I uses to live in San Francisco and had no problems with begonias growing under a porch. They always bloomed profusely and tubers always got bigger each year. Even the seeds i started in march of 2011 made tubers the size of a baseball. Albeit, we have a long growing season and the right temps. I gave them all up and moved to MA and boy is gardening different. I started with Blackmore and Langdon seeds feb of 2013 indoors in Boston. Those that survived the move outside all created tubers the size of a peanut to the size of a golf all. Out of about 100 seedlings, I culled out about half that did not have double blooms and the next spring, only about 20 made it. I kept them too cool in the basement. None bloomed summer of 2014. Mainly because I used regular topsoil with some weird mulch incorporated. They did all survive and their tubers were slightly bigger. I have started more Blackmore and Langdon seeds.mi started a batch dec 15 2014 and about 100 have been reported into 50 cell trays. I just started another batch last week just for backup. They grew really well when I had them under a 4foot shoplight on a shelf above the stove but started dying and damping off when i moved them to the basement with no bottom heat. My advice is to start very early if you want tubers and enough time to evaluate flowers to cull, being feb a the latestThey generally create tubers in the fall when day length decreases so they need enough leaves and a big enough root system to do that by September. Also give them bottom heat and use a looserepackaged potting mix. I made the mistake of potting them up in whatever was around only to find the the pH of the black mulch I used was close to 7.5

Fill, level and lightly firm the compost in your seed tray; place on the ground and water thoroughly with a fine rose on the watering can; dry the rim of the seed tray with a cloth or kitchen towel (nothing worse than sowing begonias with wet fingers), even though you don't actually touch the seed. Now, sieve a thin layer of fine compost or sand over the surface (just to fill in the pits of the seed compost.), lightly press. Take a sheet of white paper creased down the middle and place on it a small quantity of perfectly dry fine sand; onto this place a small quantity of begonia seed from your packet and stir them together with a dry implement (the tip of your knife blade, the tip of a plastic label etc. Hold the paper over the seed tray and tap it with a finger to slowly move the sand/seed down until it spills over the end and onto the compost spreading as evenly as you can. If this procedure is new to you it is a good plan to practice on another tray using the dry sand alone. Learn how to hold the paper in the fingers of one hand and tap with a finger of the same hand to move the sand gently down the crease in the paper. Don't cover the seed with any compost; place the tray in a close environment and cover with a sheet of glass or perspex and then darken off. Darkening off is not essential but is my preference. Germinate at around 70F. Very carefully turn the glass daily wiping off the moisture collected on the underside. Once germinated remove to the daylight and a temperature of around 65F. Prick off into trays of standard seed or first potting compost as soon as large enough even if it means on the point of a knife. In the region of 60,000 seed per gram. My Website

I have one Non-Stop begonia and at the end of its season last year I just stuck the pot under the sink in the guest bathroom after leaving it in the garage long enough for the stems to dry and be removed. I forgot about it until today so I added a note on my computer reminder program to check it annually on April 1 . Here it is what it looked like in 2014.

Cuttings, yes, that’s what they must be: though how they can make such vigorous plants is beyond me. The cuttings I tried to make from the fallen-off stems rotted. Must have been compromised. Unpotting and leaving the other 2 begonias’ rootballs to dry out seems to have stopped the decline. One had already dropped half its stems, the others look ok. The soil-less mix is now bone-dry. I put them back in their pots today and gave them a dribble of water. I should take any remaining flower buds off. I have often bought ready made up hanging baskets with begonias. In the fall the stems drop off normally and I bring them inside, then I can tell there is a very small tuber there. I store in the basement and a few weeks ago brought them up and outside; now I can see tiny begonia sprouts. I don’t expect much from those, it’s more of an experiment than anything else.