Tuberous Begonias: From Indoor Sprouts to Summer Giants
I used to plant tuberous begonia tubers in May. They'd bloom in July, then die in August heat. Now I start them in February indoors. Result? Flowers from May through October.
If you want 8-inch blooms all summer instead of a few weeks, you need to start early. Really early. Like when it's still snowing outside.
Why February Makes Sense
Tuberous begonias need 12 weeks from tuber to bloom. Count backward from your last frost date, then add a month. That's when you start.
Starting indoors isn't about babying them. It's about math. These plants are from the Andes Mountains - they hate heat. By the time summer arrives, you want established plants already blooming, not just getting started.
Plus, a sprouting tuber is slug food. Indoors, no slugs.
The Simple Starting Process
Get a shallow container. A seed tray works. So does a aluminum pan with drainage holes. Fill with moist potting mix.
Place tubers hollow-side up, like little bowls collecting rain. Space them 1 inch apart. Barely cover with soil - just enough to hide them.
Keep them at 70°F. This is crucial. Too cold, they rot. Too hot, they rot. 70°F is the sweet spot. Top of the refrigerator usually works.
Wait. And wait. Sometimes 3 weeks. Don't water again until you see pink shoots. One thorough watering at planting is enough. Overwatering kills more tubers than anything else.
From Shoots to Seedlings
Once shoots appear, things change. They need light immediately.
Here's what I learned the hard way: February window light isn't enough. My first batch of seedlings stretched toward the window and flopped over. Waste of good tubers.
Now? I use grow lights for seedlings as soon as shoots emerge. Nothing fancy - a basic LED shop light works. Keep it 8 inches above the sprouting tubers, running 12 hours daily. The difference is dramatic. Instead of pale, stretching seedlings, you get compact green growth.
When shoots reach 1-2 inches, transplant to individual 6-inch pots. This is also when you decide: want fewer giant flowers? Pinch off extra shoots, leaving 2-3. Want a bushy plant covered in smaller blooms? Keep them all.
The Light Balance
After transplanting, begonia seedlings need bright light but not direct sun. This is tricky in March. South windows are often too intense, north windows too dim.
LED grow lights solve this problem. Set them 12 inches above plants, run them 14 hours. Your seedlings stay compact and already budding when it's time to move outside. Meanwhile, your neighbor's direct-sown tubers are just sprouting.
Some growers keep them under lights until outdoor planting. Others move to a bright window once established. Both work. The key is consistency - don't keep moving them around.
Moving Outside (Finally)
Two weeks before your last frost date, start hardening off. Days outside in shade, nights inside. After a week, leave them out if nights stay above 50°F.
Plant in shade. Morning sun is okay, afternoon sun is death. Under trees, north side of the house, covered porch - all good spots.
Outdoor Success Tips
| Location | Success Rate | Notes |
|---|---|---|
| Under deciduous trees | Excellent | Natural filtered light |
| North side of house | Very good | Protect from wind |
| East-facing porch | Good | Morning sun only |
| South or west exposure | Poor | Too hot, flowers fry |
Keep Them Blooming
Water carefully. Soil should be moist like a wrung-out sponge. Too wet = stem rot. Too dry = bud drop.
Feed every two weeks with half-strength fertilizer. Stop in late August as they prepare for dormancy.
Deadhead constantly. Remove spent flowers before they form seed. This keeps new buds coming.
Watch the temperature. Over 85°F, they start declining. Some shade cloth or relocating pots can extend the season.
The Payoff
By starting early indoors with proper light, you get:
- 5 months of blooms instead of 2
- Established plants that handle weather better
- First flowers in May when everyone else is just planting
- Bigger blooms from mature plants
My February-started begonias are 18 inches tall and covered in buds by May. The ones I direct-planted in May? Still just leaves in July.
Storing for Next Year
When leaves yellow in fall, stop watering. Let tops die back naturally. Dig tubers, brush off soil, let dry in shade for a week. Store in paper bags in a cool (45°F), dark place.
Come February, do it all again. Same tubers can bloom for years.
FAQs
Do I really need grow lights? For February starting, yes. March starting, maybe. April starting, probably not. Grow lights for seedlings make the difference between success and leggy failures in late winter.
Can I grow these indoors year-round? No. They need winter dormancy. These are outdoor shade plants that just start indoors.
What if tubers don't sprout? Check for rot (soft spots). If firm but not sprouting after 4 weeks, try bottom heat - set tray on a heating mat at 75°F.
Can I start from seed instead? Technically yes, but it takes 6 months to bloom. Tubers are much easier.
Why are my leaves dropping? Usually overwatering or temperatures over 85°F. Begonias are drama queens about both.
The Bottom Line
Tuberous begonias aren't hard. They're just particular about timing. Start early, provide good light during the seedling stage, and plant in shade.
This weekend, order some tubers. Get a shallow tray and a bag of potting mix. If it's February or March, set up a simple grow light. By May, you'll have blooming plants while everyone else is just starting.
The secret isn't green thumbs. It's starting when there's snow on the ground.
