47 (July 1980)
Rhizomatous begonias: leaves, flower clusters
They're called "rhizomatous," but many are easier to
grow than the word is to pronounce (rye-ZOMM-us-tuss). The name means
simply that these begonias grow from a modified stem called a rhizome
(RYE-zome). Leaves emerge from the rhizome, forming a more or less
ball-shaped plant, a virtual mound of leaves, some patterned with spots
The rhizome stores water and nutrients like a
miniature canteen, making it possible for the plant to survive some
neglect, irregular watering, and temperature variation. That means a
rhizomatous begonia will survive more easily than certain other kinds. You
still must provide good care, though, if you want a handsome specimen.
Rhizomatous begonias vary in size from
windowsill-sized ones with inch-long leaves to giants requiring planting
tubs and a growing space a yard across. They include those with
multi-pointed leaves called "star begonias"; a group with heavily
textured, colorful leaves known as "distinctive foliage"; the widely
available iron cross begonia, B. masoniana; a group with frilled
lettuce-like leaves; and the "beefsteak begonia," B.
Some rhizomatous begonias you may run across include
B.acetosa, B. bowerae, B. 'Bow Nigra', B.
'Buttercup', B. 'Cleopatra', B. 'Crestabruchi', B. goegoensis,
B. 'Joe Hayden', B.manicata, B. prismatocarpa, B.
rajah, B. 'Tiger Kitten', B.'Universe', and B.
To grow rhizomatous begonias well, use a shallow pot,
preferably clay (but if your water is high in salts, plastic pots may be
better), and a standard coarse, fast-draining planting mix. Don't
overwater or overpot -- move a plant up to a pot only one size larger at a
time and only when the roots have filled the present pot. A shallow, wide
bonsai pot works nicely. During spring through fall, the time of active
growth, water only when the surface of the soil feels dry to the touch. In
winter, when some varieties become dormant (growth stops or slows), water
only sparingly until growth resumes.
Except for those with "distinctive foliage," most
rhizomatous begonias are not fussy about humidity. About 50% is
sufficient, but lower humidity does little harm. The "distinctive foliage"
types require high humidity and often are grown in terrariums and other
closed containers for this reason.
Rhizomatous begonias are "cool temperature" plants,
doing best in the range of 58 to 72 degrees F. In mild winter areas, they
may remain outdoors year-round as long as they don't freeze. In all, they
are extremely adaptable.
A "complete" fertilizer (one that contains all three
of the main ingredients, nitrogen, phosphorus, and potassium) should be
applied according to label directions unless the plant is in dormancy.
During dormancy, don't feed at all. You can tell the percentage of each
ingredient by checking the formula (such as 15-30-15) on the label.
These begonias do not attract many insects or develop
many diseases. Occasionally, you may find mealybugs. For treatment, see
page 185. Many growers feel any apparent ailment should be diagnosed
specifically before any toxic chemical is applied -- and such chemical
used only if a safer measure will not work. Some growers, however, choose
a preventive program of frequent chemical spraying.
Rhizomatous begonias grow nicely when given adequate
light without strong direct sun. Under a shade structure or a tree is
good. A shaded greenhouse or a window free of direct sun also will
suffice. You'll know when light is insufficient because the leaf stems
will elongate and the plant will grow lanky. Some varieties perform well
outdoors with morning or afternoon sun in mild areas, but too much light
will bleach leaves.
Flowers appear in airy, cloud-like clusters on stems
well above the foliage, usually late winter through spring. Some plants
bloom at other times.
Tip pinching early develops a well-rounded plant.
Pruning usually consists of replanting an old plant that is crowding
itself. Rhizomes often will grow over the pot edge. This forms a nicely
rounded plant and, unless you find it unattractive, need not prompt
pruning. Some rhizomes grow upward and may require pruning to keep the
plant in bounds. In any case, pruning yields rhizome cuttings which can be
rooted in a medium such as sand or perlite to create new plants.
B. 'Cleopatra', a widely grown
Photo by Brian Halliwell.