I my experience, tuberous begonias take between 14-20 weeks to produce large plants with buds from seed. They will flower in another 3-6 weeks.

You can start them now, and harvest tubers next fall.

Make sure you plant the plants in a place which get a fair amount of sun (not too hot afternoon sun), such as morning sun, or bright filtered sun all day. The more sun (within reason) the larger the tuber.

Also, apply a 3 months slow release fertilizer to the plants, or fertilize every 2 weeks with a diluted liquid fertilizer.

You can let the plants flower, but deadhead after blooming.

Let the foliage grow well into fall. If you only have a light frost by end of September, you can wait until it kills (blackens) the foliage, then cut the foliage back and bring indoors. Let the root ball dry before you harvest the tuber (do not pull plant from tuber. You can cut stem back to about 1" above tuber). If your first frost is typically very strong (below 25F), then bring the pot indoors before winter, and gradually withold water.

Note: to produce harvestable tubers, do not stress the plants in summer. Give as much sun as they will tolorate (the more the better) and keep flowers deadheaded. Provide regular moisture and watering.

Thanks for the info. I will plant half the seeds and save the other half until Dec, that way if decent tubers don't form all won't be lost.

You posted re fert. Is it the feeding which will help produce a good size tuber?

I could probably keep it growing into Oct by placing in the greenhouse with heat.

Plants produce energy in form of carbohydrates (actually a type of fructose). This energy is produced by converting water and fertilizer nutrients with ATP produced by sunlight. So it is the combination of sunlight, water, and fertilizer which produce energy. Excess energy is used to improve tubers. If you give your plants a reasonable amount of sun (but not so much they droop or get stressed), frequent light applications of fertilizer and regular moisture, then they will produce lots of energy. Too shady and the plant can not produce enough energy to improve the tuber (the energy will instead be drawn from the tuber).

In spring, most of the energy is used for the production of the plants and flowers. After flowering, in early fall, you should "top" (cut the top of the plant off) which will stop flower production. The remaining leaves will still be active and produce energy. This energy will instead be directed to the tuber for storage. If you want to grow begonias only for tuber purpose, I would remove the flowers consistently through the summer (or let a few flower to see color but remove the rest).

The flow of carbohydrates inside the plant is mainly from tuber to leaves in spring, bi-directional in summer, and in fall when temperatures moderate and after being topped, the flow is almost entirely from leaves to tuber.

If you can give your begonia a good long 30-60 day growing period after being topped in fall, the tubers are quickly build up nicely. Remember to provide as much light as possible and water regularily. Use a diluted fertilizer frequently.

Regards
Kenneth

Thanks Kenneth
I will put your information to good use this year and hopefully by Oct I will be able to post re the size of tuber I get, and how things turned out.
Carol