Rex Begonias
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 distinctiveness.)

   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.

   One 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.

   Rex begonias 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

Potting
 
   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 shadier locations.  

Pruning

   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 introductions.

Propagation

   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.

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