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Begonia — the garden show-offs

Seemingly infinite variety characterises the genus Begonia.

Seemingly infinite variety characterises the genus Begonia.

Seemingly infinite variety characterises the genus Begonia. They can brighten up both your indoor and outdoor gardens. Probably the most prized though not the most widely grown are tuberous begonias, whose name belies their fulsome displays of mid- to late-summer bloom. The flowers often resemble roses or carnations, and they rarely disappoint growers.

The genus Begonia includes more than 2,000 species and hybrids, and they are as varied in appearance and habit as these numbers suggest. Some are valued mainly for flowers, some for decorative leaves, some for both leaves and flowers. Begonias range in size from tiny, ground‑hugging creepers to stout‑stemmed specimens 3m tall. Yet they all share a number of characteristics. Almost all forms have asymmetrical leaves; the leaves always grow alternately along the stems; and new leaves emerge from stipules (leaflike sheaths). And many begonias do not require continuous direct sunlight, a fact that makes them particularly suitable for indoor use.

Most kinds bear flowers in clusters on short stalks arising from or near leaf axils. There are separate male and female flowers on the same plant, however, each cluster normally composed of either all male or all female blooms. Male flowers tend to be the more eye‑catching, partly because their petals are often of different shapes and sizes, whereas the petals of a female flower are more nearly alike. A distinctive feature of the female flower is the seed-bearing ovary, which looks like a three‑lobed appendage immediately behind the petals. Female flowers, although they may slightly fade, usually last for weeks or even months, but the male flowers tend to drop off within two or three days of opening.

Because the genus is so large, it is generally divided into groups based on the differing storage organs or root structures of these plants. Some have fibrous roots (as most plants do). A second group consists of species in which roots grow down from a thick creeping rhizome. A third group includes tuberous species that have a fleshy, swollen storage organ at the base of the stem. In these pages fibrous‑rooted and rhizomatous begonias are discussed together because their growth cycles and cultivation needs are similar — and are very different from those of the tuberous kinds.

FIBROUS-ROOTED AND RHIZOMATOUS BEGONIAS

There are, roughly, three different kinds of fibrous‑rooted begonia. Many species have smooth, rather woody stems marked here and there by swollen, knotted joints (nodes) a bit like the stems of bamboo. These plants usually have leaves like angel’s wings‑lobed near the top and acutely asymmetrical. A second kind of fibrous‑rooted begonia has fleshy stems, and many parts of the plant especially leaves and flowers‑are covered with hairs. These plants (commonly known as hirsute begonias) are usually bushy, like those with bamboo-like stems, although they will sometimes trail if left unsupported. So they are often grown in hanging baskets. Finally, there are the fleshy‑stemmed wax begonias, whose crisp leaves are waxy, not hairy.

Most fibrous‑rooted begonias flower profusely, and are grown both for their flowers and their decorative foliage. The flowers vary in size and colour, but most have only a single layer of petals.

The rhizomatous begonias often have a thick, fleshy rhizome, which crawls over the surface of the potting mixture, sending down roots at intervals. The plants of most species grow less than 23cm tall. The few that grow taller often need supporting with thin stakes. Many of these plants are without conventional, erect stems, but a few have much‑branching, fleshy stems. Rhizomatous begonias are prized for their foliage. Leaves may be nearly circular, roughly star‑shaped, or heart‑shaped. Some plants have miniature leaves, some huge, some in between. Flowers are always small, each with only a single layer of petals.

PROPER CARE: FIBROUS-ROOTED AND RHIZOMATOUS BEGONIAS

Light Fibrous‑rooted and rhizomatous begonias grown primarily for their fob age need bright light without direct sunlight. Those grown principafiy for their flowers need three to four hours a day of direct sunlight.

Temperature Normal room temperatures are suitable for actively growing plants. Those that have a winter rest period should be kept at about 15degC — but not below 12degC during this period. All begonias suffer in dry air. For increased humidity stand pots on trays of moist pebbles, and suspend saucers of water under hanging baskets.

Watering Water actively growing plants moderately, allowing the top 2cm of the potting mixture to dry out before watering again. During any winter rest period water more sparingly, allowing the top half of the mixture to dry out between waterings.

Feeding Apply standard liquid fertilizer every two weeks to actively growing plants.

Potting and repotting Use either a peat‑based potting mixture or a combination of equal parts of soil‑based potting mixture and coarse leaf mold. Put 3cm layer of clay‑pot fragments in the bottom of pots for extra drainage.

Move fibrous‑rooted plants into pots one size larger every spring until maximum convenient pot size (probably 15-20cm) has been reached. Thereafter, topdress annually with fresh potting mixture. Rhizomatous begonias have shallow roots and are best planted in half‑pots or pans. Move a small rhizomatous plant into the next size pot or pan only when the rlfizome has grown across the entire surface of the potting mixture; do this preferably in spring. Discard aging rhizomatous begonias in favor of attractive new plants.

When potting or repotting a begonia, simply sprinkle some mixture around the roots, and tap the container briskly to settle the mixture. Do not firm it down with the fingers.

PROPAGATION:FIBROUS-ROOTED KINDS

Take 7‑ to 10cm‑long cuttings of non‑flowering shoots in spring or early summer. Trim each cutting immediately below a leaf, carefully remove the leaf, and dip the cut end of the stem in hormone rooting powder. Plant the cutting in a 7.5cm pot of a moistened equal‑parts mixture of peat moss and coarse sand or perlite, and enclose the whole in a plastic bag or propagating case. Stand it in bright filtered light until renewed growth indicates that rooting has occurred (about three to six weeks). Uncover the rooted cutting, and begin to water it sparingly and to apply standard liquid fertilizer once every two weeks. Do not overwater, particularly not the hirsute begoriias, which will rot if kept too wet. About six months after the start of propagation, move the young plant into a slightly larger pot of standard mixture, and treat it as a mature begonia.

Many of these begonias can also be propagated from seed. Seeds are very tiny and should not be buried when sown. Mix them with a little fine sand before sowing.

PROPAGATION: RHIZOMATOUS KINDS

Cut off 5- to 7cm‑long growing tips of rhizomes and treat them like stem cuttings of fibrous‑rooted specimens (see above). Or, in spring, cut a rhizome into 5- to 7cm‑long sections, each with at least one growth point and treat cut ends of sections with sulphur dust. Plant each section half in and half out of slightly moistened rooting mixture in a 7.5cm pot or pan. Use a rooting mix of equal parts peat moss and coarse sand or perlite. Place the section either horizontally or vertically, depending on how the parent rhizome was growing in its container. Enclose each planted piece of rhizome in a plastic bag or propagating case and stand it in bright filtered light. Roots should form in four to six weeks. When two or three new leaves have appeared, uncover the little plant, repot it in an appropriate container of the recommended mixture for begonias, and treat it as a mature plant.

Most of the rhizomatous begonias can also be propagated every spring from leaf cuttings. (B. erythrophylla and B. limmingheiana are exceptions).  Take a healthy leaf with 3- to 5cm of leafstalk attached, and plant the stalk at an angle of 45 deg in a small pot of the moistened rooting mixture recommended above (or insert several leaves in a small pan or seed tray). Enclose the whole in a plastic bag or propagating case, and stand it in bright filtered light. Rooting should occur in two to three weeks, and tiny plantlets should begin to appear from each leaf after a further two to three weeks. Several plantlets are generally clustered together. When each of them has produced at least two recognizable leaves, pot the plantlets up singly in 7.5cm containers of the recommended potting mixture for mature begonias. Before treating the little plants as adults, however, dampen the mixture slightly and put the plants back in a plastic bag or propagating case for another four weeks. This will acclimatise them to normal room conditions.

SPECIAL POINTS

Some begonias, particularly B. ‘Coralhna de Lucema’, B. maculata, and the tuberous B. sutherlandii are susceptible to attack by powdery mildew, which shows up at first as small powder coated spots on stems, leafstalks, and leaves. As a preventive measure spray all begonias with a suitable fungicide at regular intervals.

Many Begonias are grown for their interesting foliage

Many Begonias are grown for their interesting foliage

TUBEROUS BEGONIAS

Tuberous begonias are grown principally for their handsome flowers. Plants truly characteristic of the group have swollen underground stems (tubers) and are deciduous, with a period of total dormancy every year. Some kinds, however, do not lose their top growth in winter even though they are tuberous. They have a deep rest period. These plants are termed ’semi‑tuberous’. There is a third group, composed entirely of winter‑flowering plants (commonly called Christmas‑flowering begonias in the Northern Hemisphere), which are fibrous‑rooted but have tuberous ancestors and the tuberous habit of dormancy after flowering. Plants of this type are usually grown as temporary indoor specimens, to be thrown away when flowering ceases.

PROPER CARE: TUBEROUS BEGONIAS

Light Give all tuberous begonias bright filtered light all year round. Light is not important during the dormant period for types that lose their top growth.

Temperature During the active growth period normal room temperatures are suitable. In temperatures above 18 deg C stand pots on trays of moist pebbles, or suspend saucers of water under hanging baskets. During the winter keep dormant forms at a temperature of about 12 deg C. Semi-tuberous forms, which retain their foliage while resting, should ideally be kept at about 12 deg C in bright filtered light during the winter. Winter flowering begonias become totally dormant in summer, but they are best discarded after flowering.

Watering Water actively growing plants moderately, allowing the 2cm of the mixture to dry out before watering again. As growth slows down, reduce amounts of water gradually. For the forms that lose their stems and foliage in winter, stop watering when the leaves begin to turn yellow. During the winter rest period of semi‑tuberous types give just enough water to prevent the potting mixture from drying out.

Feeding Apply a high‑potash liquid fertiliser to actively growing plants every two weeks.

Potting and repotting Use either a peat‑based mixture or a combination of equal parts of soil‑based mixture and coarse leaf mold. Put an 3cm layer of clay‑pot fragments in the bottom of pots for extra drainage. When potting or repotting, simply sprinkle some mixture around the tuber and roots, and tap the container briskly to settle the mixture.

Start the tubers of B. tuberhybrida forms into growth in early spring by planting several in shallow trays of moistened peat moss, setting the tubers (with the concave side upward) half in, and half out, of the peat moss.

Stand each tray in bright filtered light for about three or four weeks, when 5cm of top growth will have been made. Then move each specimen into a 7.5- or 10cm pot of the recommended mixture for adult plants. The dormant tubers of other forms are smaller, and can be planted directly in 7.5- or 10cm pots of the recommended mixture. Thereafter, treat all forms as mature tuberous begonias. The large flowered hybrids may need to be moved into larger pots two or three times during the summer, but most other kinds can spend the entire season in the same container.

The semi‑tuberous begonias should be moved into pots one size larger each spring. When repotting always keep the tuberous swelling at the same level in the mixture. After maximum convenient pot size (probably 15- to 20cm) has been reached, top dress annually with fresh mixture. Winter‑flowering begonias are normally bought as young plants and do not need moving on into larger pots during their short stay in the home.

PROPAGATION: TUBEROUS BEGONIAS

The best way to pro­pagate B. gracilis, B. grandis, and B.sutherlandii is from the small tubers usually known as bulbils‑that appear in leaf axils in autumn. Detach these bulbils when top growth dies down. Store them in a container at about 12 deg C until the following spring. Then plant each bulbil in a 5- or 7.5cm pot of the recommended potting mixture, just covering the bulbil with moistened mixture. Stand the pot in bright filtered light, and at first water only enough to make the mixture barely moist, but gradually increase the amount. Treat the rooted bulbil as a mature plant when it has made 8cm of top growth. It will not reach full size or flower profusely until its second year.

B. sutherlandii develops bulbils in some of its leaf axils during autumn. These can be detached and used for propagation.

B. dregei and B. Weltoriiensis, the semi‑tuberous forms, are normally propagated from 5- to 7.5cm‑long tip cuttings of new growth taken in late spring or summer. Trim each cutting immediately below a leaf, dip the cut end in hormone rooting powder, and plant it in a 5- to 7.5cm pot containing a moistened equal‑parts mixture of peat moss and coarse sand or perlite. Enclose the whole in a plastic bag or propagating case, and stand it in bright filtered light. After rooting occurs (generally in three to four weeks), treat the rooted cutting as a mature begonia, but do not move it into the recommended potting mixture for mature plants until it has made at least 15cm of top growth. (Winter‑flowering begonias can also be propagated from tip cuttings, but this is a difficult process not recommended for amateur gardeners.)

To propagate B. tuberhybrida forms cut a large tuber into two or more sections in spring, making sure that each has a growing point. Treat the cut ends of sections with sulphur dust, and pot each one exactly as if it were a whole tuber (see “Potting and repotting,” above).

SPECIAL POINTS

At the end of the growing season, the stems and leaves of deciduous begonias will gradually fall off. Do not pull away the stems since this could damage the tubers.

Disease problems associated with begonias include Botrytis blight and stem rot, powdery mildew, and Pythium root and stem rot. The major pests of begonias are mealy bugs, spider mites, scales, snails and slugs.

A second feature dealing with recommended varieties of tuberous, rhizomous and fibrous rooted begonias will be posted here in the near future. Visit again soon!