In Praise of Penstemons

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Penstemon glaber

As the rocky soil around the Front Range’s  Hayman fire healed, an assortment of native plants sprouted. One bright blue penstemon took the lead. Penstemon glaber, the color of lapis lazuli, joined yuccas, grounsels and wild roses. The penstemon spread more quickly and thickly than nearly any other wildflower.

Penstemons, a spectacular collection of mostly Western wildflowers, have jumped fences from wild to mild, settling into drought-tolerant landscapes. Gardeners love their brilliant colors, tough natures and unkempt appearances.

These wildflowers do have exacting needs. Some, like glaber, thrive on disturbed soil of a recent burn. Others will take to regular garden soil a little more easily. Penstemons include hybrids, which are what you’ll most likely find at garden centers, bred to be a certain height and color. Some are perennial in Colorado but not all. Many of the most striking, long-lived and drought-tolerant often are native species plants, exactly as Mother Nature designed. The key to including them into a garden is to match soil, site and penstemon.

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Rocky Mountain penstemon

Rocky Mountain penstemon, Penstemon strictus, shows up at garden centers on the natives table, closely related toPenstemon glaber. Although it’s found west of the Front Range in its native habitat, this penstemon adapts readily to ordinary garden soil providing the gardener keeps it a bit on the dry side. Even if it’s over-watered, too much saturation is more likely to shorten the life span of the plant rather than kill it outright.

An added boon is that Rocky Mountain Penstemon grows reliably from seed. Like most penstemons, a cold period is required for germination. This mimics the natural world. Penstemons produce seed at the end of summer. The seeds winter over protectively under snow and only germinate after a period of time, usually 40 days. By then, providing there’s moisture via late snows, the seed will germinate and send out roots and a slender tendril. Nearly every package of purchased penstemon seeds will state the stratification, or cold-requiring days for germination.

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pineleaf penstemon

Every penstemon gardener I’ve met has a different method for stratification. Some sow the seeds in cement troughs that can take winter temperatures. They’ll fix a mix of soil and sand–the perfect combination for penstemons that like gritty soil. A pile of snow atop keeps moisture in. Then the gardener waits until spring, never allowing the soil to dry out completely. I’ve tried this and I’m sure it works providing you dutifully remember to attend to the trough from time to time. But as my attention wanders, I usually fail to keep the seeds moist and germination never takes place.

So I’ve reverted to the refrigerator method. I dampen a paper towel, sprinkle in the seeds, roll the towel up and store it in a plastic bag that has been labeled. Then I place it in the refrigerator and wait. Every now and then, as I ferret around the vegetable bin for lettuces, I check the seeds. For weeks, there’s no change. But if the towel has dried, I’ll dampen it a bit. After 40 or so days, a green shoot emerges. One by one, the viable seeds germinate within a few days of each other. They’re ready to be potted in seedling soil without added fertilizers.

penstemonguanellaI like to grow natives in a tall seedling pot or a wider pot with room for a taproot. Too often a spindly native plant that’s drought tolerant sends down a formidable root that winds round and round, root bound too early in its seedling life. I mix damp seedling soil with some sand, just enough to give a bit of grit for drainage. Then I place the germinating seed in the soil, cover it with a lid of some kind and let the moist environment contribute to seedling growth.

By the time the penstemon’s true leaves appear, I’ll fertilize with a weak solution of fertilizer mixed in water. Few penstemons care much for fertilizer, so one-quarter strength or less will do. Keep the penstemons under lights when they are very young. But as soon as they grow an inch or more, I get them outside when temperatures rise to about 50 degrees by midday for full-strength sun. This is what they like best, although most will accommodate a interior light arrangement if need be. Most penstemons will develop sturdy roots and stems as they are tossed about by the wind, heated up by the sun and cooled off by a sudden temperature shift. Bring them in by the late afternoon. By early May, they can go into the garden.

Find a spot where you can water them regularly the first of the season. By summer, they’ll be on their own, ready for little water but enjoying full sun. The tall spires look best with other wild looking plants—ornamental grasses, daisies and sunflowers. Come fall, you can harvest the dried seeds and try growing your own seeds when mid-winter arrives.

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Penstemon secundiflorus, or the one-sided penstemon, also called the orchid penstemon

Many penstemons won’t be as compliant as the Rocky Mountain penstemon. They need a particular kind of gritty soil, or weather conditions. They’ll hate your garden soil and curl up at a drop of water. But trial and error will tell you what flourishes for you. And once you learn to grow them from seed, you can experiment for the cost of a few dollars. Penstemon barbatus, or iron maiden penstemon, is a top choice. So is Penstemon secundiflorus. Penstemon eatonii, or firecracker penstemon is worth a try.

Penstemon pinifolius, or pineleaf penstemon, is a favorite of garden centers, too. And if you have a garden that is filled with ornamentals that have high water needs, ‘Husker Red’ penstemon, (Penstemon digitalis)which hails from the East Coast, is a better choice. Few penstemons will accept heavy watering the way ‘Husker Red’ will.

If you can grow lavender, Russian sage or agastache, you can grow most penstemons. Eventually, the only difficulty may be in deciding how many to choose. Start with Rocky Mountain penstemon and go from there. Plant it in that rocky, sandy or gravelly part of the garden that is sun drenched and watch it flourish.