A Gardening Journal

Cardoons

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The astonishing silver foliage of cardoons persists well into Winter. Indeed, the challenge is that young cardoons don't know when to pack it in, and will be killed before Spring without protection. If they survive their youth, they are surprisingly tough, long-lived, distinctive, and even dramatic as adults. Here's to a plant that's a role model. 

 

Young foliage seems to emerge with its lifetime allotment of silver as well as featheriness. 

 

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By the time the leaves have expanded to their full adult size, the silver has been "diluted" by having to cover a much larger surface area. The pointed and ruffled leaves that were so delicate when small have grown to the size of palm fronds.

 

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Their great length and weight makes them prone to snapping, as has happened to most of the mature leaves in these plants. Their high moisture content makes them much more susceptible to damage from freezing, too. The mature leaves are frayed and broken, while the youngest seem unfazed even after having spent two weeks buried in snow.

 

See "How to handle it" for how to help young cardoons survive their first Winter. If they do, they'll grow a bit less dramatically, but far more sensibly. And they'll bloom in August, with immense artichoke flowers so colorful they seem phosphorescent. And they'll have every hope of doing so for decades to come.

 

Again, what a role model.

 

 

Here's how to grow this unique perennial:

 

Latin name

Cynara cardunculus 

Common name

Cardoon

Family

Asteraceae, the Aster family.

What kind of plant is it

Clumping perennial.

Hardiness

Unprotected, Zones 7 to 9; into Zone 6 with protection. Also spectacular as an annual.

Habit

First-year plants produce an enormous single rosette of arching silvery leaves that look like palm fronds. In their second and subsequent seasons, multiple rosettes arise from the root-stock, forming a denser and somewhat smaller clump of foliage. Thick branching stalks bear enormous flowers, each about half the size of a large globe artichoke, high above the foliage.

Rate of growth

First year: Very fast. Subsequent years: More measured, but still impressive.

Size in ten years

First-year plants can become huge, to four feet tall and five feet or more wide. If the plant survives its first Winter, growth from then on is, comparatively, less generous, about three feet tall and wide. Clumps that have perennialized bloom each Summer. The tall stiff stalks can reach eight feet in Zone 8 and 9 climates that combine a long growing season with cool Summers, such as that of Great Britain. In climates typical of eastern North America—with comparatively cold winters and hot summers, and also a shorter growing season—flowering spikes are usually around five feet tall.

Texture

The frond-like leaves are feathery when young, but they expand many times by maturity, by which time they seem only sparingly divided. First-year clumps have the architectural size, outsized foliage, and the open presence typical of castor beans and coppiced paulownia trees, whose hasty first-year growth races right past more measured considerations of proportion, density, or durability. Perennialized cardoon clumps are contrastingly dense and "realistic" in scale—about the size of a large peony clump.

Grown for

its foliage: Silvery when young, the basal leaves of first-year plants mature to a chilly celery green. The leaf-to-leaf gradation of silver to celery-green is most prominent in first-year clumps, and is subtle as well as showy.

 

It's also conceptually intriguing: It may well be the case that emergent leaves possess their entire lifetime supply of whatever it is that creates their thrilling silveriness; tiny surface hairs are usually the source. As leaves expand, their cells enlarge to their mature size. The leaves might not, therefore, grow in the sense of adding new cells; they may enlarge simply because extant cells become fully "inflated." This enlargement means that the silvery component—those tiny hairs, say—becomes distributed over a larger and larger surface area. Then there would be more and more space between hairs, allowing more of the underlying color of the leaf (green, of one hue or another) to show through unmediated by the surface silveriness. The silvery color has become, literally and visually, diluted by the leaves' taking on more and more water to expand their cells. It might also be the case that the silvery component isn't as long-lasting as the leaf that bears it; leaves would seem to lose their silver hairs by "dilution," and would also lose them by outright disappearance. If a leaf were enlarging more from the creation of new cells than from the full inflation of the ones that leaf were "born" with, it would be more probable that the silveriness of the leaf would remain more constant through its lifespan.

 

Mature first-year leaves can be four feet long, and are the epitome of voluptuousness. A first-year plant looks as much like a trunkless silver-leaved palm as a perennial.

 

Leaves of established clumps are somewhat smaller, but much more numerous because established clumps produce multiple rosettes of foliage simultaneously. This means that the showy gradation of silveriness from emergent to mature foliage isn't nearly as well displayed as it is in first-year clumps; the foliage is just too crowded to serve such a specialized aesthetic goal. Nonetheless, perennialized cardoon clumps are still impressive, both in foliage and overall scale, especially when compared to the size and foliage of more typical perennials.

 

See "Where to use it in your gardens" for help in siting cardoons to their best advantage, whether using them as annuals or perennials.

 

 

its high-Summer flowering, which is automatically the high point, in coloring and in scale, of any garden's perennial display. Fist-sized buds are only about half the size of globe artichokes but, by any other standard, they are huge. They top themselves with a half-dome of ray flowers, which are unusually vibrant because the long lavender petals project far above a tightly-packed plate of the other flower parts, whose overall color is raspberry. As is typical for thistles, the flowers are favorites with bees, hummingbirds, and butterflies.

 

 

its permanence: Cynara cardunculus can be a challenge to establish, but if it can be perennialized it's as permanent as peonies. Clumps can grow without division for decades.

 

 

its imperviousness to browsers:  As is typical for thistle-like species, Cynara is not at risk from browsers. This is achieved, in part, via subtle but irritating spines. It's best to wear gloves when handling cardoons.

Flowering season

High Summer: August, here in New England.

Color combinations

The silver foliage, lavender ray-flower petals, and raspberry flower parts place Cynara cardunculus firmly in the world of pink-friendly horticulture. Blue, white, rose, burgundy, and grey are its best companions. The flowers are showy from a long distance; site even farther away than usual from orange, red, and yellow.

Partner plants

for cardoons used as annuals: First-year plants' quick size, hunger for full sun, and dangerously-cantilevering foliage all make Cynara challenging to use as an annual. But there's nothing else, hardy or tender, cheap or dear, that can provide their "silver palm tree" look. They also need easy access to plenty of loose and nutritious soil, so can't be used as later-season filler amid Spring bulbs and ephemerals, or as an accent plant "punched" into a sea of vinca. Instead, plant cardoons and companion plants at the same time, as part of a unified display that will, as a whole, continue to luxuriate through hard frost. Although the cardoon won't flower, its architectural look and scale would make a mockery of companion plants with "pretty" flowers, even if they would otherwise be appropriate in habit and seasonality. (Here's hoping none of us ever have to see a cardoon underplanted with impatiens.) Further, although part of the appeal of a first-year cardoon is its overall size, the plant is engaging in its details. Companions that are low and mounding, at least by comparison, will be able to celebrate their own qualities without seeming to be distracting from the nuances of the bigger cardoon show nearby.

 

Companions with few but telling details work best. For perhaps the first time ever, I can't think of how variegated foliage would help. The cardoon "fronds" bring all the pattern and color gradations needed. Solid colors, please!

 

What about a large foreground of one of the heart-leaf forms of purple sweet potato? If small plants of Ipomoea batatas 'Blackie' were sited four feet in front of the tiny starter of Cynara, both will have space, heat, and soil so that by August they'll have each revved up into monsters. Don't hesitate to add a mid-ground of Asparagus sprengeri, which can tolerate both sun and shade, and won't flinch when the occasional Cynara leaf crashes down. Or use the asparagus fern as the entire foreground, adding a pot or three of Philodendron 'Black Cardinal' as mid-ground. Wait to add the philodendron until August, when the cardoon should be big enough to provide the dappled shade 'Black Cardinal' usually requires. 

 

If using cardoon as part of a seasonal container planting, choose as large a container as possible, and underplant the cardoon simply, with shade- and sun-tolerant mounding and cascading plants. Asparagus fern and ornamental sweet potato both thrive in containers, and can tolerate what is, inevitably, the much closer association with the cardoon than would be advisable when planting in a bed. Use as rich soil as you can, and don't hesitate to use fish emulsion when you water.

 

 

for cardoons used as perennials: Companion plants to perennialized cardoons have almost antithetical opportunities, caveats, and responsibilities to those for first-year cardoons. Cynara clumps have the wide-ranging and aggressive roots needed to fuel their massive growth, so the center of the clump of even near neighbors shouldn't be closer than three feet away. Because the excitement of the clump's foliage, now springing from many tightly-spaced rosettes, doesn't depend on maintaining the simple and pure geometry of a single palm-like fountain, there's nothing wrong with fronting with full and even large perennials; the cardoon foliage will be seen just "in the tip," not in its entirety. And because the cardoon will, itself, be bringing fantastic flowers to the table, neighbors that bloom are now very welcome. Lastly, the good drainage and somewhat leaner soil that will help the cardoon perennialize suggest perennial neighbors that also require the same circumstances. Only my recommendation against variegation seems to hold.

 

What about fronting perennial Cynara with Papaver orientalis and Eremurus robustus? Let bronze fennel, Foeniculum vulgare 'Purpureum' self-seed to cover the poppy and foxtail lilies. Back cardoons with something tall and dark, to highlight both the Cynara foliage and flowers. A coppiced purple smokebush, Cotinus coggyrgia 'Velvet Cloak' has just the sun-loving constitution, simple and dark foliage, and vertical habit needed. To the side of the cardoon, plant whatever medium-small crape myrtle is hardy for you; across its hardiness range, it is available in forms whose raspberry flowers will harmonize with those of the cardoon.

Where to use it in your garden

First-year foliage is uniquely large and silvery and, held as it is in a palm-like array, stunning both in detail and overall aspect. Site Cynara cardunculus that is used as an annual towards the front of a bed. You could also center a cardoon in a very large container. See "How to handle it" and "Partner plants."

 

The foliage of perennialized clumps—those that have survived their first Winter and, therefore, are likely to flower each Summer thereafter, and increase in bulk and vigor for decades to come—is often a third smaller than that of first-year clumps. The clump can also become a little bare at the bottom. Further, the remarkable flowers are heavy (remember, each is as big as a half-sized artichoke), and the spikes they are held on are branched just enough that the clump will usually look (and be) top-heavy. Perennialized cardoons inevitably need to have their flower stalks staked, and unless you use your thickest bamboo or tomato stakes for the job, they'll be snapped. Accentuate the positives—the upper half of the foliage and the immense flowers on tall spikes—by siting perennialized cardoons well back in the bed, or even at the very back.

Culture

Full sun. When used as an annual, there's no need to worry about Winter drainage, so provide soil as deep and rich as you would for tomatoes, dahlias, or melons. If used as a perennial, good Winter drainage is paramount in Zone 7 or colder, even at the expense of richness and moisture-retentiveness in Summer. Cardoons are tolerant of heavy soil or merely average drainage only in Zones 8 and 9.

 

See "How to handle it: Another option—or two!" for strategies on growing Cynara cardunculus as a perennial.

How to handle it: The Basics

Cynara is easy an annual. Plant young plants when you would tomatoes. With luck, they will increase is size exponentially as Summer burgeons. For the most size, keep the growth of any large neighboring plants from shading the young cardoons.

 

The leaves of first-year cardoons are regularly renewed over the Summer. New ones continue to emerge from the center and old ones continue to mature and then, unfortunately, fall over around the periphery. Fallen leaves are big enough to shade out companion plants, and heavy enough to crush them in their descent. If leaves don't release right at their base, they are so heavy and long that their central stalk, which is as thick as any of celery, snaps partway up the shaft. It isn't practical to try to support the clump to keep older foliage intact and, given that new foliage is ever-appearing, it also isn't necessary. Instead, be proactive in visiting first-year cardoons weekly, to harvest outer leaves even before they topple.

 

Yes, you could also counteract the "Timber!  Look out!" foliage by growing annual cardoons in drier and less nutrient-rich soil. But this would also restrain the overall size of the plant and, so, seem a Pyrrhic victory.

 

When using cardoon as an annual, there's no need to implement Winter protection strategies. Given how watery and large the growth, plants still have remarkable resistance to cold, and will maintain a credible presence even after the first snowfalls of the season. Unless you keep a tidy garden in Winter, just let the plants expire gradually. By April, there's usually little left. 

How to handle it: Another option—or two?

Cynara cardunculus can be established as a perennial. If you're gardening in the warmer reaches of Zone 7—on the East Coast, that would be from Maryland south—first-year rosettes will probably survive on their own. Further north, make use of as many of the following tactics as you can.

 

To help Winter drainage, plant in a bed that is higher than its surroundings, and in soil that is leaner than usual; amend your current soil with plenty of sand and even gravel to help reduce its ability to retain moisture.

 

Through early Winter, let first-year clumps tough it out unattended, so they can shed the large outer foliage (which will have the highest water content and, therefore, the greater susceptibility to freezing) on their own. When the clumps are close to reducing, mostly, to the silver emergent leaves, be ready for action on a day that is sunny and mild. Let the rosette bask in the sun so surface water evaporates. Then tie the leaves up together, as if you were going to blanch them—and then tie the upper third of them back down one side of the gathered leaves, so the top of the bunch now has an overlapping little "roof" of the upper sides of the leaves. Mound dry mulch thickly and widely around the rosette, then top the mound with evergreen branches, which will muffle the wind but also permit air circulation. Lastly, top off with a large piece of cardboard—three feet square isn't too big—with one crease in it, as if you were putting a slanting roof atop a split-level suburban house. Anchor the cardboard by placing a few logs of firewood atop it. It's fine that breezes can reach under the cardboard, via the eaves of your "house."

 

As the promise of Spring is fulfilled—when forsythia is in bloom, say—remove the cardboard and the evergreens, to locate the tied-up foliage. It may have remained intact or, despite your efforts, may have found enough moisture to rot. Don't despair: Cardoons produce new rosettes from below-ground even when growing in mild climates. What's important is that the roots below the foliage are still alive.

 

Scuffle back the mulch as the weather warms, so that the sun hits the ground around the original rosette and helps encourage formation of new ones. Be patient; new growth might not emerge until well into May.

 

If you've been successful at overwintering your young cardoon in its first year, the plant is likely to be much hardier the next. As before, let the season's foliage tough out early Winter on its own, but still mulch, evergreen-bough, and, if you have the energy, "split-level" the clump with cardboard as before.

 

In subsequent years, you may be able to omit all protection but a heavy mulch. Cardoons germinate very easily from seed, and grow with extraordinary speed; even if you do need to start over, there won't be a gap in your display.

 

 

The thick flowerstalks will need staking, and it's easier to put in a couple of stakes at the beginning of the season, when the cardoon foliage is still small, than to attempt it when the stakes are needed. Then, the lush but fragile foliage of the cardoon and its neighbors may well make access tricky. Because the stakes will need to support top-heavy stems that, by August, can be four feet tall and taller, they'll be visible when pounded into place in May. Use new five-foot tomato stakes each year—my local lumberyard is (amazingly) happy to mill mine to order from mahogany—and pound them to the same depth so the tops are even, and so that nearly four feet of stake is above ground. Large cardoon clumps might need three each. Tie flower stalks loosely to the nearest stake with heavy twine, looped to or three times around the stake and stalk both to minimize any danger that the twine will cut into the stalk. Stalks won't normally need staking until the heavy flower buds are developing; it's the weight of the buds that causes the stalks to lean.

 

Spent cardoon flowers are modestly showy, but eventually you'll want to cut them off, at the very least, so you can retrieve those expensive tomato stakes. Save the stakes for less showy jobs, using a fresh crop for next year's cardoon staking. In some mild and dry climates, Cynara can self seed. Check with your local office of the USDA Cooperative Extension Service to determine if self-seeding is a problem where you garden, and dead-head accordingly.

Any (other) quirks or special cases?

None.

Downsides

If first-year cardoons were less committed to being evergreen, they would be hardier. That would require a more realistic sense of the imminence, length, and potential severity of the coming Winter. Ah, the bravado of youth.

 

Despite their impressive thickness, the flower stalks rarely escape the need for staking.

 

In some warm and dry climates, Cynara can self-seed, and needs to be dead-headed responsibly.

Variants

There are many cultivars of the globe artichoke, Cynara cardundulus var. scolymus, no doubt on account of its greater importance as a vegetable. The foliage of globe artichokes isn't as silvery as that of the ornamental artichoke, which, in contrast, is rarely available in any form other than the straight species, Cynara cardunculus. 'Porto Spineless' is, indeed, spineless, but looks the same. The striking foliage of C. cardunculus ssp. flavescens is extremely divided and aggressively spiny. Judging from pictures, the plant is smaller, with a well-armed look similar to that of one of the spiny cardboard palms, Encephalartus horridus.

 

If you have room or energy for only one form of cardoon, go with C. cardunculus itself.  

 

Availability

On-line and, sometimes, at "destination" retailers.

Propagation

By seed and by division. 

Native habitat

Cynara cardunculus is native to Morocco.  

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