Start tuberous begonia tubers indoors 8 to 10 weeks before your last frost (Iowa State), hollow side up and barely covered. Plant them outdoors after all danger of frost, once the soil is warm (Iowa State, UMN), at the same depth they grew in the pot, 12 to 18 inches apart (NC State) in partial shade with morning sun. They are hardy only in zones 9 to 11 (NC State), so most gardeners dig and store the tubers each fall.
Tuberous begonias (Begonia × tuberhybrida) are hybrids of species from tropical South America (NC State), grown for big single or double flowers in red, orange, yellow, white, salmon and pink (UMN). UMN notes they bloom all summer in shady spots where few other showy flowers will. They are slow starters, so the indoor head start is what gets flowers by early summer: Iowa State says bloom begins about 12 to 14 weeks after planting.
Tuberous begonia timeline
| Step | When |
|---|---|
| Start indoors | 8 to 10 weeks before your average last frost (Iowa State); NC State says 2½ to 3 months. Tubers started in February bloom in June, and those started in March or early April bloom in July (UMN) |
| Harden off | Begin about 2 weeks before the last frost date (UMN) |
| Plant out | After all danger of frost, once the soil is warm (Iowa State, UMN): late May to early June in Minnesota (UMN) |
| First flowers | About 12 to 14 weeks after the tubers are planted (Iowa State) |
| Dig in fall | When the foliage yellows or within a few days of the first killing frost (UMN, Iowa State) |
Our planting calendar shows your average last spring frost. Count back 8 to 10 weeks for the indoor start, which lands near the time many gardeners start tomato seeds indoors (GardenPal note).
Starting tubers indoors
- Medium: a flat of moist peat moss or vermiculite (UMN), or pots of well-drained potting mix with drainage holes (Iowa State).
- Depth: just covered (UMN), or under ½ to 1 inch of mix (Iowa State), concave side up.
- Warmth and water: about 70°F, kept moist but never wet (Iowa State, UMN).
- Light: once shoots appear, move them to a sunny window or under lights (Iowa State). UMN says brighter light gives short, sturdy stems instead of leggy ones.
- Pot up: when stems are about 1 inch tall, after roughly a month, move tubers into 5 to 6 inch pots or their summer containers (UMN).
- Feed: a dilute fertilizer every 2 to 3 weeks once growing (Iowa State).
- Bigger blooms: UMN thins shoots at about 3 inches to 2 or 3 stems on small tubers and 3 to 6 on large ones. Don't thin hanging-basket types, which are grown for lots of smaller flowers.
Planting out: depth, spacing and light
| Tuberous begonias | |
|---|---|
| Harden off | Start about 2 weeks before the last frost, moving plants out to shade by day and in at night, then gradually into more light and wind (UMN) |
| Depth | The same depth they grew in the container; deeper planting can rot the buried stem (Iowa State) |
| Spacing | 12 to 18 in apart in beds, or 3 to 4 tubers per container (NC State) |
| Light | Partial shade, about 2 to 4 hours of direct sun, ideally morning sun and afternoon shade (Iowa State). Oregon State calls it bright shade: too much sun burns them, too little makes them leggy |
| Soil and shelter | Moist, well-drained soil, sheltered from strong wind (Iowa State, UMN) |
Watering, feeding and deadheading
- Water with care. UMN says they are not drought tolerant, and Iowa State waters beds weekly in dry weather and checks pots daily or every other day. But Oregon State says overwatering is the main cause of failure, so feel the soil an inch down and water when it has dried (Oregon State, UMN).
- Feed steadily. They are heavy feeders (UMN, Oregon State). Iowa State works 10-10-10 into beds before planting and feeds pots every 2 to 4 weeks with a dilute fertilizer; Oregon State prefers slow-release fertilizer. Clemson warns that too much water or fertilizer makes flower buds drop.
- Deadhead spent flowers for a neater plant and more blooms (Iowa State).
- Hot, humid summers: NC State says they dislike heat and humidity, and Clemson finds they grow best in the cooler upper Piedmont and mountains. Clemson suggests the more heat-tolerant Non-stop series for the South.
Tuberous begonias in pots and baskets
Containers, window boxes and hanging baskets are where tuberous begonias shine (UMN, NC State). UMN says moss-lined wire baskets give the drainage they need, and lets potting mix dry slightly between waterings. Oregon State notes baskets outdoors may still need water every day in hot weather. Trailing kinds have stems 12 to 18 inches long and suit baskets (Clemson).
Digging and storing tubers
- Slow down: cut back watering and stop fertilizing at the end of August (UMN).
- Dig when the leaves yellow or within a few days of a killing frost (UMN, Iowa State), leaving a little soil on each tuber (Iowa State).
- Cure: cut stems to about 1 inch (Iowa State) and dry the tubers in a cool, dry spot for 2 to 3 weeks, then shake off the soil (Iowa State). UMN leaves longer stems and waits until they come off easily. Don't wash them (UMN).
- Store in peat moss, vermiculite or sawdust (Iowa State) at 40 to 50°F, and never let them freeze (Iowa State, UMN). Clemson gives 45 to 55°F.
- Check tubers now and then and throw out any that rot or shrivel (UMN). Potted plants can also go dormant in their containers in a frost-free place (NC State).
Rot, mildew, slugs and pet safety
NC State lists aphids, mealybugs, whiteflies, slugs and snails as occasional pests, plus powdery mildew and stem rot. Too much water causes tuber rot, and crowding invites leaf disease, so give plants room for air (NC State). For safety, the ASPCA lists begonias as toxic to dogs, cats and horses because of soluble calcium oxalates, with the underground parts the most toxic. Keep stored tubers away from pets (GardenPal suggestion), and call a veterinarian if one is chewed.
Tuberous begonias near the vegetable garden
Vegetables want full sun, so tuberous begonias belong in the shady spots around them: a porch, a north-facing window box or under a tree beside the beds (GardenPal suggestion based on Iowa State light needs). They pair well with caladiums, which like the same shade. For sunny summer bulbs, see when to plant dahlias and when to plant elephant ears.
Frequently asked questions
When should I start tuberous begonias indoors?
8 to 10 weeks before your average last frost (Iowa State). Tubers started in February bloom in June; March or early April starts bloom in July (UMN).
When can tuberous begonias go outside?
After all danger of frost, once the soil is warm (Iowa State, UMN). Harden them off for about two weeks first (UMN).
Which way up do you plant a begonia tuber?
Concave, dented side up, where the pink buds form; the rounded side is the bottom (Iowa State).
How much sun do tuberous begonias need?
Partial shade, about 2 to 4 hours of direct sun, ideally in the morning (Iowa State).
Can tuberous begonias stay in the ground over winter?
Only in zones 9 to 11 (NC State). Elsewhere, dig them after frost, cure them and store them at 40 to 50°F (Iowa State).
Are begonias poisonous to pets?
Yes. The ASPCA lists begonias as toxic to dogs, cats and horses, with the tubers the most toxic part.
Enter your frost dates to see your last spring frost and count back to your begonia start date.
Open itSources
- Virginia Cooperative Extension, "Companion Planting in Gardening" (SPES-620P)
- University of Maryland Extension, "Vegetable Succession and Companion Planting"
- Oregon State University Extension, "Companion Planting"
- Penn State Extension, "Herbs Make Good Plant Partners and Companions"
- University of Minnesota Extension, "Planting the vegetable garden"
- USDA Plant Hardiness Zone Map
- NOAA NCEI U.S. Climate Normals (frost/freeze data)
- Iowa State University Extension, "Planting and caring for tender perennials"
- Iowa State University Extension, "When should I start tuberous begonias indoors?"
- Iowa State University Extension, "How do I overwinter tuberous begonias?"
- University of Minnesota Extension, "Tuberous begonias"
- NC State Extension Gardener Plant Toolbox, "Begonia, tuberous types"
- Oregon State University Extension, "Five steps for saving tuberous begonias as frost approaches"
- Clemson Cooperative Extension HGIC, "Begonia"
- ASPCA Animal Poison Control, "Begonia"
Pairings are summarized in our own words from extension-service guidance. Research support for many companion effects is limited, so treat them as good habits, not guarantees.