I am going to start them in Pro-Mix which is mostly peat and vermiculite.
The main thing is good drainage so they are kept moist,but not wet.
Keep them warm and in good light,but not direct sun for the first month.

PS: I am 70 so I do not lug sand around any more, because it is too heavy.
The vermiculite works fine. It is a good lighter alternative.
Good luck with them.
Caroline

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.

One thing more, I will put them in indirect light when I get leaves. Now they are on top of the armoire for warms and out of the way. Getting the light right is important or they will stretch. 77 is nothing, determination everything and you have that. I hope I am privileged to get to your age and beyond.
I bought some outdoor potting soil from Kellogg by mistake, stinky�.stinky. I think I will use it later to pot some of these up and go organic. I have been listening to podcasts about soil structure.

Hi
I too am starting some begonia bulbs, very precious to me, as they are said to be scented.
what I am going to do...and this sounds ODD,but it has worked for me before (when I had some plants in extremely tall containers, and the soil would become compacted, even good potting soil) I am keeping old styrofoam cups, and I will chip them up into tiny bits, as a soil additive. They turn dark soil-color almost immediately, and don't show up. It makes the lightest fluffiest planting medium you can imagine.
Ruth

I am not that brave to let Styrofoam lose in potting soil. I may give it a try in a little pot. I have some big pieces that I put in the bottom of pots or use soda cans or plastic bottles to lighten big pots. Many plants stay to the top anyway. People that don't know think perlite is Styrofoam. Yes it's ODD but I think I will give it a try. Thanks

I am 67 and have grown tuberous begonias since I was 14.I started growing california tubers which I obtained from Antonelli in California.They are out of business.Now I get them from England.The two types of tubers are started in different ways.California ones are only started when the pink sprouts canbe discerned and they are then put in a flat UNCOVERED.British tubers are immediately placed in a flat and COVERED with less than a half inch of soil.I used regular potting soil when starting the tubers and when I go to pot them I use Pro-mix.

franklyn you are posting good information. No pictures yet! How are the English begonias better? This is my first year with too many begonias, will learn a lot. I will look around to see if Pro-mix is local.
Wondering about soil for other purposes, begonias are all planted. I am starting more hanging baskets and they weight a TON. I don't care for perlite or Styrofoam in soil but willing to trial and error. I use Turface to add drainage but wouldn't want to walk under baskets with Turface in them.
Begonias are pretty economical flowers. You pay up front but then they will last for years. I also like the not having the burden of wintering them through. I drag in the lime, brugmansia, cane begonia, coleus on and on. There is a little start up trouble and all over the house to begin with. Thanks