Rex Begonia Care: Humidity, Light, Water and Dormancy

Give a rex begonia bright but filtered light with no direct sun, humidity above 50 percent, a shallow pot of airy mix, and water only when the top inch of soil is dry. The New York Botanical Garden (NYBG), whose care guide is the most detailed of the sources we checked, adds a steady 70 to 75 F and no feeding in winter, when many plants rest.

In the sources below, most problems trace back to one of three things: dry air, soggy soil, or the normal winter leaf drop being mistaken for dying.

NYBG says a relative humidity above 50 percent is important for this group, and a few plants need as much as 80 percent. Logee's gives 50 to 70 percent as the preferred range and says the plants can survive the 40 to 50 percent of an average home, so expect crisp edges rather than death in drier rooms.

Ways to raise it, from the sources:

• Group plants together, run a humidifier, or use a tray of wet pebbles with the pot sitting above the water (Anoka County; NC State Extension).
• Grow it in a bathroom or kitchen with suitable light (NC State).
• Do not mist the leaves. NC State says to avoid misting to prevent disease, and Clemson's Home & Garden Information Center says misting is not recommended; spray the surrounding air instead.

Bright, filtered light with no direct sun is the rule in every source. Anoka County warns that direct sun scorches the leaves, and Logee's says it also fades their colours. NYBG's placement advice is specific: near a north-facing window or several feet back from an east-facing one, turning the pot a quarter turn at each watering so it grows evenly, and moving plants a further foot back from the glass in winter. Logee's also suggests east or west windows. Keep it away from drafts; Anoka County says rex begonias dislike them.

Water when the top inch of soil is dry, and water sparingly (Anoka County). NYBG adds three details: use tepid water, water in the morning, and tip away any water left in the saucer after 15 minutes. In winter, let the mix get drier without drying out completely.

For the mix, NYBG uses equal parts coco-coir potting mix, small orchid bark (1/8 to 1/4 inch) and coarse perlite. Anoka County and Logee's describe the same idea: a light, airy potting soil amended with perlite, peat or orchid bark. NC State notes a preference for acidic soil, below pH 6.0, rich in organic matter.

Small, shallow pots. Rex begonias grow from a rhizome (NC State), and NYBG recommends shallow containers, moving up no more than one pot size at a time; it says repotting may be needed as often as every year when the plant gets crowded. Logee's recommends clay pots because they let moisture escape, and Anoka County insists on drainage holes: the pot should never sit in water.

Match the symptom to the cause:

• Leaves dropping in mid-winter, few or no new leaves. Probably dormancy, not death. NYBG says rex begonias often go dormant in mid-winter for about two to three months. Water less, stop feeding, and keep the plant around 60 F until new growth starts in spring.
• Soft, collapsing stems or a mushy rhizome. Root rot from overwatering or poor drainage (NYBG; Logee's). Let the mix dry more between waterings and check the drainage hole.
• Crisp, brown leaf edges. Dry air, or minerals in the water; Logee's suggests rainwater or distilled water, and NYBG links leaf spots to high mineral content and splashing.
• White powder on the leaves. Powdery mildew, which NYBG says is common in high humidity with poor air flow. Logee's home remedy is 1 tablespoon of baking soda with 3 drops of mild liquid soap in a gallon of water, sprayed every 10 to 14 days.
• Grey, fuzzy, moldy blotches. Botrytis (NYBG); NC State advises removing dead leaves and providing good air circulation.
• Faded or scorched patches. Too much light (NYBG).
• Sticky or cottony spots, fine webbing. Mealybugs, scale, aphids or spider mites (Anoka County); act as soon as you see them.

Yes in warm climates, and for summer elsewhere, in shade. NC State lists rex begonias as hardy in USDA zones 10a to 12b, where they grow in landscapes, containers and hanging baskets, and treats them as indoor plants in most other climates. If you move one outdoors for summer, keep it out of direct sun, which scorches the leaves (Anoka County), and bring it in before nights drop below 50 F, the minimum Logee's gives. Check for pests before it comes back inside.

The sources agree on dilute feeding during active growth and none in winter, and differ only on frequency: every two to four weeks at half strength (Anoka County), every two to three weeks at half strength from spring to early autumn (Logee's), or every two to four weeks from February to October with a dilute 15-30-15, tapering after mid-summer (NYBG).

Propagation is easy and a good backup for an ailing plant:

• Leaf cuttings: Logee's keeps 2 to 6 inches of leaf stalk, holds them at 70 to 75 F, and sees new plantlets in about 12 weeks.
• Stem cuttings: 2 to 3 inch sections in spring, rooting in about four weeks (NYBG).
• Rhizome pieces: 1 to 3 inch segments, rooting in about three weeks (NYBG).

NYBG keeps cuttings humid in a propagation case or under a plastic bag.

The ASPCA lists begonias as toxic to cats, dogs and horses. The toxic principle is soluble calcium oxalates, the reported signs in cats and dogs are vomiting and drooling, and the underground parts are the most toxic. NC State rates the severity as low and adds that the sap can irritate skin. Keep the pot where pets cannot dig in it. If a pet may have eaten part of one, call your vet or the ASPCA Animal Poison Control Center at (888) 426-4435.

Research and drafting for this page were done with the help of AI, and every figure was then checked line by line against the sources listed at the end. Where sources disagree, we say so rather than pick one silently. If you find a mistake, tell us and we will correct it.