A Gardening Journal

Summer Beauty Acanthus

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When most other plants have dropped their leaves, Acanthus foliage is still fresh green.  Only days before Thanksgiving, the plant's exciting flowers of Summer are but a fading memory.  It's the foliage that's the thrill now.  It tolerates a remarkable amount of frost, week after week, when so much of the rest of the garden is bare stems and bare ground.

 

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Acanthus is a triumph—translation, a struggle—here in New England, even through 'Summer Beauty' is one of the hardiest cultivars.  No surprise, given that it's a native of Spain and Morocco, whose Winter climates may be cool and rainy, but rarely even frosty.  I'm grateful for this lush foliage, then, in the way that folks who garden farther south could never be, where Acanthus can be just a workhorse and even a thug. 

 

There are many other Acanthus to lust after, experiment with, and—if you can Winter them over and then convince them to flower—crow about.  See "Variants" below for highlights of this remarkable plant genus, and "How to handle it" for strategies to welcome as many as possible to your own gardens.

 

 

Here's how to grow this impressive beauty:

 

Latin Name

Acanthus 'Summer Beauty'

Common Name

Summer Beauty acanthus

Family

Acanthaceae, the Acanthus family.

What kind of plant is it?

Flowering perennial; evergreen in mild climates.

Hardiness

Zones 6 - 8

Habit

Clumping and broad.  Growth is dense enough to work as groundcover; that ability is enhanced by the large and somewhat overlapping leaves.  Tall spikes of flowers soar far above the foliage.  Although the specifics of foliage and flower (let alone hardiness) are quite different, in these general aspects, Acanthus 'Summer Beauty' looks and functions like a large hosta.

Rate of Growth

Slow to establish unless circumstances are ideal.

Size in ten years

Size is dependent on congenial siting and climate.  Slower and smaller in colder climates.  Where its happiness is well assured—in solid Zones 7 and 8, plus in almost any soil that has excellent Winter drainage, and with some afternoon shade—you can expect a foliage clump five or more feet across and over two feet tall.  The spikes of flowers can soar to more than five feet.

Texture

Dense but not boring; authoritative as well as detailed.  Even the tall flower spikes have authority, with none of the ethereal fragility (as thrilling as that is) of other "spikers" like, say, the delphinium.  See more details in "Grown for," below.

 

Almost no description—including this one—fails to enthuse that a well-grown Acanthus brings an architectural presence to any garden.

Grown for

its vigor:  Acanthus is a strong groundcover—perhaps too strong in climates where it thrives, where it can spread easily by both roots and seeds.

 

its mounding foliage:  The dark green leaves are shiny (shiniest when young, true) as well as toothily notched and lobed.  They spring directly from the ground, and then arch up, out, and over.  Their size, geometry, and overlapping array were inspiration for the capitals of Corinthian columns starting in the 4th century BC in ancient Greece, and continuing today, through millenia of imitations, homages, and parodies in civic, commercial, residential, and royal architecture world-wide.

 

its tall spikes of flowers:  With Acanthus, "spike" has two meanings.  The snap-dragon-like flowers crowd the top majority of a tall and unusually self-supporting stem, the flower spike.  Also, each flower is hooded as well as "subtended" (hooded from underneath) by a bract tipped with a spine that's sharp, rigid, and long enough to draw blood.  The bracts of 'Summer Beauty' are a dusty purple, but the flowers themselves are white.

Flowering season

Mid-Summer on the rare occasion when I get any flowering at all.  Spring in milder climates.

Color combinations

Although the white flowers, alone, would go with anything, their dusty-purple bracts call for neighboring plants that feature blue, pink, grey, and white.  But because the plant is so tempting but also so temperamental at the cold end of potential hardiness—lower Zone 7 down into Zone 6—just getting it to survive and bloom usually trumps aesthetics.  If my best potential location for Acanthus were already spoken for by plants that bloom in red as well as deep yellow, I'd still plant 'Summer Beauty' there.

Partner plants

If your climate and your garden's specific conditions—see "How to grow it," below—permit Acanthus to be so solidly hardy that you can focus on its aesthetics rather than its mere survival, then the wider possibilities of the sculptural foliage, vigorous and even aggressive energy, and impressive "Don't mess with me, dude" flowers beckon. 

 

The large leaves suggest more finely-detailed partners such as ferns, ornamental grasses, thread-leaf perennials or Japanese maples, or small-leaved shrubs like box-leaf honeysuckle, Lonicera nitida, or (duh) box itself.  If their foliage can be in a contrasting color, so much the better.  Acanthus, like hosta, welcomes some afternoon shade, too, so better to think of partner foliage that's brighter, not darker.  Don't forget to choose partner plants that also appreciate afternoon shade, instead of sulking because of it.  Additionally, the Acanthus bracts are dusty purple because there's really a dash of pink in them, so your partner plants will be more congenial visually if they veer into white or grey variegation rather than yellow.  All together, now: Shade-tolerant, with brighter but non-yellow colors if possible.  Painted ferns, not Russian sage.  Leucosceptrum 'Silver Angel', not 'Golden Angel'.  Variegated liriope or carex, not Hakonecheloa macro 'Aureola'.  Lonicera nitida 'Silver Beauty' instead of 'Baggesen's Gold'.  If I were gardening in Zones 8 and up, I'd vote for planting Pittosporum 'Marjorie Channon' near my Acanthus.  

 

Acanthus partners equally well with larger details, from the stone slabs of walls and walkways, to wide expanses of gravel, to the trunks of large shrubs and trees, for where its shade tolerance is particularly helpful.  

 

Because the foliage is engaging right out to the tips of the downward-arching leaves, Acanthus is best with foreground details, if any, of minimal height.  A mulch of black river stones would be powerful, as would a carpet of mondo grass, Ophiopogon planiscapus.  If there's enough light for it to maintain thick growth, black mondo, Ophiopogon planiscapus 'Nigrescens', would be high drama, indeed.  Other prostrate or very low foreground horticulture possibilities for part-shade, whose coloring also goes with dusty purple, include Cephalotaxus harringtonia 'Prostrata', Lysimachia nummularia just in its green species form not the chartreuse, and, of course, vinca and pachysandra.

Where to use it in your garden

If hardiness isn't a challenge for you, Acanthus can be a space-filling groundcover near large trees or along paving.  But its striking foliage, form, and flowers also make it worthy of prominent and even focal siting.  Perhaps as a high and mighty groundcover for an exceptionally ornamental tree?  What about a silver-leaved pear, Pyrus salicifolia 'Pendula', or a pollarded silver-leaved willow, Salix alba 'Sericea'?  Or at the feet of oak trees, whose dappled shade and deep roots (the antithesis of large maples on both counts) welcome shade-comfortable underplanting.

 

The full and arching foliage, let alone the striking spikes of flowers, also suggest using Acanthus in large containers, even if only for the warm months.  This is also an end-run around all the challenges of growing Acanthus in-ground where it's marginally hardy.  See the strategy for "How to handle it—another option or two" under 'Skyracer' molinia

Culture

Almost any decent soil as long as Winter drainage is impeccable.  Acanthus is not the plant for lean soil, even though it would make that sensational Winter drainage all the easier to achieve.   

How to handle it: The Basics

Drainage, drainage, drainage:  The rule of thumb for beech trees—never plant on level ground!—is in effect for Acanthus as well, whose survival is very affected by culture, and primarily by winter drainage.  Plant Acanthus on a slope, then, no matter how gentle.  Full sun is fine unless the location is particularly brutal in the Summer, in which case afternoon shade would be welcome.  Acanthus is shade-tolerant, as well, so if it's readily hardy for you, you can plant it almost anywhere you'd plant a hosta.

 

Think first before you plant, though, because Acanthus is impossible to eradicate without using herbicides:  The least bit of root left behind will sprout a new plant.

 

Clear away the foliage after Winter has finally killed it.  If you're growing Acanthus where it's not reliably hardy—anywhere in Zone 6 and even, for the truly adventurous, dipping into Zone 5—it's even more important to protect it from Winter wet than Winter cold.  Merely mulching the plant, in fact, could just help it rot faster, because the mulch itself will absorb and hold water, and act like a spongy reservoir to keep the soil it covers even moister than ever. 

 

Do mulch, but on a sunny day, and with mulch that's dry.  Use big nugget mulch, which won't absorb much water, doesn't pack down, and allows for some cross ventilation.  Make a mound of it eight inches high right over the clump, and sloping outward to the ground all around; the mound will be two feet across or even a bit wider. 

 

And then put some sort of impermeable shield atop it, to keep the mulch dry through the Winter.  This could be a short section of something rigid—PVC pipe or drainage conduit—cut in half lengthwise to make a tunnel.  The pipe or conduit would need to be at least 18 inches in diameter; a section two feet long would be enough.  But much easier and cheaper is to use cardboard from a box you've opened out into a flat sheet.  Center one of the seams right over the Acanthus clump, as if you're giving the mulch a gently-sloping roof like that of a split-level house.  Use a rectangular sheet of cardboard big enough to slope right out to the ground on two sides, leaving the mulch at other two sides exposed.  This lets air into the mulch, and even under the cardboard, but not sharp wind or icy rain.  Keep the cardboard in place by covering it with more mulch.

 

In early Spring, after danger of ice and snow is past, you can remove the cardboard and, scuffling down throught the nugget mulch with your fingers, check if the clump is resprouting.  Don't worry if it's not; Acanthus are slow after what, for it, is a serious Winter.  Cover the clump back up—including the cardboard top—and check back in two or three weeks.  When you finally uncover fresh growth, leave the cardboard off, but leave the mulch in place around but not over the new shoots.  After Spring is truly underway and serious frost is no longer a danger, remove all the mulch so the sun can warm the soil, too.  Your Acanthus is ready to welcome the new season.

 

See "Downsides," below, for how this Fall and Winter protection increases the chances of flower production in Spring. 

How to handle it: Another option—or two?

Grow Acanthus in large ornamental containers.  See the strategy for "How to handle it—another option or two" under 'Skyracer' molinia.

 

Quirks or special cases

Growing your Acanthus in containers year-round isn't enough?  "Extreme" mulching and cardboarding isn't enough?

Downsides

In locations where it's solidly hardy, Acanthus can easily become too much of a good thing, spreading by both roots and seeds.  Worse, those same mild climates are where slugs and snails can be particularly nasty, and they enjoy nothing as much as a good munch on Acanthus leaves.  This is yet another reason to consider growing Acanthus in large containers, regardless of whether it's hardy in-ground for you or not.

 

Where Acanthus is only borderline hardy, the foliage may still emerge faithfully, and even lustily, but flowers are rarely produced. Those new leaves are probably a sign that the entire base of the plant is regenerating itself from portions of roots that had penetrated too deep to be killed by the cold. (This ability to regrow from deeply-buried root remnants is a talent also shared by dandelions and horseradish.) But where Acanthus is solidly hardy, it flowers in Spring, meaning that at least the nubs of those flowers were formed during the Winter or even the previous Fall. And, alas, of the base of the clump is killed over the Winter, so are those precursors to its Spring flowers. Increase the chances of successful overwintering of the base of the plant, not just its lower root tips, by trying the "extreme mulching" described, above, in "How to handle it: Another option—or two!"     

Variants

With about thirty species in the Acanthus genus—and an easy and almost incestuous familiarity among them when it comes to spontaneous hybridizing—there are more than a few Acanthus to consider.  A. spinosus has foliage that's particularly toothed, lobed, and palmate—let alone spiny—and it's a full zone hardier than A. mollis: well down into Zone 6 provided that Winter drainage is perfect.  A. 'Jeff Albus' is probably the same as 'Rue Ledan', with large foliage and white flowers as well as bracts; finally, Acanthus blooms to partner with deep yellow or red.  A. hungaricus is midway in foliage between A. spinosus and A. mollis.

 

A. mollis 'Holland's Gold' has solid chartreuse leaves, for the spiky-leaved coloring—and similar sun and shade tolerance—of a 'Sum and Substance' hosta.  Here's the Acanthus foliage to partner with deep yellow and red.  

 

Acanthus m. 'Tasmanian Angel' is the first of the variegates, whose young leaves are heavily marked in white, and with bloom spikes strongly marked in bubble-gum pink.  No, really.  'Whitewater' foliage is even whiter, and the flowers and bracts are almost pure white, with the flower stems brightly suffused with plum.  Incredible, both.  But because they're bred for looks not toughness, they're not likely to thrive unless conditions are uncompromisingly ideal. 

 

Although I doubt there's any Acanthus that's truly ugly, hybrids are usually selected, first, for hardiness, and only second for aesthetics.  And with greater tolerance of cold as well as high-humidity heat that these hybrids offer—or, at least, say they offer—gardeners farther and farther north and south have hope of more easily satisfying their Acanthus hunger.

Availability

On-line.

Propagation

Division in the Spring.  Sections of the roots will also sprout plants; like yucca or horseradish—or dandelions—the smallest pieces left behind after an attempt to transplant (or eradicate) will produce new plants. 

Native habitat

Acanthus 'Summer Beauty' was hybridized in China, and is probably a mixture of A. mollis, native to countries bordering the western Meditteranean, and A. spinosus, also native to countries bordering the western Meditteranean.

 
 
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