Bulb · Begoniaceae
Tuberous Begonia
Begonia × tuberhybrida
Give it shade, even moisture, and good airflow, and the tuberous begonia answers with huge summer blooms.
Partial shade
Every 2–4 days
55–75°F · 13–24°C
98–120 days
Overview
The tuberous begonia earns its place for the flowers, which are among the biggest and richest any shade garden can grow. It comes from a fat underground tuber rather than a fibrous root, and that tuber is the key to everything: it powers a long summer of bloom, then rests through winter ready to start again. You can grow it in a bed, a pot, or a hanging basket.
These are warm-season plants that hate both frost and harsh sun, which makes them a shade gardener's flower. They run the color range from soft pastels to deep reds, in forms from camellia-like doubles to trailing types that spill over a basket. Most stay compact, reaching 12 to 24 inches (30 to 61 cm) tall.
None of it is hard once you respect two things. The tuber rots in soggy soil, so drainage and a careful hand with the watering can decide whether the plant lives. Get the light and water right and it carries big blooms from midsummer to frost.
Care
The short version: partial to dappled shade, soil kept evenly moist but never soggy, rich and well-drained ground, and a cool, breezy spot so the foliage stays dry and disease-free.
Light
Give them partial to dappled shade, the kind of soft light under a tree or on the cool side of the house. They take a little gentle morning or late-afternoon sun, but hot midday sun scorches the leaves and stalls the flowers.
Water
Keep the soil evenly moist but never soggy, watering every 2 to 4 days as the surface starts to dry. The tuber stores its own water, so a soaked pot rots it from below. Let the top dry slightly between drinks.
Soil
Plant in rich, well-drained loam with plenty of organic matter worked in. A pH from 5.5 to 7.0 suits them, and sharp drainage is the part that matters most, since standing water is what kills the tuber.
Temperature
They are happiest between 55°F and 75°F (13°C to 24°C). Heat above that range stresses the plant and drops the buds, and frost ends the season outright.
Humidity
Average humidity is fine, so there's no number to chase. Airflow matters far more than moisture in the air, because still, damp conditions are exactly what mildew and gray mold need to take hold.
Feeding
These are heavy feeders. Through the growing season, from spring into late summer, give a dilute balanced fertilizer about every two weeks to keep the big flowers coming.
Pruning
Pinch the growing tips early to push bushier growth and more blooms, and pull spent flowers as they fade. As late summer arrives, cut the plant back and ease off the water to walk it into dormancy.
Repotting
Treat them on a yearly cycle. Lift the tubers each fall, store them cool and dry over winter, then start them in fresh medium in spring rather than leaving them in tired, crowded soil.
Size & spacing
Plants reach 12 to 24 inches (30 to 61 cm) tall and spread 12 to 18 inches (30 to 46 cm) wide. Give each one that much room so air moves freely and the leaves stay dry.
When to plant
Tuberous begonias work on a yearly cycle built around the tuber. Start the tubers indoors in late winter, set the plants out only after the last frost has passed, and enjoy the bloom from midsummer until cold weather sends the tuber back to sleep.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Sow: Start tubers indoors February to April; sow seed December to January for summer bloom
- Plant: After all danger of frost passes, typically late May to early June
- Harvest: Mid-summer through first frost (July to September)
Hardiness
USDA hardiness zones: 9–11
Temperature: down to -7°C
Troubleshooting
Most begonia trouble traces back to water or stale air. Watch the tuber, the lower leaves, and the flowers, since that's where the first signs turn up. Match what you see below and step in early.
A soft, mushy tuber or rotting stem base
Tuber rot is the most serious thing that goes wrong with these plants, and it almost always starts with too much water. A foul smell and a plant that wilts even in moist soil are the tells. Lift the tuber, cut away the rot, dust the cuts, and replant in fresh, fast-draining mix.
Fix:
- Reduce watering immediately
- Lift and inspect the tuber; cut away rotted tissue and dust cut surfaces
- Repot or replant in fresh, well-drained medium
Dusty white film spreading over the leaves
This is powdery mildew, a fungus that settles in when the air sits still around crowded plants. The coating wipes off but keeps coming back until the plant can breathe. Space things out, water at the base, and pull off the worst leaves.
Fix:
- Improve air circulation and spacing
- Remove and discard affected foliage
- Avoid wetting the leaves when watering
Fuzzy gray mold on the flowers and leaves
Botrytis blight, also called gray mold, takes hold in cool, damp, stagnant air and rides in on wet leaves and fallen petals. Affected tissue browns and caves in fast. Clear away the moldy parts and any debris, then open the plant up so the foliage dries between waterings.
Fix:
- Remove infected flowers and leaves promptly
- Increase air circulation and reduce leaf wetness
- Clean up fallen plant debris
Buds yellow and fall off before they open
Bud drop is the plant's response to stress, usually swings in watering or a stretch of heat and humidity. Keep the soil evenly moist without letting it flood or bake. A cooler, shaded, breezy spot settles the plant and the next round of buds holds.
Fix:
- Maintain even soil moisture without extremes
- Provide shade during hot afternoons
- Move container plants to a cooler, protected spot
Toxicity
This is a toxic plant to keep clear of pets and children. Every part holds soluble calcium oxalates, and the buried tuber is the most toxic piece of all. A bite brings a burning mouth, heavy drooling, and vomiting, so keep the plant, and especially any cut tubers, well out of reach.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Mild
A cat that bites the leaves or tuber gets a burning mouth and starts drooling hard. Vomiting and pawing at the face usually follow.
Dogs Mild
Expect drooling, an irritated mouth, and vomiting if a dog chews any part. The tuber underground is the most toxic piece, so digging dogs are the ones to watch.
Horses Moderate
Grazing brings the usual mouth irritation and drooling, but a large dose is more serious for a horse and can push the kidneys toward failure.
Rabbits Mild
The same oxalates sting a rabbit's mouth and turn it off feed. Keep cut tubers and trimmings well out of reach.
Livestock Moderate
Cattle, goats, and sheep get a sore mouth and drooling from a nibble, and a heavy graze on the tubers can strain the kidneys.
Birds Caution
There's little reported on birds, but the calcium oxalates that irritate other animals make this a plant to keep away from them.
People Caution
No part is meant to be eaten, and the sap and tubers can irritate skin and the mouth. Wash your hands after handling and keep the plant away from small children.
Wash hands after handling tubers and sap, which can irritate skin; keep tubers and plants away from pets and children.
Helpline numbers
United States
ASPCA Animal Poison Control: (888) 426-4435 (fee applies)
Poison Control: (800) 222-1222
Canada
Poison Centres: 1-844-764-7669
Varieties
Tuberous begonias come in a wide range of flower forms, and a few groups cover most of what you'll meet. These are the ones worth knowing by name.
Camellia (Camelliflora) types
These carry the showiest flowers of the lot, full and double and up to 3 inches across, looking a lot like a camellia.
Picotee types
Each double bloom is edged with a thin band of a contrasting color, so the petals look outlined.
Pendula (hanging/trailing) types
Instead of standing upright, these trail their stems over the edge of a pot, which makes them the choice for hanging baskets.
How to propagate
You can make more of these plants three ways, from splitting a tuber to rooting a cutting to the slow business of starting from seed.
DivisionModerate
Spring when restarting dormant tubers · Several weeks to sprout and root
In spring, as you wake the dormant tubers, cut one apart so every piece keeps at least one shoot. Dust the cut surfaces, since an open wound on a tuber is an easy way in for rot. Each division takes several weeks to sprout and root before it gets going.
Stem cuttingsModerate
Spring as new shoots develop · 3 to 6 weeks to root
Once new shoots push up in spring, take a basal or stem cutting and set it in a moist, well-drained mix out of direct sun. Give it 3 to 6 weeks to root into its own plant.
SeedDemanding
December to January for summer bloom · 14 to 16 weeks before ready to plant out
Seed is the hard road here. The seed is as fine as dust, so scatter it on the surface and keep it warm and humid rather than burying it. Sow in December or January and count on 14 to 16 weeks before the seedlings are ready to plant out for summer bloom.
Myths
Begonias are perfectly safe houseplants for homes with pets.
The ASPCA lists begonia as toxic to cats, dogs, and horses thanks to soluble calcium oxalates, and the buried tuber is the most toxic part of all. A pet that chews it gets an irritated mouth, heavy drooling, and vomiting. Keep the plant, and especially the tubers, out of reach of animals that chew.
Tuberous begonias need plenty of full sun to bloom well.
It's the opposite, really. These plants want partial to dappled shade and sulk in hot, direct sun, scorching their leaves and dropping buds. A little gentle morning or late-afternoon light is all they care to take.