Begonia care: rex and cane begonias indoors

Light

The University of Illinois Extension lists begonias as medium-light plants, around 150 foot-candles, but most grow and color better a step brighter. Clemson places foliage begonias in a bright spot away from direct sunlight, and that is the right home for a rex: a foot or two back from an east window, beside a north window, or behind a sheer curtain on a west exposure. Missouri Botanical Garden adds a quarter turn each week if light comes from one side, otherwise rex leaves all tilt toward the glass.

Cane begonias want more. Clemson notes that many cane and shrub types flower best with some direct sun, and that early morning sun in an east window works well, with several hours of winter sun a bonus. The warning sign is on the leaves themselves: an angel wing that gets too much hot sun loses its silver spots and then scorches, according to Clemson's outdoor begonia factsheet. Pale, stretched growth with long gaps between leaves means the reverse.

Water

Clemson's instruction for indoor begonias is to keep the soil evenly moist and let it dry only slightly between waterings, and never to leave the pot sitting in a saucer of water. In practice that means touching the surface: when the top of the mix feels dry to a fingertip and the pot has lost some weight, water slowly until it drains, then empty the saucer. Missouri Botanical Garden phrases the rex version as letting the surface become almost dry before the next drink.

The margin for error is narrow with rex begonias. Their rhizome sits right at soil level, and NC State lists begonias as highly susceptible to root rot, along with Pythium stem rot. Water at the base and keep the leaves dry, which also discourages the powdery mildew and botrytis that spot rex foliage. Cane begonias, with fibrous roots and tougher stems, recover from a missed watering or a slightly soggy week far more readily.

Cut back in winter. Clemson tells growers to reduce water once days shorten, and Missouri Botanical Garden explains why with rex types: many cultivars stop growing in fall unless kept under lights, and some drop part or all of their leaves. A dormant rex in wet mix is the classic way to lose one. Give just enough water to keep the rhizome from shriveling until new leaves unfurl in spring.

Soil and pH

Use a light, porous mix with plenty of organic matter. NC State recommends moist, well-drained soil with a pH between 5.5 and 6.5, which matches a standard peat-based houseplant mix. For rex begonias, cutting in a generous handful of perlite keeps air around the rhizome. Missouri Botanical Garden specifically asks for a shallow pot, since the roots stay near the surface and a deep pot holds a cold, wet layer the plant never uses.

Humidity

This is where rex and cane begonias part ways most clearly, and the sources do not agree on a number. Missouri Botanical Garden asks for more than 50% humidity for rex begonias, adding that some cultivars want more than others, while NC State says rex plants need humidity above 75% and decline when it drops. Most heated homes sit under 30% in winter, according to the University of Illinois Extension, so a rex on an open shelf is usually struggling. Clemson asks for a humid environment but advises against misting the plant itself; spray the surrounding air instead, or better, run a humidifier, group plants together, or grow a small rex in a terrarium. Cane begonias tolerate ordinary room air with little fuss.

Temperature

Clemson recommends 66 to 73°F (19 to 23°C) for foliage begonias, with nights no lower than 55°F (13°C). NC State gives the same floor, and Missouri Botanical Garden says rex begonias do best around 70°F by day and 60°F at night, surviving cooler rooms with slower growth. Keep plants off cold glass in winter and away from heating vents, which dry the air as well as the leaves.

Fertilizer

Feed only while begonias are actively growing. Missouri Botanical Garden suggests a balanced fertilizer at quarter strength every two weeks for rex begonias once spring growth starts, tapering in fall and stopping in winter; a controlled-release product applied every three months is its alternative. Clemson uses half-strength balanced liquid fertilizer at alternate waterings for both flowering and foliage types. Pick one routine and stay with it, and skip feeding entirely during a rex's winter rest.

Work out the dose for your watering can with the fertilizer dilution calculator →

Repotting

Repot in spring, Clemson advises, and divide plants that have become pot-bound at the same time. A rex rarely needs a much bigger pot; more often its rhizome has grown past the rim with bare sections, which Missouri Botanical Garden suggests cutting back. The cut piece can be rooted as a new plant. Move cane begonias up one size when roots circle the pot or the plant wilts between normal waterings, one of the signs the University of Illinois Extension lists. In spring or early summer, cut overlong canes back to push new shoots from the base, and stake tall stems.

• Roots circle the inside of the pot or poke from the drainage hole
• A rex rhizome has crept over the rim with bare, leafless sections
• Cane begonias wilt between normal waterings

Propagation

Rex begonias root from leaf or rhizome cuttings, as Clemson's outdoor factsheet notes. A healthy leaf with an inch of stalk, pressed into damp mix under a clear cover in bright, indirect light, forms plantlets at its base; a section of rhizome laid on the mix roots faster. Cane begonias grow easily from stem cuttings of 3 to 6 inches with at least two nodes, the University of Illinois Extension guideline. Spring and early summer give the quickest results.

Methods: Leaf or rhizome cuttings (rex), Stem cuttings (cane), Division at repotting · Best season: spring to early summer.

Common problems

Most begonia trouble comes down to water and air. A soft, darkening rhizome means rot from mix that stayed wet; crisp brown leaf edges on a well-watered rex point to dry air. White powder or gray fuzz on rex leaves is mildew or botrytis, which Missouri Botanical Garden recommends preventing by removing dead leaves promptly and improving air circulation. Mealybugs, whiteflies and spider mites are the pests Clemson lists for indoor plants. Leaf loss in late fall on a rex is often just dormancy, not disease.

Not sure which one it is? Try the plant problem diagnosis tool →

Is the begonia toxic to cats and dogs?

Begonias are toxic to cats and dogs. The ASPCA lists the genus and rex begonia separately, both with soluble calcium oxalates as the toxic principle, and notes vomiting and salivation in dogs and cats; the most toxic part is underground, so a pet digging in the pot is the bigger risk. NC State rates human severity as low, with possible skin irritation from the sap. Keep begonias on high shelves if a pet chews plants, and call your veterinarian or the ASPCA Animal Poison Control Center after any ingestion.

Check any other plant against the ASPCA lists →