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

Thank you for your post. I have had tuberous begonias before but this year I am getting serious. I will tell what I know and then others can correct me. More tuberous begonias are killed with over watering and over potting then anything else (me last year). You warm the tubers up uncovered until the 'pinkies appear' and then you cover them with 1' to 1-1/2' of soil. I am using Kellogg potting soil and Turface. Turface like vermiculite or sand just for drainage. Personally I think you will do okay with just potting soil just be very careful with the water. I wet my soil and then put it in the container to have better control over wetness. I am laying my tubers on the wet soil and do not cover them until the 'pinkies' appear. Mine are up high in the warm computer room. Small guys in cups and larger in a big tray. I plan to repot them in their end containers when 6" high and point the leaves towards the front of the pot. I learned last year to be very patient. All came up successfully but a few very slowly. I plan to put some in larger pots with other plants. I can't find any info on this most are planted alone as specimen plants. A little plant in a big pot is deadly because the root system is too small to take up and use all the moisture and so rots. I am familiar with the deadly results. I drown my plants with love and so have ordered a moisture meter. I ordered various types of tuberous begonias and will plant them in baskets, urns and little pots, big pots and directly in the soil. This year I will learn, next year it will be easy.

I don't use pots because I am not lucky with pots. However, using als gritty mix does work, but still the ground is better. I use compost made from food waste and starbucks coffee grounds and bagged sheared wood. But, compost does not work in pots. I think it's the compost, but on the other hand, Antonelli Brothers did great in pots. They used a green house environment and feed them with Romeo brand food the red colored one. One tip is avoid wetting the leaves as much as you can, for fungal problems and they did spray them with fungicide and they used small sticks to prop up the larger flowers. Doubles can sometimes droop or break off. So clearly compost and the ground is not a requirement. I would order at least one double and try it. I found ones I just buy like at any old place like home depot wonâÂÂt be up to par at all. You have to be on top of the small sticks/stakes because as it get bigger it will begin to sag and then break off, so you need to stake before it sags. They use small green bamboo stakes cut up to the size they want. They do not need a cold night, they donâÂÂt actually like cold. I could be wrong, but we donâÂÂt get cold nights here in San Francisco, normally 50âÂÂs. If you can, increase the humidity, because sun rooms can be dry. They like humidity, but then you donâÂÂt want wet leaves, just humidity in the air. Water deeply until it runs out of the pot, but then donâÂÂt water until it is dry again. You want to flush out any salts from the pot. If you only water just a bit, the salts from the fertilizer can build up. I would repot often if possible. I am not feeding mine with the romeo stuff, since I have so much compost, I use dyno grow foliage, because I have alkaline soil and water and they like acid soil. If you feed with a high middle number which is good for flowers you risk increasing alkaline soil. Your soil choices sounds ok for a begonia, I donâÂÂt have enough experience to judge, but the peat moss can get packed down so maybe some perlite? They appeared to grow them in peat with perlite, and I mean, a lot perlite, as I recall. I also use treatments to increase the acidity sulfur is good, it is also a plant food as well. Or aluminum sulfate, but donâÂÂt use too much. When you repot look careful at the tuber and check to see it is firm and healthy. If it is soft the plant is rotting. You can sort of get the hang of how a healthy tuber looks, itâÂÂs like picking a good potato, not rock hard, but not mushy. Then always make notes on what you did so it something works better you can do that again. I got the orange one to bloom but it is just a later begonia then the other one. It has big flowers, I took a photo today of it.

I have the composted bag chicken manure. What is your thinking, too fine a texture? I thought I would have to use it sparingly anyway or might get too much foliage. Potting soil in general I don't feel I have gotten under control. I made the mistake of using Costco Miracle Gro moisture plus last year. When it's wet it feels like ground cardboard. It never dries out but works great for impatiens. I have a drip system and of course pots on a system have a mixture of sunny and shady exposures. Dry loving plants in the shade never dried out. This winter I tried putting together Al's mix, 1-1-5 and already I can see it drains too fast, if I even got it right. I am trying Kellogg brand now. I add 15% Turface because I don't like perlite. The extreme summer temperature in the valley are a challenge. I use Osmocote but organic gardeners think fertilizer salts kill the soil microbes. What do you plant your begonias in Al?