Tuberous Begonias
by Brad Thompson
Tuberous Begonias are probably the most commonly
grown begonias worldwide. The tuberhybrida types are always
popular and are commercially grown in many countries. Unlike the
other types of begonias, tuberous begonias grow from a bulb like
structure and most go dormant part of the year. 
These die back completely, leaving only the
tuber, which requires special storage. There are a few begonias
placed in the tuberous class, which don’t have tubers but form a
caudex. There is some debate as to whether these should be in
this class but currently they are commonly called semi-tuberous.
More correctly they should be called caudex forming begonias.
Tuberous begonias were one of the first types of begonias
to be hybridized. Combining aspects of the various species,
hybridizers have developed types with large double flowers. This
type is called tuberhybrida. There are also a few dozen species
with various unique qualities. There are also various hybrids
between tuberous and other types such as Riegers, which don’t have
tubers but have some tuberous qualities. For purposes of this book, the
tuberous types are divided into five main types, tuberhybrida, species,
caudex forming, hiemalis and cheimantha.
Tuberhybrida Type Tuberous
This is by the far the most widely grown type of tuberous
begonias. Over many decades of hybridizing work, various species
traits were combined to create what we know as the tuberhybrida type
today. Much work continues today in Europe, Australia, New
Zealand and North America to come up with even more improved
varieties.
The most attractive characteristics of the tuberhybrida
are their double flowers. The flowers come in every color but
blues. There are also various styles of flowers from ruffled
petals to rose forms. There are even picotee type tuberhybrida
that have petals with edgings of different colors. Most are
upright growing but there are also hanging types with trailing stems
called pendula tuberhybrida. The flowers range in size from a
couple inches as in the commercial Nonstop varieties to named varieties
that can have dinner plate sized blooms with proper culture.
There are also scented varieties available.
All tuberhybrida go dormant for the winter. All
upright varieties usually require staking to support the flowers.
Unstaked plants can break off at the tuber easily. There are
probably more books available on this type of begonia than any other
type. Since this is a specialized type of begonia, further
reading is recommended.
Species
Tuberous (and first generation hybrids)
There are a few dozen types of species tuberous
begonias. There are also as many first generation hybrids between
the various species. Many require specialized care or are unique
so may require more research into the care for specific species.
All grow from a tuber or similar root structure. They come in a
wide range of sizes, flower colors and sizes, and leaves. Several
species types form bulbils in the leaf axils that can be collected and
grown into new plants the next year. At least one variety, B.
grandis variety evansiana, is winter hardy even in cold climates.
Most require similar care to the tuberhybrida but may be more tender or
difficult to grow. Examples of this type are B. sutherlandii and
B. boliviensis.
Caudex
Forming Tuberous
This type is commonly called semi-tuberous but caudex
forming is more proper. Nearly all are variations or hybrids of
B. dregei. There are a couple that aren’t but all need similar
care. The caudex is 
a unique root structure that looks like a large
tuber (which is how they got lumped in this group). It is usually
half out of the soil. From this caudex small stems arise.
During stressful times or bad weather this type can lose all their
foliage down to the caudex. Unless the caudex is rotted by
overwatering, new stems will come back from the caudex when conditions
improve. This type does best in clay pots to avoiding keeping
them too wet. The caudex forming are commonly grown as a natural
bonsai and do well in the shallow bonsai pots. This type is very
mildew prone, especially during cool wet times of the year, but also
nearly any time of year. They may require routine spraying with a
fungicide to be successful with them. Examples of this type are
B. dregei var partita and B. dregei var glasgow. All have small
leaves and most have white flowers like B. suffruiticosa pictured to
the left.
Hiemalis
Type Tuberous
Most of this type are not truly tuberous but have tuberous
parentage and some tuberous qualities. They are hybrids between
various tuberous types and other types such as rhizomatous. There
are some 
with tubers but these are not commonly grown
anymore and may not exist. Most have tuberous qualities such as
double flowers but don’t have a tuber so grow all year. The most
common of this type grown are the Riegers. The Riegers are ever
blooming if given enough light. They can be trimmed any time of
year to keep them compact, if grown indoors. Outdoors, grow and
prune as you would shrubs. The Riegers may require winter protection or
additional light during short days. They are common florist
plants and are most commonly grown indoors or in greenhouses. New
varieties will do well outdoors however and some recent developments
have created some that will grow in full sun in some areas.
Cheimantha Type
Tuberous
This type of tuberous is not commonly grown, at least in
the US. They are hybrids between B. socotrana and B. dregei or
other types. They are winter blooming.
Tuberous
Culture
Potting
Tuberous begonias will grow in a variety of pot
types. The most commonly used pots for tuberous are
plastic. Some species such as B. boliviensis, which has a very
large tuber, may do better in a clay pot to avoid any chance of
overwatering.
Although tuberous begonias can be started directly in the
pots they are to grow in, they usually are started in flats or smaller
pots first. Most will require at least an eight inch pot but
starting them directly in that size is more risky than starting them
first and transplanting. Large numbers of tubers are usually
started in shallow nursery flats filled with mix or oak leaf
mold. If you’re starting a small number of tubers, you can
substitute four inch pots. The tubers shouldn’t be placed into
flats or pots for starting until the eyes on the tubers have started to
sprout. This is a sure sign they are ready to grow.
Planting sooner runs the risk of rotting tubers. The top of the
tuber is the side with the depression in it. When starting them,
cover them shallowly and keep slightly moist until you have shoots a
few inches tall. Once you have shoots, they can be transplanted
directly into eight inch pots. All upright growing tuberous
usually require staking. It’s best to put the stake in when you
do the transplanting to avoid accidentally pushing the stake through
the tuber later. When transplanting, plant them an inch or so
deeper than you started them at. This will cause more roots to
form above the tuber and make for a sturdier plant.
If planting the pendula hanging type tuberhybrida, it’s
best to use at least three tubers per basket to make a full
plant. Tuberous begonias can be grown easily in beds but the
tubers should be started first before planting in the beds. They
should have soaker type watering. Overhead or spray watering may
cause mildew problems or damage the blooms.
Pruning and
Staking
Tuberous begonias don’t require pruning. Some of the
species and pendula type tuberhybrida should have the tips pinched out
when they are a few inches tall to promote fuller growth. The
other tuberhybrida and most species shouldn’t be pinched. The
tuberhybrida are usually grown as single stemmed plants except for
Nonstops. Pinching these will force branching but reduce the
flower size. Most of the upright growing tuberous
require staking to keep the stems from breaking off at the tuber.
It is best to insert a stake when planting the tuber so you can place
the stake next to the tuber. If you wait to stake later, make
sure you insert the stake far enough away from the stem that you don't
push it down through the tuber. It is best to use the plastic
stretch type tie since the stems are soft and can be damaged if wire
ties are used.
Watering
and Fertilizing
Tuberous begonias usually require more water than some
other types but should be allowed to dry slightly before
watering. They will let you know if they are improperly watered
by dropping flowers or wilting. They should be fertilized weekly
with quarter strength fertilizer all during their active growing
period. Stop fertilizing when plants shows signs of winding down
or a month or so before you expect them to do dormant. Double
flowered tuberhybrida will put out single or undersized flowers if not
fed properly. Avoid getting water on leaves and flowers except
for occasional necessary rinsing for grooming purposes.
Light
and Heat
Tuberous begonias require good light in order to grow or
bloom properly. Morning sun or filtered light all day is
preferred. They won’t grow in extremely hot or dry areas.
They grow best in coastal areas of California and northern
climates. They either won’t grow or won’t grow well in most areas
of Florida and Texas where other types of begonias are commonly grown
because they can't tolerate the high temperatures.
Pests
and Diseases
Since tuberous begonias go dormant part of the year, they
shed most of their pests at that time so aren’t usually pest
prone. The only pest that is a common problem are thrips.
Any time you see distorted new leaves or flowers you should suspect
thrips. They can sometimes be seen moving quickly if you pull
flowers apart but are extremely small. Thrips can only be
controlled with an insecticide. Routinely spraying tuberous
begonias is recommended to avoid a thrips problem. Waiting till
you see damage will usually result in having bad looking plants for the
rest of the season. They will survive but once the damage is done
they probably won’t look pretty again that season unless caught
early. Since thrips hide in the buds and flowers, a systemic
insecticide works best since it’s nearly impossible to hit them
directly with any other type spray. The tubers can sometimes have
mealy bugs or borers on them so should be sprayed or dusted before
storage.
Tuberous begonias can be mildew prone in various locations
or during poor weather conditions. Some may require a fungicide
to keep mildew in check. Some fungicides may discolor or damage
flowers so should be tested on one plant first.
Propagation
Tuberous begonias can be started various ways. They
can be grown from seed, from stem cuttings, from tuber sections or from
leaf cuttings. Some that form bulbils from leaf nodes can also be
started from those. Seed should be started early enough in the
season so plants have time to get some size before moving
outside. Plants grown from seed started early enough will perform
nearly as well as plants grown from tubers but usually aren’t their
best till their second year. After the first year, the tuber may
only be pea sized so you may have to look carefully after they go
dormant to find the little tuber. The tuber will increase in size
every year.
Tubers can be sectioned like potatoes with at least one
eye per section. These sections should be allowed to heal before
planting and dusted with a fungicide. Since many tubers are very
expensive I don’t recommend sectioning the tubers. If you’d like
to try that method of propagation, practice with cheaper tubers before
trying it on any expensive tubers. Some species form clusters of
small tubers. These can be separated into individual plants each
year.
Tuberous begonias grow easily from stem or leaf
cuttings. Young growth works best and cuttings should be taken
early in the year. The plant grown from the cutting needs time to
grow it’s own tuber before it goes dormant the next fall, so the
earlier the better. Tuberous begonias don’t root well in water,
except for the smaller stemmed types like the caudex forming types and
B. sutherlandii. Most tuberous stem cuttings will start directly
in potting mix. They may look wilted initially but most will
eventually root and recover. It’s best to keep them in shade
until they have rooted. Leaf cuttings should be started as you
would rhizomatous begonias.
Special
Uses, Tips, and Winter Storage
You can find tuberous begonias to fill nearly every niche
except full sun or deep shade areas as long as you have a climate where
tuberous begonias will grow. Some begonia growers have
collections exclusively of tuberous begonias. They put on a
magnificent show during the summer and fall and since they are dormant
for the winter, the grower gets a rest too. In Southern
California tuberous begonias aren’t commonly entered in shows so this
is a good category for new growers to try in those areas.
A few varieties such as B. sutherlandii, it’s hybrids and
also B. dregei varieties can be kept growing through the winter by
growing in terrariums or under lights. Most tuberous varieties
however will go dormant and must be stored for the winter. In
late fall your plants will start to wind down and foliage become
yellow. Once it’s apparent they are stopping growing for the
season, stop all watering and put in a location where they won’t get
rained on. Eventually the plant will die back completely.
You can store the tubers in the pots they grew in but it’s
better to remove all the soil mix and dry the tubers further in open
flats or boxes. Tubers planted in beds should also be dug up and
dried. Many growers dust the tubers with a fungicide or
insecticide powder at this time to eliminate any pests or diseases
before storing the tubers. They can be stored in cardboard boxes
or paper bags. Some growers store them in sawdust, vermiculite or
peat but that isn’t necessary. Whatever they are stored in should
be dry and be able to breathe. Plastic bags aren’t recommended
since they hold moisture and cause diseases that can do damage to the
tubers.
Tubers should be stored in a cool dry location.
Storing them in overly warm areas may cause them to sprout prematurely,
too cold may cause rotting. Tubers should be inspected routinely
over the winter to check for rotting or insect damage. Prompt
attention may save tubers with problems if caught early. Rotted
parts can be cut out and the remaining tuber treated with a
fungicide. Storing each tuber in it’s own paper bag will
keep potential problems from spreading to other tubers. When
you’re almost ready to plant the tubers, you might move them to a
warmer place to help them sprout out sooner.
If you’re really lazy and live in a temperate climate like
southern California, you can leave the tubers in the pots they were
growing in and just tip them on their sides for the winter to keep them
dry. In the spring when they should be starting to sprout, turn
the pots up again and start watering. This method isn’t as good
as proper storage but better than doing nothing and most tubers will
usually come back though you will lose a few.
