Gesneriads such as African violets, gloxinias (Sinningia speciosa) and Streptocarpus have long been favorites of indoor gardeners who appreciate their relatively easy cultivation and extended periods of bloom on windowsills or under artificial lights. A few genera are suitable for outdoor rock gardening and have proven hardy through zone 5. These include Ramonda myconi, which forms a rosette of hairy, dark green leaves and produces African violet-like blue-purple flowers in spring (pink and white-flowered forms are also known). Haberlea rhodopensis, native to Greece and Bulgaria, also can be grown in a rock garden setting with excellent drainage.
Besides these Usual Suspects, are there other gesneriad possibilities for the perennial garden in temperate regions? Surprisingly, the answer is yes.
I discovered one cold-hardy gesneriad by accident some years ago when I planted a grouping of Sinningia tubiflora in a partly sunny garden bed. This species grows from stout tubers. Flowers, about three inches long and usually white (mine, grown from seed are pale pink) appear along a slender stalk two or more feet high for at least a month in summer.
They have a moderate sweet fragrance, unusual among gesneriads. For a couple of years I dutifully harvested tubers in fall and kept them in cool dryish storage over the winter. One spring a sprout emerged from the S. tubiflora planting area, from a tuber I had evidently left in the ground. It had survived winter lows near zero, without mulching! Since then I’ve had similar experiences with winter survival. It turns out that S. tubiflora has elsewhere reportedly endured temps to 0F; I can vouch for survival under mulch after a winter low of -9F. Plants do well in partial sun with good drainage and should optimally be cited near the front of the border to appreciate the delicate flowers and fragrance. S. tubiflora can readily be raised from seed; a recent sowing of freshly harvested seed has produced 110% germination, or at least it seems that way.
Another Sinningia suitable for outdoor growing (I haven’t yet attempted to overwinter it outdoors) is Sinningia sellovii. This gloxinia relative enjoys conditions similar to S. tubiflora (partial to full sun, reasonably good drainage) and can get larger - heights when in flower up to 4 feet or more have been reported, though mine were typically no more than half that. Nodding bells in shades of pink, dusky red, purple and even yellow depending on the cultivar are produced along the tall flower stalks. Plant Delights reports one type surviving in zone 7b, which means it’s within my experimental range.
There are other gesneriads said to stand a fair degree of cold, such as members of the genera Lysionotus, Aeschynanthus, Achimenes and Titanotrichum. The latter genus has a single species, Titanotrichum oldhamii, found in moist mountainous regions of China and Taiwan. Earlier references described it as tropical, but it’s been found to be hardy to 0F in temperate climates. In habit it resembles Digitalis (foxglove), with 1-2 foot spikes of yellow flowers with deep reddish-brown throats. It’s on my short list of hardy gesneriads to try.
Sinningia “Towering Inferno”, selected from a cross between Sinningia 'Krezdorn Yellow' and S. 'Apricot Bouquet', is reported to be hardy to zone 7b. It has spikes of what one seller describes as “bright red” flowers, though ones on the plant I acquired this fall are more of a reddish-pink. It’s said by the folks at Plant Delights to produce flowering spikes up to 4 feet tall from mid-June into October, which makes it something I want to try toward the front of the border as a “see-through” plant. Whether or not it comes true from seed remains to be seen.
Certain Achimenes can handle considerable winter cold. The cultivar “Purple King” is reportedly hardy to zone 7b. I have grown it and another rhizomatous gesneriad, Gloxinia nematanthodes “Evita” as summer bedding but never tried to overwinter it. Both are very sensitive to cold early in spring, as I found out one year when temps dipped below 40 in a coldframe and I lost most of my stock. Achimenes make a fine addition to the front of the border. This year I grew a couple of hybrids in a partially sunny location. They don’t need special attention, other than to make sure plants never dry out, which can send them into premature dormancy. Gloxinia nematanthodes "Evita" makes a good accent at the edge of a border, yielding abundant small red tubular flowers for months during warm weather.
They’re not candidates for overwintering but I’m tempted to try African violets in a mostly shaded location in-ground. They tolerate cool temperatures better than what the reference books say (I’ve had a couple refugees in pots survive near-freezing weather) and the prohibition against overhead watering is overblown in my opinion. It’d be cool to see AVs in flower outside in the summertime. Species types with spreading habits and small but prolific flowers might be especially nice.
Lastly, non-hardy gesneriads are fun to try in pots as temperennial accents. Hanging basket candidates include Aeschynanthus and Columnea. Hybrids of Columnea are a better bet to provide relatively continuous bloom.