Tuberous Begonias from Seed
water (still covered by plastic) to wick up moisture from below (preferred). A very dilute fertilizer solution can be used after the seeds have germinated to avoid them stalling in the flat.
Transplanting
30 days after germination, gradually remove the plastic cover from the young seedlings. With the pointed edge of sharp knife gently lift out a small clump of growing media below the seedling. Transfer these clumps to separate pots and gently squeeze them into small pre-made holes in the potting soil. Then water the pots from below to settle the soil around the roots.
If the seedlings have been growing close together, you can lift out a group of seedlings to the table and gently pull them apart by handling them by the leaves and roots. Do not touch the weak stems. Although the seedlings are small, they are incredible tough.
After transplanting, I cover the new pots with clear plastic for a few days to retain humidity which seems to aid in their recovery. Some loss should be expected, nevertheless.
Growing to Maturity
After the initial transplanting, grow the young seedlings under fluorescent grow lights. Make sure they have amble room to spread out before they get crowded or root bound. 4"-5" pots are good final homes before setting out. If you prefer to keep them potted outdoors, 8-10" pots can be used also.
Initially, the seedlings can be spaced pot to pot, but eventually they may need to be spaced 6-8" between the pots to avoid stretching. Foliage should not touch.
Fertilize weekly with initially 100 ppm from a nitrate based fertilizer, and increase this to 200-250 ppm after transplanting. You can read more about using PPM here.
Finishing Up
About 18-22 weeks after sowing, flower buds will start to form. Switch to a blooming fertilizer at this time and start to gradually harden the seedlings off to go outdoors.
When watering the begonias, try to water infrequently but allow enough water to drip through the drainage holes at the bottom. The begonias can do with remarkably little water, and overwatering often result in more problems.
Fertilize begonias every 2-3 weeks with a half strength soluble liquid plant fertilizer outdoors.
The plants do very well in partial shade; especially with morning sun. Too hot a position can lead to bud drop, but too much shade result in tall unattractive plants which do not flower well.
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