Tuberous begonias
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If you appreciate plants that have no hesitation in boldly stating their presence with huge, almost artificially perfect flowers, then tuberous begonias are for you. While some may find them rather too overstated, downright brazen even, if you like colour, and plenty of it, with subtlety an option rather than compulsory, then look no further.
Flowering from late spring, as the days exceed 13 hours long, until well into autumn when the foliage dies back, these tuberous-rooted perennials have been extensively hybridised and refined to the point where the larger-flowered forms are nothing short of spectacular. For those with greenhouses or a very mild winter climate there are even types that flower well into winter, if not right through the cooler months.
Description and classification — groups
Begonias are among those convenient plants where the proper name is also the common name. The genus is found in the tropics and subtropics, particularly in the Americas, and is made up of around 900 species with 130-odd in cultivation, from which many cultivars and hybrids have been developed. These garden forms have been divided into 8 main categories. They are:
1. Cane-like, which have narrow, upright stems with conspicuous leaf nodes and evergreen foliage.
2. Shrub-like, which are bushy plants with branching stems.
3. Thick-stemmed, which resemble the cane-like but with much heavier stems that sometimes form a trunk.
4. Semperflorens, which are the fibrous rooted types usually treated as summer-flowering bedding annuals.
5. Rhizomatous, which may be evergreen or deciduous and which are often grown as much for their foliage as for their flowers.
6. Rex-cultorum, which are the fancy-leaved Rex begonias that are usually grown as house plants.
7. Tuberous, which have thickened tuberous roots and which usually die back over winter or, in the case of the winter-flowering types, which may be dried off at some other time.
8. Trailing Scandent, which are pendulous, ever-blooming plants often with long internodes and which should not be confused with the trailing tuberous begonias.
So group seven it is, yet although quite a few begonia
species have tuberous roots, when we talk of tuberous begonias we’re generally
referring to the fancy-flowered group of garden-raised plants known as Begonia
× tuberhybrida. Developed mainly from South American species, they first
appeared in Europe in 1867, just three years after the introduction of the most
influential of the early parent species, B. pearcei. Since that time
thousands of hybrids have been raised and we now have tuberous begonias in a
wide range of sizes and styles of flower and growth habit.
There are small- medium- and large-flowered hybrids; they
may have single, semi-double or fully rose- or camellia-like double flowers;
they may be small and mound-forming, trailing or upright to nearly a metre
tall. And while the flowers are spectacular, don’t ignore the foliage. Because
although unlike say the Rex begonias, tuberous begonias are never grown for
their foliage alone, their velvety, deep green leaves add a rich luxuriance
that is the perfect foil to the flowers, which would definitely be diminished
without the contrast of the leaves.
Cultivation
So, tuberous begonias are beautiful. I don’t need to tell
you that, the pictures speak for themselves, but how do you get the best out of
them? Well, as garden plants they’re not for everyone and not for every
location, though with careful selection and siting you may be surprised at just
how well they grow outdoors.
Begonias have a preference for cool, moist conditions and a
climate that doesn’t suffer from extremes of summer heat or winter cold. They
need bright light to flower well but should be out of direct sunlight,
especially during the heat of the day, and they also need shelter from strong
winds or the flowers may brown at the edges and the soft foliage may be torn or
bent. Tuberous begonias flower best with humus-rich soil, plenty of moisture
and regular feeding.
Given those requirements it’s not surprising that many
gardeners choose to cultivate tuberous begonias indoors, as conservatory,
shadehouse or cool greenhouse plants. However, if you have a bright southerly
facing position in your garden or a shaded spot facing north, then begonias
will thrive outdoors too, particularly in areas that don’t often experience
drought in summer.
Strong sun and wind, especially hot dry winds, are the main
enemies; light soil that dries out quickly doesn’t help either. But in a
lightly shaded, sheltered position with soil that has been thoroughly prepared
with plenty of well-rotted compost tuberous begonias will flower from early
summer to the first frosts. And all that you need do is to stake the tall
growers to bamboo canes (specialist nurseries stock wire frames), remove any
spent flowers, keep the soil moist and add a little liquid fertiliser every
week.
If you find that the super-fancy large-flowered forms are
simply not tough enough for your garden, don’t give up. Instead try some of the
smaller-flowered hybrids. The little Multiflora types, commonly known as
Flamboyant Begonias, are very resilient. Grown as massed bedding or in
clusters, they’re most commonly seen with bright red flowers, which often
almost hide the foliage, but also occur in orange and a somewhat weaker
yellow-flowered form.
Nonstop begonias are crosses between the Multifloras and the
larger flowered types. As you’d expect they’re of intermediate height and
vigour. They flower continuously, even in winter if kept indoors, and are
available in a wide range of colours. Nonstops are F1 hybrids so there is no
point in saving the seed and any seedpods should be removed to keep the plants
flowering. Reiger begonias, developed from B. × hiemalis, are similar.
And if open beds don’t seem to work, consider growing your
begonias in pots so that you can find just the right place for them. The
upright types flower and grow well but are rather brittle, necessitating
staking. The trailing types, often B. boliviensis hybrids, have more
flexible, pendulous stems and when grown in hanging baskets they are easy-care
plants that make a great show. Trailing begonias usually do best in
sphagnum-lined wire baskets rather than solid pots, their roots appreciating
the cool moist sphagnum.
Disbudding and deadheading
Begonias have separate male and female flowers. Usually one
large male flower is subtended by two smaller female flowers. Removing the female
flowers before they mature will allow the showy male flowers to reach their
full size and will also prevent the development of seedpods that could lessen
the plant’s vigour. Old flowers should be removed once past their best. They
snap off easily and doing this not only encourages new blooms to form, it also
helps prevent fungal diseases that could develop among the decaying petals.
Pests and diseases
Begonias are neither particularly susceptible to nor
resistant to pests. Slugs and snail relish the young shoots and the mature
foliage, various caterpillars may chew the foliage, rasping and sap-sucking
creatures such as thrips, aphids and mealy bugs may be present, but with a
little attention and routine care, pests can usually be stopped before they get
out of control.
More of a problem are fungal diseases, especially soft rots,
mildew and botrytis. Damaged stems can quickly become soft, watery and rotten
and this may lead to the eventual collapse of the plant. Almost inevitably the
foliage will develop mildew in late autumn — it’s just part of the winter
die-back process — but mildew can also occur during the growing season. Good
ventilation goes a long way towards controlling the severity of fungal
diseases, keeping the foliage and stems reasonably dry also helps, though
spraying with a fungicide will probably be necessary too.
Winter care
As flower production lessens from mid-autumn, cut back on
watering and feeding and allow your begonias to dry off. While the foliage
should dry, brown and fall away without too many problems, do keep an eye open
for any fungal diseases that could spread to the tubers.
Once the foliage has dried, the tubers may be lifted or
removed from their pots for winter storage. This isn’t always necessary in mild
winter areas, but where hard frosts or prolonged wet conditions are likely it’s
a good idea. The tubers can be stored in barely moist sawdust or any other
fairly dry, inert medium, such as damp, shredded newspaper. Replant them
(concave side up) in spring as the new shoots appear. Cover the tubers with a
few centimetres of soil, as they sprout roots from the tops too.
The exceptions to the process are the winter-flowering
B × hiemalis hybrids, which result from crossing B. × tuberhybrida
with B. socotrana, a species from an Indian Ocean Yemen. Widely sold as
‘blush’ begonias, these plants start to flower from late summer and will remain in
leaf and flower until spring. Obviously these plants, which are completely intolerant
of frost, need a very mild, benign winter to grow outdoors. However, they are very
adaptable to indoor cultivation and are a great choice for winter-flowering house
plants that can spend the summer outdoors in the garden.
Propagation
There are several ways to propagate tuberous begonias, the
method used varying with they type of plant.
Seed
Sow bought seed to produce F1 hybrids such as the Nonstops
or to get a new crop of vigorous young plants.
Begonia seed is very fine, dust-like in fact. It’s so fine
that it doesn’t usually come in seed packets, where it would be lost in the
folds, but in glass phials that have to be snapped open before sowing. Pelleted
seeds are much easier to handle, thought they are not always readily available.
The seed needs warmth and light to germinate. It should be
sown in spring, uncovered, in heated trays. Keep the seed just moist until it
germinates. The young seedlings grow quickly and are soon large enough to pot.
When small they are sensitive to draughts and temperature fluctuations and
should be kept under cover until spring is well settled into early summer.
Prepare your garden beds with high-humus compost and organic
fertiliser and because the foliage will be tender, plant out when the weather
is not too hot and sunny.
Division
Mature plants have large tubers that divide readily and
division is a good way to quickly produce established, sturdy plants. Divide
the tubers in spring when replanting. They slice easily with a sharp knife but
because finding the growing ‘eyes’ can be difficult keep your divisions on the
large side. To prevent fungal diseases, dust the cut surfaces with sulphur
powder and allow them to dry before planting.
Cuttings
Most tuberous begonias will grow from cuttings and this is
an especially good method of building up large stocks of the small-flowered
Multiflora types for massed bedding. The fresh spring and early summer shoots
make the best cuttings and will strike very quickly under mild humid
conditions. You can continue to take cuttings well into summer but unless the
new plants can develop reasonably sized tubers before winter they will probably
not survive until the following spring.
Like many of the house plant begonias, B × hiemalis
is often raised from leaf cuttings. This involves removing a mature leaf,
slicing across its veins and pinning the leaf down on moist soil. A warm humid
environment, such as an enclosed propagation tray, is essential. Also, you
should start in spring so that the young plants are well-established before
winter.
Begonia displays
Several public gardens use tuberous begonias in their
displays. Although the best-known are probably those at Pukekura Park in New
Plymouth and in the Townend House at Christchurch’s Botanic Gardens, smaller
centres such as Oamaru often devote greenhouse space to begonia displays. If
you’re visiting Australia in late summer, the begonias at Ballarat are
absolutely brilliant.
Copyright Geoff Bryant
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