47 (July 1980, page 184)
Rex Cultorum begonias: gloriously gaudy
The Rex Cultorum group
-- whose members commonly are known as
"rex begonias" -- is aptly named.
These "kings" of the begonia world
display wildly varied leaves streaked,
bordered, spotted, and splotched by many
colors. They also flower, but usually the
flowers are overshadowed by the striking
Rex Cultorum begonias
are derived from the species B. rex,
whose dark leaves are marked with a prominent
silvery-gray band. B. rex was
discovered in Assam and introduced into
cultivation in the 1850s. Its immediate and
widespread use in hybridizing gave rise to
countless cultivars that boosted foliage
begonias as indoor plants.
Most rex begonias grow
from a thickened stem structure called a
"rhizome." They are not classed with
other rhizomatous begonias, however, because
of their bold leaves and more exacting growing
requirements. Some varieties are upright and
branching rather than creeping.
Some of the oldest rex
cultivars, such as B. 'Abel Carriere'
(1878) and B. 'Louise Closson' (1889),
are still in cultivation. Other cultivars
include B. 'Fireworks', B.
'Helen Lewis', B. 'Lady Frances Jean', B.
'Merry Christmas', B. 'Purple
Petticoats', and B. 'Woodriff's
Tricolor'. Several hundred named cultivars are
grown today, and many more are hybridized and
sold but never named. Rexes hybridize readily,
and, of the several hundred seedlings that may
grow from a single cross, no two will be
Rexes are not "easy
care" plants. They require high humidity
(more than 50% -- some cultivars want more
than others), porous planting mix, a shallow
pot, heavy fertilization during growth, and
care to avoid overwatering. Once you hit the
proper combination of growing conditions, the
stunning color display will make it
Your watering technique
should permit the soil surface to become
almost dry between waterings. Stick a finger
into the planting mix to check.
In spring, when new
growth has started, a balanced complete
fertilizer (23-19-14 or similar formula)
should be applied quarter strength every two
weeks. Or a controlled-release fertilizer can
be applied every three months. Taper off in
fall and stop in winter.
Provide plenty of light
without putting the plants in direct, hot sun.
Spring morning sun or filtered sunlight may be
acceptable in mild areas. If light comes from
one side, give each plant a quarter turn
weekly. Rexes do best if day temperature
hovers around 70 degrees F., and 60 degrees at
night. If it is cooler, they usually will
survive but growth will be slow.
In fall or winter,
unless grown under lights, many cultivars
enter dormancy -- they stop growing and might
even drop some or all of their leaves. If this
happens, water only sparingly until spring,
when new leaves will emerge.
Most don't need pruning
unless they are "upright rexes" or
the rhizome has grown too long for its
container and has unsightly bare sections.
Pruning is simple: just cut the rhizome back.
It will develop new leaves and may even
branch. You can root the rhizome cutting and
grow another plant. Tip pinching earlier will
result in beneficial branching.
The primary enemies of
rex begonias are mildew and botrytis, both
fungus diseases marked by white spores. The
systemic fungicide benomyl is a good
preventative. Many growers use a fungicide
containing karathane to kill the diseases once
they have started. As with all garden
chemicals, follow label directions exactly.
Occasionally, the insect
known as mealybug may appear as a small
cottony-looking mass tucked in the joint where
a leaf joins the leaf stem or the stem joins
the rhizome. To kill the bugs, just dip a
cotton swab in rubbing alcohol and touch it to
each mealybug. A large infestation can be
treated with malathion.
The best way to prevent
insects and diseases is to keep a plant well
groomed, removing dead leaves and any debris
on the surface of the planting mix.
The Begonian of
March 1980, a special issue called "The
Regal Rexes," is a source of detailed
information on the Rex Cultorum group.
Propagating information is contained in an
article in the June 1980 Begonian.
An unnamed B.