The tuberous kind of begonia should be allowed to be touched by frost...a killing frost that does in their foliage. At that time you then remove the foliage and dig up the bulb, let them dry for a day or so in your garage or shed, then store them in your cool basement, away from light, away from heat, away from water. Make a note of which end is UP...store them in a way you know which is which. Then in March, bring them out and you can put them ontop of the refrigerator--there they might begin to grow a root. Whatever, pot them up, just barely under the soil surface...their tips might even be seen, water them, give them sunlight and pretty soon a little bit of fuzz shows on the surface....it might look like like mildew...but its not...its the bulb beginning to grow. Keep the soil damp...but not wet. Tuberous begonia is one of the slowest forming bulbs and it might be June or July before they come to sizable growth and bloom. The bloom they produce is worth the wait.

juju, it's best not to use manure in pots, even composted one. and certainly not raw! begonias are very prone to all kinds of fungal issues and you are taking a big chance. use seaweed/fish emulsion instaed, if you want organic fertilizer. and it is best to start them in small pots and then uppot a couple of times until you reach a 12" pot. you started yours outdoors way too early. when tubers do not have enough roots they rot easily when temps are below 55f. temperatures ARE of issue. both low and high. they hold foliage well and even bloom in the 50s, but they need a well developed root system to do that. and also need to be kept on the dry side. but... when plants are young and tender, roots need higher temps to grow faster - 60s at least. your plastic covers are a good idea! the big one looks good - but you need to stake it, or it can break off very easily at the base. the link below is the most detailed guide to cultivation i've ever found. they are grown for cut-flower competition, so you won't necessarily do the same, but there is a lot of detail that is very useful. Here is a link that might be useful: british grower diary

I also have no experience with growing these type of begonias; just received 6 tubes and they have sprouted the little pink sprouts. Now, don't laugh to hard but is this little pink sprout a leave sprout or a root sprout... which ends up??? So pink sprout up or down??? Thanks granny

Beautiful! I am so envious because here in Tampa Bay, we cannot grow them - way too hot and humid. Of course, I have 8 foot angel wings and rhizomatous in the landscape 5 feet around ....but those tuberous are ribbon-worthy! Greg in New Port Richey zone 9b/10a