooming variety, ‘Pink Pearl’ is a compact new pink-bloomer, and the Summerlong series comes in five colors with extra-long bloom time. Most grow two to three feet tall. (Some agastaches are borderline winter-hardy in the Harrisburg, so check plant labels for USDA Cold Hardiness Zones of 6 and lower.)
‘American Gold Rush’ is one of the best black-eyed susans, a popular type of rudbeckia.
George Weigel
Black-eyed susan (
Rudbeckia
)
This long-time favorite native produces showy, golden flower petals around a central dark cone and also blooms for up to two months in mid-summer.
‘American Gold Rush’ is hard to beat for flower power and disease-resistance. It grows two feet tall.
Blanket flowers (Gaillardia) are some of the heat-toughest native perennials. This one is ‘Heat It Up Yellow.’
George Weigel
Blanket flower (
Gaillardia
)
Another U.S. native, blanket flower is one of the longest-blooming perennials, producing red, gold, or red/gold daisy-like flowers most of the summer – especially when spent blooms are dead-headed.
‘Fanfare’ and ‘Goblin’ are two good 18-inchers.
Compact versions of Russian sage make a long-blooming choice as a heat-tough perennial. This variety is ‘Jelena.’
George Weigel
Russian sage
This one’s been catching on lately because of its heat- and drought-toughness and new, compact varieties, which make Russian sage garden-friendlier than older, floppier three- and four-footers.
Plants produce purple to blue-purple spikes for weeks. Compact two-foot versions include ‘Denim ‘n Lace,’ Lacey Blue, and the even shorter Little Lace.
Their succulent leaves make sedums durable in heat and drought. This variety is a dark-leaved one called Rock ‘n Grow ‘Back in Black.’
Walters Gardens
Sedum
This cold-hardy succulent comes in groundcover forms, but for the perennial garden, bushy two-foot types that bloom pink in late summer are solid, heat-tough, drought-tough choices.
Good varieties include ‘Neon,’ ‘Brilliant,’ ‘Autumn Fire,’ and the dark-leafed Rock ‘n Grow ‘Back in Black.’
Hardy hibiscus ‘Midnight Marvel’ has dark leaves and giant red flowers.
George Weigel
Hardy hibiscus
Our native hardy hibiscus is a big, head-turning, shrub-like perennial that can grow four feet tall and wide with huge, dessert-plate-size flowers of red, pink, or white in July.
The whole Summerific series is excellent, and ‘Midnight Marvel’ offers bright red flowers set on a dark-leaf background.
Coreopsis ‘Zagreb’ is a heavy bloomer in addition to tolerating heat well.
George Weigel
Coreopsis
Coreopsis or “tickseed” is another heat-tough native and one that produces coin-sized gold, yellow, or red daisy-like flowers in early summer. When cut back after bloom, most of the thread-leaf types will bloom again in late summer.
‘Zagreb’ and ‘Golden Gain’ are two of the best varieties. They grow about 18 inches tall.
Daylilies come in a variety of bloom colors and deal with hot summers well.
George Weigel
Daylily
It’s hard to kill a daylily, even after deer repeatedly eat them and the increasingly problematic leaf-streak disease turns them brown soon after blooming.
When all goes right, though, daylilies produce lots of trumpet-shaped flowers in all sorts of colors and color blends. Many of them rebloom.
You’ll find tons of varieties, and most of them are at least good performers.
Native to the Southwest, gauras are good at coping with high heat. This variety is ‘Pink Panache.’
George Weigel
Gaura
Native to the hot U.S. Southwest, gaura is a little known perennial with butterfly-like pink or white flowers growing at the tips of wiry stems in July.
Good varieties include ‘Siskiyou Pink’ and ‘Whirling Butterflies’ (pink bloomers) and ‘Crimson Butterflies,’ a pink bloomer with red-tinted foliage. Gaura grows 18 to 28 inches tall, depending on variety.
Coneflowers are native perennials that tolerate high heat and bloom for weeks. This variety is ‘Pica Bella.’
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Coneflower
A top native choice for pollinators, coneflowers are heat-lovers that traditionally bloom in purple-pink in mid-summer. New varieties come in many other colors as well, including light and dark pink, white, red, and orange. It’s hard to go wrong with any variety you’ll find.
Coneflowers range in height from 18 to 36 inches tall, depending on variety.
Perennial salvias are both deer-resistant and heat tough — and most rebloom somewhat in early fall if cut back. This one is the popular ‘May Night’ variety.
George Weigel
Salvia
One of the few flowers that deer will hardly ever even sample, salvia is a reliable perennial that produces spikes of purple, blue-purple, and sometimes pink. May is the main bloom month, but after a cutback, most salvias will bloom again in late summer.
‘May Night’ and ‘Caradonna’ are two of the best, ‘Blue By You’ is a heavy-blooming newcomer, and ‘Marcus’ and Sensation ‘Deep Blue’ are foot-tall compact choices vs. the usual 18- to 24-inch heights.
Catmint is one of the longest blooming perennials, and it’s also virtually bullet-proof against heat and deer browsing.
George Weigel
Catmint
One last heat-loving and virtually bullet-proof perennial is catmint, a blue-blooming, low, mounding plant that isn’t aggressive as most true mints can be. The foliage is silvery-gray – a nice contrast to the little blue, long-lasting blue flowers.
‘Walker’s Low,’ ‘Cat’s Meow,’ Junior Walker, and ‘Purrsian Blue’ are among the top performers.
5 other heat-tough perennials for shadier spots
Barrenwort
. A perennial with heart-shaped, red-tinted leaves and pink, yellow, or white spring flowers.
Hosta
. Leafy plants come in all sorts of shades, including many variegated options. (For non-deer-plagued areas only.)
Lenten rose (
Helleborus
)
. Very early to bloom, these produce nodding, bell-shaped flowers of pink, white, burgundy, and more.
Coralbells (
Heuchera
)
. The wide leaves that come in a range of showy colors (gold, caramel, rose, nearly black, etc.) are the main attraction.
Bleeding heart
. Native fringe-leafed types bloom pink or white for weeks, while old-fashioned bleeding hearts bloom in early spring and go dormant from summer on.
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Read George’s post on 12 annual flowers that can take the heat
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Read George’s post on 10 shrubs that can take the heat
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Read George’s post on trees that can take our future heat
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Read George’s post on how to garden in the “new normal” that climate change is bringing
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