Blooming beauties - growing begonias - includes list of nursery sources

Flower & Garden Magazine, Feb-March, 1994 by Margaret A. Haapoja

NOBODY EVER CALLED NORTHERN Minnesota a tropical paradise, but among the flowers we can grow are some that match the flamboyant blossoms and lush foliage of southern latitudes.

Tuberous begonias originated high in the Andes of Peru, Ecuador and Bolivia. Although they emigrated from the tropics, those native tubers grow at elevations of 10,000 to 12,000 feet where daytime temperatures are moderate and nights are distinctly cool. Started indoors under grow lights and set out after the last frost, these blooming beauties brighten the shady spots in our Zone 3 landscape.

Dubbed "begonia" by the monk Charles Plumier, who collected species in the Antilles in 1690, these plants were named for Michel Begon, a French botanist who was the governor of the Dominican Republic. The first begonia species to reach England in 1777 was a fibrous begonia with tiny pink flowers. Tuberous begonias arrived at Kew Gardens in England around 1804. The tuberous begonia as we know it today (Begonia x tuberhybrida) is essentially a 20th century development, and much of the hybridizing takes place in the mild coastal climate of central California. By the 1940s, Pacific Coast Hybrid tuberous begonias were world famous. Truly old favorites, tuberous begonias have been around for quite a while.

Among the most dependable of my perennial flowers, these plants can be started from either seed or tubers. Flowers grown from seed will not come true, however, so I always begin with tubers. Purchased tubers vary from about an inch to three inches in diameter, but the size of the tuber does not determine the size of the flowers. Flower quality is a genetic characteristic and so-called large tubers, which are always more expensive, will not give better flowers though they might produce more of them on larger plants.

The tubers are usually flat and round with a convex bottom and somewhat concave top. A smooth circular scar on the upper surface of the tuber indicates the place where the stem was attached the previous growing season. New stems originate from tiny pink or white growth buds that emerge from the top of the tuber.

About mid-February, I start my begonias indoors under grow lights, using shallow wooden seed flats. I use peat moss and sand as a starting medium although begonia experts recommend well-rotted leaf mold. Above all, the planting medium should be loose and well-drained and should not contain fertilizer or manure. Don't start the tubers until they've developed sprouts at least 1/4 inch long; starting them sooner encourages rot.

Press the sprouted tubers into the starting medium about 3 inches apart, making sure that the indented side of the tuber is up and the rounded side down. Bury the tubers; it's essential to good root development that the tubers are completely covered because the roots develop from the tops and sides. Keep the rooting medium evenly moist. but don't overwater. Place the flat 8 inches below a timer-regulated grow light for 16 hours a day. Keep the tubers warm while sprouting.

When the sprouts are 2 to 3 inches long, the tubers are ready to be potted. Generally I move them into 4- to 5-inch pots at this time, except for the hanging basket tubers, which I plant directly into their baskets. Three or four tubers make a nice, full 10-inch hanging basket. The potting mixture must hold moisture yet drain readily. According to Antonelli Brothers, begonia breeders from California, the most important factor to consider in preparing any soil for tuberous begonias is perfect drainage.

The potting mixture for tuberous begonias should also be rich in organic matter. My recipe calls for one-third peat, one-third garden soil, somewhat sandy, and one-third leaf mold, compost or well-rotted manure. To this mixture I add a generous amount of bone meal, a natural slow-release fertilizer. Then I cover the tuber with about 1/2 inch of potting mix and place the pots in a cool location in the house where they receive full light but not direct sun.

Once potted, keep the plants moist and feed them weekly with half-strength water-soluble fertilizer (Peter's 20-20-20, Miracle-Gro 15-30-15 or RaPid Gro Bloom Builder 19-24-18). Before the stems grow too tall, I stake my plants to grow them upright if they become top-heavy. I continue tying all season long. Allowing only one stalk to develop on each tuber produces larger but fewer blossoms.

Tuberous begonias are very tender and susceptible to frost. They shouldn't be moved outside until after tomatoes have been planted -- usually around June 1 in my area. I start hardening off my begonias two or three weeks before setting them out. If they're planted directly into the garden, the soil must be well-drained and mixed with compost or leaf mold. I also mix a handful of bone meal into the soil under each plant.

Since the plants' leaves and flowers always face the same direction, set the plants so the tips of the leaves face the source of greatest light. Begonias love to have their foliage sprayed with water; in warm weather I try to give them a daily shower. Leo Newman, who raises several hundred exhibition begonias each year in Illinois, recommends removing the first three or four flower buds to allow the root system to develop before the plant blooms.