Rieger begonias. Photo: http://www.swflorist.com
Rieger begonias (B. × hiemalis), often called Elatior begonias, are hybrid begonias, originally derived from a cross between summer-flowering tuberous begonias (B. × tuberhybrida) and a rarely cultivated winter-flowering species, B. socotrana starting as far back as 1883. The name Rieger, now applied to all of them, comes a series introduced in 1955 from German hybridizer Otto Rieger. The alternate name, Elatior begonia, likewise derives from a once popular strain otherwise largely forgotten.
Originally, Rieger begonias were winter-flowering (that’s what hiemalis means!), needing short days in order to bloom, and they were, in fact, commonly called winter begonias. But over time, everblooming begonia species were added to the hybridization palette and most modern hybrids, like the Solenia, Valentine, Dragone, Frivola and Glory series, are day-neutral and essentially everblooming.
Due to their original winter blooming habit, they were first grown as houseplants, often for the Christmas market, but the modern hybrids capable of bloom all summer long have now transitioned into a new role as garden plants, although they still do fine indoors and are commonly sold as gift plants for indoor use during the winter months.
Rieger begonias are densely clustered plants producing multiple stems. Some have a mounding habit, while others tend to be trailing and are better suited to hanging baskets. Plants typically measure about 10 to 18 inches (25 to 45 cm) tall and wide. The shiny, rounded to heart-shaped leaves are green or bronze. The flowers can be single, semi-double or double; the double ones often look a lot like roses. The flowers are of a variable size: typically about 1 inch (2.5 cm) in diameter, although some, like those in the Solenia series, are up to 3 inches (7.5 cm) wide, almost as large as some tuberous begonias (B. × tuberhybrida). They come in a nearly full range of colors: red, pink, white, orange, purple, etc.; only blue is missing. They’ll bloom non-stop for months at a time, usually until well into fall if purchased in the spring. No wonder they’re becoming so popular!
Overwintering Is a Problem
The problem with Rieger begonias is overwintering them. Not that nurserymen are much concerned. “They’re annuals,” they insist. “Let them freeze!” Yes, a non-plantperson would say that. But they aren’t annuals, they’re subtropical perennials. If you’re like me, you won’t want to give up on any plant that is salvageable.
The difficulty is that Rieger begonias have retained a genetic disposition to going dormant in the fall, a trait inherited from their tuberous parents, yet they don’t produce any tubers to go dormant into. So, if they do die back to the state of full dormancy, they simply end up dead, certainly not what you want!
Rieger begonias can be overwintered, but how to do so seems to be a secret professionals just don’t want to share with home gardeners. Certainly, I’ve found pretty much nothing on it in gardening books or on the Web. So, I had to learn on my own, losing more Rieger begonias than I care to think of over the years. But I’ve finally got them figured out.
Here’s what I do:
When I bring my Rieger begonia back indoors in the fall, I cut them back harshly, about a 1 to 2 inches (3-5 cm) from their base. Yes, they’re often still in full bloom, but they seem to need a shock just before they would instinctively start to go dormant. This sudden attack seems to push them into survival mode and, instead of dying back, they start to produce new stems.
Keep them in bright light and water only moderately as new stems grow back, remembering they won’t need much water until they start producing new leaves. They seem to prefer cool temperatures (45-55°F/7-13°C) at this time. By the New Year, they should be coming back strongly. At this point, I suggest making a few backups, so cut off and root some extra stems, just in case. 3-inch (8 cm) stems root quite nicely: plant them 3 or 4 per 6- to 8-inch (15-20 cm) pot for a nice full plant.
Keep both the cuttings and the original plant fairly cool, but in the brightest light you can imagine: this is winter, after all, and sun is at a premium. You can water more generously as they begin to fill in.
By spring, your Rieger begonias should be coming along just fine. They don’t mind a bit of shade, but indoors, at least at this season, full sun seems best. You can start fertilizing them, lightly (they’re not heavy feeders), as days grow longer.
Soon buds appear, then flowers. And there you go: you’ve overwintered your Rieger begonias and brought them into bloom again.
Older cultivars were quite susceptible to powdery mildew and often needed special fungicide treatments to maintain healthy leaves. Better disease resistance has been bred into today’s hybrids, but do keep them aerated and don’t overwater, just in case. If they are hit, just remove the diseased leaves and they usually fill in again rapidly.
You can, of course, grow these begonias indoors all summer if you want. (Few people seem to do so anymore, but they grow really well that way.)
Rather, most people these days want them as garden plants. If so, when all risk of frost has dissipated, acclimatize them carefully to outdoor conditions (a week in shade and a week in partial shade before daring full sun). Outdoors, they prefer cooler conditions and will do best in partial shade in many climates. In my cooler summer area, they’re perfectly happy in full sun. The Solenia series was actually developed for better resistance to full sun and would be a good choice for hotter climates.
You can grow Rieger begonias in pots or in the ground. And these are no longer the delicate “winter begonias” your grandfather grew, but are, in fact, tough, easy-to-grow plants. Just give them basic “annual flower” care and avoid overwatering. They’re pretty much sterile, almost never producing seeds, and are essentially self-cleaning, so no deadheading will be necessary.
In fall, just bring them in and start over!
Best of luck with your Rieger begonias!