The renowned rock gardener Geoffrey Charlesworth wrote that “penstemons are the glory of the North American flora. . . . If you value the alpine look, the showy shrub, the glorious border plant, the wild garden colonizer, the difficult rarity, you can find them all within this genus.” He’ll get no argument from me. I’ve seen maybe one or two drab little species; the rest are as close to living jewels as anything I’ve encountered in the plant kingdom. Alas, they can be a challenge for gardeners who live east of the 100th meridian: many of the most gorgeous penstemons are denizens of the arid West, growing in alkaline, mineral soils under skies whose elemental blueness results from low atmospheric moisture. Transplant these children of a harsh landscape to the humusy soils and summer sogginess of the Northeast or Midwest, and they degenerate into pale travesties of their original selves, if they deign to grow at all.
One dazzling exception is Penstemon heterophyllus, or at least the seed strains derived from it, such as ‘Blue Springs’, ‘Heavenly Blue’, ‘True Blue’, and ‘Züriblau’ (often listed as ‘Blue of Zurich’). The species itself is native to the Coast Ranges of California and the foothills of the Sierra Nevada, but ‘True Blue’ thrived even in my former Boston garden, perhaps because the process of selection involved in creating a seed strain has made it more tolerant of the soft, pampered life of the mixed border. ‘True Blue’ is the form I had in Boston. I started it from seed sowed indoors in January or February (no cold stratification needed) and grew the seedlings under fluorescent lights, which yielded plants large enough to set out by late summer. Don’t expect too much their first full season. The plants will fill out and bloom a little, but they won’t bowl you over, and you may be tempted to write them off. Just wait. Next spring, 18-inch flowering stems will begin to form on each shrubby clump, and by late May the wands of small, snapdragon-like flowers will almost obscure the foliage, and the display will continue throughout all of June. “Sky blue” is one of the most abused descriptions in the horticultural literature, but these flowers look like a cloudless noon made tangible. When the floral fireworks start to peter out, shear back the flowering stems to keep the plants from looking bedraggled (but let one or two ripen seed so that you have a source of new plants; penstemons tend to be short-lived).
In my Portland garden, I grow ‘Blue Springs’ and ‘Jean Grace’. It’s impossible to say which is the more beautiful—‘Blue Springs’ looks as though someone plugged it into an electrical outlet, it’s that dazzling; ‘Jean Grace’ is just as brilliant but ever-so-slightly darker, approaching the blue we associate with gentians.
Penstemon heterophyllus doesn’t need heaps of well-rotted manure and will get by quite happily on a minimum of water once established. What it does need is full sun and excellent drainage. If your soil is extremely acid, add a handful of ground limestone at planting time. Subzero temperatures can cause die back, so a winter mulch is a good idea in USDA Zone 6—it’s a small price to pay for being able to gaze for weeks at the true blue of the Far West.
A version of this article first appeared in Horticulture magazine, vol. 100, no. 3 (May/June 2003).