by Brad Thompson
This type of begonia is officially called Rex Cultorum but commonly
just called rexes or rex begonias. This type name is somewhat of
a misnomer however since many varieties of begonias went into their
creation. The reason they were given this type name is that the
one thing they all have in common is B. rex somewhere in their
ancestry. B. rex, a species from India, is not considered a rex
cultorum begonia itself, it is considered a distinctive foliage
rhizomatous. Rexes have been hybridized for decades and were one
of the first types of hybrids created. (rexes are also rhizomatous
begonias but were given their own classification because of their
Newly discovered rhizomatous
begonias from the same part of the world may eventually broaden the
definition of rex cultorum. These as yet unnamed species have
been used recently to create many new hybrids with rex cultorum type
coloring and qualities. Though they lack the B. rex parentage
that originally defined this type distinguishing these hybrids from
true rex hybrids is nearly impossible. Since all rexes have very
mixed parentage anyway, I think similar type plants should also be
eventually added to this classification.
Also, rexes have been crossed with various upright jointed rhizomatous
begonias. The resulting hybrids would fit into either
classification easily. It may be that eventually all the upright
jointed rhizomatous will also be moved to the rex cultorum group.
Especially if the definition could be broadened to include begonias
with rex coloring regardless of parentage.
trait common to all rex begonias is their showy foliage. Their
leaves come in every conceivable color combination and pattern.
They also come in every size from miniatures to dinner plate size
leaves. There are also myriads of leaf shapes including
spirals. It is commonly said that there is no such thing as an
ugly rex begonia. I find that to be true. Even
non-gardeners find it hard to walk past a well-grown rex without
stopping to ooh and aah at it. Because of their fabulous
coloring, they are always a showpiece.
can be more challenging than some other types but are worth the effort
and not really that hard if a few requirements are met. There are
various types of rex classifications but culturally most have the same
requirements. For purposes of this book they are divided into
just two categories, rexes and miniature rexes. The following
culture can be applied to growing both types. However, miniature
rexes usually have B. dregei in their parentage so are more mildew
prone and sensitive. For this reason, most miniature rexes are
grown in greenhouses or terrariums. If grown outdoors they should
at least be brought in for the winter.
Rex Cultorum Culture
Rex begonias require the same basic care as the other types of
rhizomatous in regards to pots. They do best in clay pots or
wooden pots, at least in California. They will grow in the same
potting mix as all other begonias. They should be repotted and
revamped yearly. They don’t seem to be long lived plants unless
refreshed yearly. Most have large leaves but avoid using pots
that are too large. Like with the other rhizomatous, they do best
in shallow pots. Rex begonias will do very well when grown in beds in
Rexes don’t require much pruning if revamped yearly but overgrown
plants can be trimmed back to within the bounds of their pot.
They can be pinched to encourage fuller growth but you should avoid
this on the larger leaved varieties. They could become too
crowded and not have room for leaves to open to full potential.
Plants with one large rhizome however may benefit from pinching to
force out more size growing rhizomes.
Watering and Fertilizing
Avoid keeping rexes too wet. Allow the mix surface to dry
slightly before watering again. Rex leaves are delicate and
disease prone. You should avoid over head watering or getting
water on the leaves except for an occasional rinsing for grooming
purposes. Only do this during a time of day where the leaves will
dry off quickly. Avoid getting the leaves wet during hot weather
or leaves my burn, even in shady areas. You should fertilize
weekly with quarter strength fertilizer any time plants are actively
growing. Rexes planted in beds do best with soaker type
watering. I have seen nice beds of rexes where they were watered
with sprayers, but leaves can be damaged with this type of watering,
especially during cool weather.
Light and Heat
Although rexes bloom, they aren’t grown for the blooms. In fact
many rex growers keep the blooms removed. For this reason rexes
can grow in shadier locations than some other types that need good
light to bloom. Although pure shade isn’t recommended, you can
use rexes in those areas that aren’t suited to your blooming
types. Filtered light most of the day is preferred.
It is a common urban legend that rexes rest for the winter and go into
semi dormancy. I think this is just a myth. Rexes do have
the habit of defoliating and not growing during the winter
months. This is more due to an intolerance of cool wet
conditions. They drop their leaves as a survival method but not
by choice. Most rexes will continue to grow and stay nice if
given protection for the winter. Many growers move them into
greenhouses or put up temporary shelters for the winter. As long
as you keep them from staying soggy wet from winter rain, even if they
defoliate, the will recover with warm weather. Miniature rexes
should always be brought indoors or into greenhouses, they are almost
impossible to winter over outdoors in California. It may be
different for Florida growers.
Rex begonias are not
very heat tolerant. In areas that get very hot, they may require
special conditions if you are to be successful with them. Growing
close to the ground as mentioned will help but you may also need some
type of misting system to help keep the air temperature down.
Pests and Diseases
Rex begonias are prone to the same pests as most other begonias.
The main one being mealy bugs, the scourge of all begonias. As a
group, they are disease prone however. They are especially prone
to powdery mildew and most will require some type of routine
spraying. Unlike most plants where mildew appears on top of the
leaves, rexes can have the undersides get covered with mildew
too. When grown low to the ground, this will also help with
mildew problems. If they seem especially prone in one area of
your yard you can also try moving them to other areas to see if other
microclimates in your yard are more satisfactory. Rex hybridizers
are continuously working on new varieties especially ones that might be
less mildew prone or heat sensitive. If you haven’t done well
with older varieties, you may look into trying more recent
Rex begonias can be started in all the same ways as the other
rhizomatous begonias. Refer to chapter on rhizomatous for more
information. If you don’t care about named varieties you can also
find many sources of rex seed to grow your own. If color and
quantity are what you’re interested in, seed can be the best way to
go. Many rex hybrids are circulated without names anyway.
Special Uses and Tips
Most rexes do best when grown on or close to the ground. Most rex
growers grow them in pots either sitting on the ground or on very low
shelves. It’s believed they prefer the coolness and greater
humidity at that level. I think that is true. Regardless of
the reason, the do grow best there. You may be able to grow them
in baskets in a greenhouse but it wouldn’t be recommended for outdoors
because of the mildew problem you’d have.
Pictures and more updates to this page coming soon.