Polka dot begonia

Identification and origin

Begonia maculata belongs to the begonia family, Begoniaceae. It is native to southeastern Brazil and has become naturalized in Argentina, Cuba, the Dominican Republic and Mexico.

It is a cane-type begonia: it has fibrous roots and upright, smooth, sparsely branched stems with swollen nodes that resemble bamboo. Like those of all begonias, the leaves are lopsided; they are elongated, shaped like a stretched heart or an “angel wing,” with a wavy edge. The upper surface, dark green to olive green, is scattered with silvery gray dots; the underside is red to orange-red.

The white to pale pink flowers hang in drooping clusters, mainly in summer and into fall. Male and female flowers grow on the same plant: the males drop after a few days, while the females, recognizable by the three-part ovary behind the petals, last for several weeks and are more numerous.

Kept in a pot and maintained, the plant usually reaches 12 in – 3 ft tall and 12 in – 2 ft wide; left unpruned, its canes can exceed 6 ft. It is also sold under the names Begonia argyrostigma or variety wightii, both now included in the species.

Light

It needs bright but filtered light to build sturdy stems and to bloom. From spring through fall, avoid all direct sun: prolonged exposure scorches the leaves. An east-facing window is ideal; in front of a south-facing window, move the plant back from the glass. In winter, when the sun is lower and weaker, a few hours of direct sun near an unobstructed window are beneficial.

Give the pot a quarter turn each time you water and make any change of location gradually. Stems and leaf stalks that stretch and droop are a sign of too little light.

Watering

Water when the top 1–1 in of the mix feels dry to the touch. Then soak the whole root ball, ideally in the morning and with tepid water, and empty the saucer after about fifteen minutes: the plant does not tolerate soggy soil. Avoid wetting the foliage.

In winter, let the mix dry a little more between waterings, but never let it dry out completely. Pale, soft, floppy leaves most often point to overwatering or poor drainage.

The air should stay moderately humid, around 40 to 60 percent relative humidity. Set the pot on a tray of damp gravel, with the water kept below the surface of the stones, or use a humidifier. Above 60 percent and without air movement, the plant can rot and powdery mildew sets in, so don’t neglect ventilation.

Potting mix

Choose a light, peat-free, well-drained mix rich in organic matter, with an acidic to neutral pH. An indoor potting mix lightened with perlite works well. A terracotta pot with a drainage hole helps excess water escape.

During active growth, apply a balanced liquid fertilizer every two to four weeks, or a high-potassium fertilizer from late spring to early fall to support flowering. Feed only while the plant is actively growing.

Temperature

It grows best between 59 °F and 72 °F, with an optimum around 66 °F; the lower end of the range suits the night. Don’t let it drop below 55 °F in winter, and keep it away from cold windows, air conditioning, cold air currents and heating vents.

It can spend the summer outdoors in light shade, protected from direct sun, but it cannot take frost. In the ground, it is grown only in USDA zones 10a to 11b.

Repotting

Let a newly bought plant settle in for a few weeks before repotting it. Spring is the best season. This begonia develops a larger root system than many others and needs a good-sized pot to stay upright, but move up only one pot size: that is how it keeps the best shape and blooms best. Spread the fresh mix around the roots without packing it down.

Pruning

Regularly pinch out the tips of young stems just above a node: the plant branches, stays compact and sends up new stems from the base. On plants more than two years old, prune in spring, as growth resumes and before flowers form: cut back some of the oldest canes that have turned woody to four or five nodes, or to two or three buds if they are too long, just above an outward-facing bud. Avoid hard pruning in fall, when bringing in a plant that has spent the summer outdoors. Tall stems can be staked.

Propagation

Tip cuttings are the easiest method. In spring or summer, take a 3–4 in piece with two to four nodes from a non-flowering stem, remove the lower leaves and insert it in a light mix of perlite or seed-starting mix. Keep it warm, ideally with bottom heat of 64–70 °F, in bright light without direct sun.

Cuttings also root in water: stand the base of a 2–3 in piece, stripped of its lower leaves, in water and pot it up as soon as roots appear, after about three to four weeks.

The species can be grown from seed, but the seeds are tiny: sow them on the surface without covering them, at 70–75 °F, under a clear cover. Hybrids do not come true from seed.

Common problems

• Dry, brown leaf edges: air too dry or too much heat.
• Pale, soft leaves that rot or drop: overwatering, poor drainage, humidity too high or too little light; cold and stagnant air make it worse.
• Yellow leaves: too little light, or too much or too little water.
• Bud, flower or leaf drop: dry air, lack of water, cold air currents or sudden changes in conditions.
• Diseases: powdery mildew (white coating), botrytis (gray, moldy patches), stem rot and root rot, encouraged by soggy soil and still air.
• Pests: whiteflies, thrips, spider mites, mealybugs, aphids and vine weevils; check the undersides of the leaves regularly.

The same symptom can have several causes: look at light, watering, temperature, air humidity and the condition of the roots together before acting.

Toxicity

Begonias are toxic to dogs, cats and horses. They contain soluble calcium oxalates, concentrated mostly in the underground parts. Ingestion causes vomiting and drooling in dogs and cats and can lead to kidney failure in grazing animals. For people the toxicity is low, but the sap can irritate the skin, so wear gloves when pruning. Keep the plant out of reach of pets and young children, and if any is swallowed, contact a veterinarian or a poison control center.