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Tuberous Corydalis for the Woodland Garden

Making predictions can be a risky business, but what the heck: the Next Big Thing in spring gardening is going to be the tuberous species of the genus Corydalis. Not that pleasant weed, C. lutea, which seeds itself into every square centimeter of unoccupied soil, and not the disappointing selections of C. flexuosa, which, though irresistibly attractive with their electric-blue flowers, defy most attempts to keep them longer than a year or two. And certainly not that fabled rarity C. cashmeriana, which is growable only by angels and few lucky Scotsmen. Unless you’re a very determined gardener with top-notch sleuthing skills, you’ve probably never seen most of the species I have in mind. But they are slowly becoming more widely available, and they are so beautiful, and so easy to grow, that they will leave you dumbstruck with awe and gratitude. We’ll talk about the minor inconveniences (like how much they cost) later.

Family Traits

The genus Corydalis is a good-sized one, with some 440 species. Depending on your botanical proclivities, it belongs either to the poppy family (Papaveraceae) or to the fumitory family (Fumariaceae), a name that suggests that its members were once either smoked or burned, for whatever obscure purposes. (Experiments in this area are firmly discouraged.) Many corydalis share a family resemblance, being smallish plants with delicate, finely divided, somewhat fernlike foliage (like that of their near relatives, the dicentras) and racemes of small but abundant long-spurred flowers with conspicuous, flaring upper and lower petals. Some species, like the ubiquitous C. lutea and C. flexuosa, are fibrous-rooted, while others form a small tuber. The tuberous species can be further subdivided into those that thrive in woodland conditions, with humusy soil and regular moisture, and those that come from the steppes of Asia, and require a long, hot summer baking. While the latter group boasts some highly attractive members, it’s the woodlanders that contain the most dazzling gems and that, serendipitously, are the most tractable in cultivation. Most grow from 8 to 12 inches tall. The tuber, incidentally, is a clue to the plants’ life cycle: these are spring ephemerals, which bloom early in the year—in my Boston garden, they emerged just after the snowdrops—and then go dormant. Fortunately, they go dormant quickly and neatly, without the prolonged death agonies of, say, daffodil foliage. They are thus ideal for planting near clumps of hostas, which will just be emerging as the corydalis are blooming, and whose fully expanded leaves will keep the hapless or forgetful gardener from inadvertently digging up or skewering a precious corydalis tuber.

Some Choice Species

Corydalis solida, a widely distributed Eurasian species, isn’t exactly a newcomer on the gardening scene, but, until recently, most of the forms that popped up in the bulb catalogs had flowers that could charitably be described as puce, and there’s no reason on earth why anyone would bother to grow them. The one ray of light was the famous cultivar ‘George Baker’, a late-blooming selection with flowers of a glowing pinkish crimson, but it was hard to find and there were many impostors around. Now, however, all that has changed. Thanks to the efforts of Latvian nurseryman Jānis Rukšāns and others, scores of delectable forms are becoming more widely available. There are pure whites, like ‘Snowstorm’, ‘White King’, and ‘White Knight’; clear, tender pinks like ‘Cantata’, ‘Pink Smile’, and ‘Zarja Povolzya’; pearly blue-violets like ‘Blue Dream’ and ‘Christina’; and—best of all—scintillating reds and crimsons like ‘Rivendell’ and the ruby ‘Latvian Zwanenburg’. The species has a tendency to self-sow moderately, and if you’re diligent about roguing out inferior seedlings, within a few years you’ll have a carpet of your favorite colors.

As much as I like the white forms of C. solida, my favorite white-flowered species is C. malkensis, which used to masquerade under the name C. caucasica ‘Alba’. For the gardener’s purposes, C. malkensis is the superior plant, with comparatively large, frilly-lipped flowers that open from chartreuse buds. Native to the central and northwestern Caucasus, it is a natural tetraploid, with the ability to transmit its large lip to hybrid offspring.

From icy white, let us move to warm, sunny yellow. That a plant as lightsome as C. bracteata should be native to the bleak region of Novosibirsk, in Siberia, would seem to be one of nature’s little jokes; well, at least the locals have something to cheer them up. As you might expect, C. bracteata actually relishes harsh, continental winters, and will sometimes fade away, or bloom unreliably, in areas with a mild, maritime climate. If you can give it the no-nonsense winter it craves, you’ll be rewarded by dense racemes of clear yellow, slightly darker at the center of the lip. And if conditions in your garden are sufficiently like those in Moscow, you may even get a self-sown colony.

Artistic soul that you are, you will surely want to give C. bracteata pleasing companions, and for that we must take flight into the realms of blue. And oh, what thrilling blues are to be found among the tuberous corydalis! Forget ‘Blue Panda’ and its fussy cohorts; these stalwart Russians make the flexuosa crowd look like a bunch of sissies. Let’s begin with C. fumariifolia, whose natural distribution ranges eastward from the lower Amur River in eastern Russia all the way to the Japanese island of Hokkaido. A highly variable species, with numerous forms and subspecies, it can be the palest of sky blues, a rich mid-delphinium blue, or an indifferent bluish purple. To avoid disappointment, pay close attention to the catalog descriptions. This species is sometimes offered, inaccurately, as C. ambigua.

Next in line is C. ornata, known only from a single collection in southeastern Russia. It’s perhaps stretching things a bit to claim this as a blue-flowered species, since it is as likely to have white, lilac, or muddy purple flowers. But the blue is a very good blue, and the white has a thin pencil mark of clear turquoise on the upper and lower petals, which can provoke shudders of aesthetic bliss.

I’ve saved the best for last. No gentian, no delphinium, no Himalayan poppy can equal the brilliance of C. turtschaninovii in its best named forms. Another member of the southeastern Russian gang, it almost gives off sparks when it bursts into bloom (somewhat later than C. fumariifolia and C. ornata). As a bonus, some forms, such as ‘Eric the Red’, bear coppery, purple, or reddish foliage, which contrasts thrillingly with the flowers. If ordinary gorgeousness is enough for you, look for the aptly named ‘Blue Gem’. Easy as pie to grow, C. turtschaninovii also has the charming habit of increasing well, producing a modest number of new tubers each year though never to the point of becoming a nuisance (if such a thing were possible).

Counsel and Caveats

The various tuberous corydalis species I’ve grown have been among the least demanding and most reliable plants in my entire garden. Authorities like Jānis Rukšāns warn that the tubers of woodland corydalis should remain out of the ground for as short a time as possible—advice that I’ve heeded to the letter. As soon as the tubers arrive, I plant them a couple of inches deep in humusy, well-drained soil in the shade of deciduous trees. And although I’m not sure whether these morsels appeal to the appetites of the local rodents-in-residence, I take no chances: each cluster of tubers gets covered with a square of hardware cloth, which I then conceal with a sprinkling of soil. So far, so good—I haven’t noticed any winter disappearances.

The comparative rarity of these plants does make for certain inconveniences, such as a complete lack of hardiness information. Extrapolating from where they occur in the wild, a lower limit of USDA Zone 4 would seem a fairly safe bet. In fact, you may run into trouble if it isn’t cold enough where you garden. Nurseryman Tony Avent has told me that the tuberous woodlanders don’t do well much south of Raleigh, North Carolina. (Update: Now that I live in Portland, Oregon, I’m finding that the Siberian species don’t seem to get sufficient winter chilling to thrive. Phooey.)

I hinted earlier that these plants weren’t exactly a bargain. In fact, they can be staggeringly expensive—as much as 30 or 40 euros for a single tuber. Presumably, this rather daunting state of affairs will ease as they become more widely grown. Until that happy day arrives, I urge you to take whatever economies may be necessary to add a few to your garden. Cut back on the single-malt scotch. Brown-bag it for lunch. Keep the old Toyota for a few thousand more miles. Beauty as rare and perfect as that offered by this fascinating group of plants doesn’t come along every day.

A version of this article first appeared in Horticulture magazine, vol. 104, no. 1 (January/February 2007).