How to Grow Begonias: Tuberous Need 65°F Soil, Wax Types Tolerate Full Sun, Rex Need 50% Humidity — Complete Type-by-Type Guide
Tuberous begonias need 65°F soil, wax types tolerate full sun, rex want 50% humidity. This type-by-type begonia guide covers light, soil, water, feeding, and seasonal care with extension-backed data.
With roughly 1,500 species and three completely different root systems, begonias are the most biologically diverse genus most US gardeners will ever grow. What trips people up isn’t begonias in general—it’s applying generic care advice to a type that needs something entirely specific. I’ve seen gardeners drown tuberous begonias by watering them like wax types, and watch rex begonias crisp in a sunny south window that would make a semperflorens thrive.
Wax begonias tolerate full sun because bronze-leaved varieties evolved a three-layer defense involving anthocyanins, leaf folding, and thickened cuticles—the kind of sun tolerance rex begonias simply don’t have. Tuberous types produce dinner-plate double blooms but evolved in cool Andean cloud forests and fade when nights stay above 70°F. Rex begonias grow on Indian forest floors where sunlight barely filters through canopy.
This guide covers each type in order: what it needs, why, and the single most common mistake that kills each one. If you already know your type, jump straight to that section. If not, the identification table below takes 30 seconds.
Know Your Type Before You Plant
The fastest way to identify your begonia is to look at roots, leaves, and flowers together:
- Wax (fibrous-rooted): Compact, rounded shape. Small waxy leaves in green or bronze. Continuous small flowers in red, pink, or white. Sold in spring bedding six-packs.
- Tuberous: Large showy flowers, often double, 2–6 inches across. Fleshy stems. A potato-like tuber underground. Dies back completely in winter.
- Rex (rhizomatous): Grown entirely for dramatic metallic or iridescent patterned leaves. Flowers are small and secondary. Spreads by a horizontal underground stem (rhizome) rather than a tuber.
| Type | Root | Sun tolerance | Humidity | Main use | Zones (outdoors) |
|---|---|---|---|---|---|
| Wax (B. x semperflorens) | Fibrous | Full sun to shade | Average (40%+) | Bedding, containers | Zones 9a–11b; annual elsewhere |
| Tuberous (B. x tuberhybrida) | Tuber | Partial shade | Moderate (50%+) | Pots, hanging baskets | Zones 9–11; lift and store in Zones 3–8 |
| Rex (B. rex-cultorum) | Rhizome | Bright indirect only | High (50–70%) | Houseplant (foliage) | Zones 10–12; houseplant everywhere else |
The full begonia types guide covers Angel Wing, cane-stemmed, and shrub begonias if you’re trying to identify a more unusual variety.
Wax Begonias: The Sun-Tolerant Workhorse
Wax begonias (Begonia x semperflorens-cultorum) bloom continuously from late spring through the first frost with very little attention. They’re the only begonia group you can reliably plant in full sun, and they generate more than $130 million in annual US sales for good reason—they work in almost any bed or container from Zone 3 to Zone 11 as a warm-season annual [8].
Why Bronze Leaves Handle Sun Better Than Green Leaves
Not all wax begonias handle sun equally, and the reason is measurable. Bronze-leaved varieties outperform green-leaved ones in full sun because of elevated anthocyanin content in their leaf tissue. Anthocyanins function as both a UV light filter and an antioxidant—they absorb excess radiation before it damages the photosynthetic machinery beneath [8].
A 2025 peer-reviewed study published in PMC found that heat-tolerant wax begonia genotypes accumulated up to three times more anthocyanin under high-light stress conditions compared with susceptible commercial varieties [8]. Their leaves also folded to reduce exposed surface area under peak irradiance, cutting direct radiation exposure by over 60%. Green-leaved varieties lack both defenses, which is why NC State Extension recommends them for morning sun and afternoon shade rather than all-day full sun [6].
Practical rule: Bronze-leaved varieties (Cocktail series, Ambassador Bronze) for full-sun beds. Green-leaved for partially shaded spots, where they produce deeper color and more lush foliage.
Light, Soil, Water, and Fertilizer
- Light: Bronze-leaved = full sun to partial shade. Green-leaved = morning sun, afternoon shade. Both grow in deep shade but flower sparsely.
- Soil: Moderately humus-rich, well-drained, and moist. Soil pH 5.5–6.5 [9]. Illinois Extension notes that planting into cold soil—even when air temperature is adequate—causes stunting [10]. Wait until both air AND soil temperatures are consistently warm before transplanting.
- Transplanting timing: After last frost, once soil reaches at least 60°F [9]. Space 12 inches apart in garden beds [7].
- Watering: Keep soil evenly moist. Use drip irrigation or water at the base early in the morning—wetting hot foliage causes scorch [7]. Wax begonias tolerate mild drought better than tuberous or rex types once established.
- Fertilizing: Apply a balanced slow-release fertilizer at planting. From June onward, switch to a high-potassium feed such as tomato fertilizer. High potassium supports sustained flower production by strengthening cell walls and regulating water movement through stomata during hot weather [1].
Tuberous Begonias: Spectacular Flowers With a Cooling Requirement
Tuberous begonias produce the most dramatic flowers in the genus—double blooms up to 6 inches across in colors from white to deep red to apricot. But they evolved in cool, cloudy Andean cloud forests, and they show it. They struggle when summer nights stay above 70°F and stop producing new blooms entirely when soil temperatures climb above 80°F. Getting their temperature requirements right matters more than anything else.
Starting Tubers Indoors
Start tubers indoors in March, 8–10 weeks before your last frost date [11]. Technique matters: place tubers in a shallow tray of peat or potting mix with the concave (indented) side facing up—the growing point emerges from the hollow [4]. Cover with half an inch of mix. Maintain soil temperature at 65–68°F (18–20°C) [4]. Below 65°F, activation is slow and rot risk rises.
Move plants outdoors in May once soil reaches 60°F and nights consistently stay above 50°F [11]. Choose dappled light or morning sun. Direct afternoon sun bleaches flowers and stresses plants already sensitive to heat.
Watering and Bud Drop
Bud drop—where formed buds fall off before opening—is the most frustrating tuberous begonia problem, and it’s almost always caused by overwatering, dry air, or excess fertilizer [2]. The mechanism: when roots sit in waterlogged soil, oxygen displacement kills root cells and the plant responds by shedding developing buds to reduce metabolic demand. Water at the base, never overhead. Allow the soil surface to begin to dry before watering again.
Lifting and Storing Tubers
In Zones 3–8, tuberous begonias must be lifted before the first frost. Watch for leaves to yellow and stems to soften—that’s the plant entering dormancy [2]. Reduce watering when you see this. After lifting:
- Wash off soil and let tubers dry for 1–2 weeks in a warm, ventilated spot.
- Remove dried stems and foliage.
- Store in peat moss or dry sand at 45–60°F (7–15°C) [4].
- Check monthly during storage and discard any that show rot.
For a variety-by-variety guide covering all 18 types including rare collector finds, see 18 Common and Rare Begonia Varieties: Tuberous, Wax, Rex, Cane.
For zone-specific planting dates, the guides for Zone 3–4, Zone 5, and Zone 6–8 include exact start and move-out dates.
Fertilizing Tuberous Begonias
Use a light touch. Excess nitrogen in early summer pushes leafy growth at the expense of blooms, and too much fertilizer at any stage causes the bud drop described above [7]. Apply a balanced slow-release fertilizer at planting, then switch to a low-nitrogen, high-potassium formulation once flowering is underway.
Rex Begonias: Foliage Drama With Specific Indoor Demands
Rex begonias (Begonia rex-cultorum) originate from the forest floors of northeastern India and Southeast Asia—deep shade, consistently warm temperatures, and high humidity year-round. In US homes, that translates to conditions the other two types couldn’t survive.
Light
Bright indirect light is the target: an east-facing window, or a spot several feet back from a south- or west-facing window [5]. Too much direct sun bleaches the metallic pigments that define rex leaf patterns. Too little (under about 2,000 lux, typical of a north-facing window in winter) causes the plant to enter semi-dormancy and drop leaves. Fluorescent or full-spectrum LED grow lights supplement low winter light effectively—run them 10–12 hours per day.
Humidity: The Make-or-Break Factor
Rex begonias need ambient humidity of 50–70% for consistently healthy foliage [5]. Most US homes with forced-air heating run 20–30% in winter—well below what rex plants tolerate without browning at the leaf margins [1]. Brown, crispy leaf edges are the first signal.
Three methods that reliably raise local humidity:
- Pebble tray: Fill a shallow tray with gravel, add water to just below the surface, and set the pot on top. Evaporation humidifies the immediate microclimate.
- Grouping plants: Plants transpire constantly. Grouping several houseplants raises local humidity measurably.
- Bathroom or kitchen placement: These rooms run higher ambient humidity from daily water use.
Don’t mist rex begonias directly. Wet leaves invite powdery mildew, and rex types are particularly susceptible [5]. The guide to increasing indoor humidity covers all methods and their trade-offs in full.
Temperature and Winter Dormancy
Rex begonias grow actively between 65–75°F. Growth slows noticeably below 60°F [5], and most plants enter partial dormancy in winter as days shorten—leaf production slows, and older leaves may drop. Reduce watering during this period (keep the mix barely moist rather than evenly wet) and stop fertilizing from October until new growth appears in spring [5]. This is normal and not a sign of failure.
Fertilizing Rex Begonias
Rex begonias are grown for foliage rather than flowers, so they need a high-nitrogen fertilizer—not the high-potassium formulation used for wax and tuberous types [1]. Use a quarter-strength water-soluble fertilizer every 2–3 weeks during the growing season (spring through September) [5]. Full-strength fertilizer burns the fine rhizomatous root system. Stop feeding October through early spring.
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→ View My Garden CalendarContainers and Potting Mix
Rex begonias grow from horizontal rhizomes and spread outward rather than downward. Use shallow, wide containers rather than deep pots—a deep pot traps excess moisture below the roots and promotes rot [5]. African violet mix or a light soilless medium drains better than standard potting soil and suits the fine root system. The guide to potting composts for houseplants covers how to build the right soilless mix for containers like rex.
Toxicity
Rex begonias contain calcium oxalate crystals in their underground parts, which are harmful to dogs and cats if ingested [3]. Keep plants out of reach of pets and young children. The ASPCA lists begonias as toxic to cats and dogs.
Soil and pH by Type
| Type | Best mix | pH | Avoid |
|---|---|---|---|
| Wax (outdoor) | Moderately humus-rich garden soil with compost | 5.5–6.5 | Clay without amendment; waterlogged spots |
| Tuberous (starting) | Peat-based seed-starting mix then well-draining garden bed | 5.5–6.5 | Heavy garden soil in containers |
| Rex (houseplant) | African violet mix or soilless perlite blend | 5.5–6.5 | Standard potting soil (too dense); deep pots |
For heavy clay garden soils, incorporate compost or coarse sand to 8–10 inches depth before planting wax begonias outdoors. The drainage test: pour water into a 6-inch hole—if it hasn’t drained within 4 hours, the site needs amendment before planting any begonia type.
Seasonal Care Calendar: February Through November
| Month | Wax | Tuberous | Rex |
|---|---|---|---|
| Feb–Mar | Start from seed indoors at 70–75°F; 16 weeks before last frost [9] | Start tubers at 65–68°F, concave side up; shallow tray [4] | Reduce watering; no fertilizer; watch for new growth signal |
| Apr–May | Harden off; transplant after last frost when soil ≥60°F [9][10] | Move outdoors in May once nights consistently ≥50°F [11] | Resume quarter-strength fertilizer as growth restarts [5] |
| Jun–Jul | Switch to high-K feed; water base early AM; deadhead if not self-cleaning | Peak watering; stake large-flowered types; avoid overhead water | Peak growth season; water when top inch dries; feed every 2–3 weeks [5] |
| Aug–Sep | Continue high-K feed; watch for powdery mildew in humid climates | Ease watering as nights cool below 60°F; watch for bud drop signals | Continue care; prepare for growth slowdown as days shorten |
| Oct–Nov | Remove after first frost or pot up for indoor overwintering | Lift before first frost; dry and store at 45–60°F in peat or sand [4] | Stop fertilizing; reduce watering to barely moist; move from cold windows |
Diagnosing Problems: Symptom, Cause, and Fix
| Symptom | Most likely cause | Fix |
|---|---|---|
| Flower buds drop before opening | Overwatering, dry air, or excess fertilizer (tuberous most affected) | Reduce watering frequency; raise humidity; cut fertilizer by half |
| Brown, crispy leaf edges | Low humidity (all types; rex most sensitive) [1] | Pebble tray; move away from heating vents; group plants together |
| White powdery coating on leaves | Powdery mildew (Erysiphe spp.) | Improve airflow; apply neem oil or horticultural oil [5]; never water leaves |
| Soft brown stem base; wilting | Pythium root rot from overwatering [3] | Remove affected stems; repot in fresh dry mix; reduce watering |
| Silvery stippling; fine webbing on leaf undersides | Spider mites (especially rex in dry conditions) [5] | Increase humidity; spray with insecticidal soap or neem oil |
| White cottony masses in leaf axils | Mealybugs [5] | Dab individual insects with rubbing alcohol on a cotton swab |
| Mushy grey areas on leaves or flowers | Botrytis blight (poor airflow, high humidity) [3] | Remove affected material; improve ventilation; reduce overhead watering |
| Distorted, bronze-stippled new growth; stunting | Thrips (vector for INSV and TSWV virus) [7] | Remove badly affected plants; yellow sticky traps; spinosad-based spray |
For a deeper dive into specific begonia problems, the begonia problems guide covers powdery mildew, leaf drop, and stem rot with photo identification.
Propagating Begonias by Type
Each type propagates differently. Using the wrong method wastes weeks:
- Wax from seed: Start 16 weeks before last frost at 70–75°F [9]. Surface-sow without covering—seeds need light to germinate. Germination takes 15–20 days; seedlings need 14 hours of light per day after germination [9]. The only type worth the seed-starting effort.
- Wax and tuberous from stem cuttings: Take 7–10 cm cuttings with a leaf node; root in water or moist perlite. Roots form in 3–4 weeks [9][1].
- Tuberous from division: Divide dormant tubers before starting in spring, ensuring each division has at least one visible growth point. Dust cut surfaces with sulfur powder to reduce rot.
- Rex from leaf vein cuttings: Select a mature leaf; cut into pie-wedge sections, each with a main vein running through the center [11]. Press sections vein-side down into moist propagation mix. At 18–24°C (64–75°F) with high humidity [1], plantlets emerge from the vein cut within 4–6 weeks. This technique works specifically because of the rhizomatous growth habit.
- Rex from rhizome division: Cut the horizontal rhizome into sections, each with 1–2 leaves and visible roots. Pot in shallow wide containers [5].
Step-by-step instructions with timing and rooting hormone guidance are in the begonia propagation guide.
Frequently Asked Questions
Can begonias grow in full sun?
Bronze-leaved wax begonias tolerate full sun once established and when adequate moisture is available [6][10]. Green-leaved wax types need morning sun with afternoon shade. Tuberous and rex types should never receive direct afternoon sun—it bleaches flowers, fades rex leaf patterns, and compounds heat stress in tuberous types already sensitive to high temperatures.
How long do begonias live?
Wax begonias are grown as annuals in most of the US but are perennial in Zones 9–11. Tuberous begonias live indefinitely as long as tubers are stored correctly each winter—tubers grow larger each season and some gardeners maintain the same stock for a decade or more. Rex begonias are long-lived houseplants; well-maintained specimens produce new growth for many years, with rhizomes periodically divided to keep plants vigorous.
Are begonias toxic to pets?
Rex begonias contain calcium oxalate crystals, particularly in underground rhizomes, which are harmful to dogs and cats if ingested [3]. The ASPCA lists all begonia species as toxic to cats, dogs, and horses. Keep all begonia types out of reach of pets and children, and treat any suspected ingestion as requiring veterinary attention.
Can you grow begonias indoors year-round?
Rex begonias are natural houseplants and grow well indoors year-round with adequate humidity and indirect light. Wax and tuberous types can be overwintered indoors—wax as a compact potted plant near a bright window, tuberous as stored dormant tubers. Neither wax nor tuberous types actively thrive through a US winter indoors without supplemental grow lighting. The guide to reviving dying houseplants covers recovery steps if your indoor begonia has declined.
When do tuberous begonias bloom?
Tuberous begonias started indoors in March typically begin blooming June–July when moved outdoors. They bloom continuously through September as long as nights stay cool. Above 75°F at night, flower size decreases and bud drop increases. Zones 3–5 often get the longest display because cool summer nights extend the bloom window; in Zones 8–9, summer heat frequently forces a mid-summer pause.
Sources
- Royal Horticultural Society: Begonias: Houseplants
- Royal Horticultural Society: Begonias: Outdoors
- UF/IFAS Gardening Solutions: Begonias
- UF/IFAS: Tuberous Begonia (FP062)
- UConn Cooperative Extension: Rex Begonia
- NC State Extension: Begonia Wax Types
- Clemson HGIC: Begonia
- PMC/NCBI: Investigating Morphological and Physiological Responses to Stress in Begonia semperflorens
- UMN Extension: Begonia
- Illinois Extension: Wax Begonia, Fibrous Begonia
- Iowa State Extension: Growing Begonias in Iowa
