I have three plants in a large terracotta pot - I suppose about 10" across. I did this last year and they filled it and spilled over the sides beautifully. I was going to add - that an extra plant in a 14" basket would be fine, but I hesitated because it's a basket - would 4 in there have enough space for their roots? Could you add some smaller trailing plants like Lobelia so that they do have enough room?

Mine are in multi-purpose compost with added granules, as per last year, and all was well.

Thanks Spritzhenry. So they must get quite congested together then really. The type of begonias I have are predicted to grow to about a foot high and the same wide. T and M recommend planting them so that there is a 'gap' between each plant which air can circulate around. Last year I grew begonia (Semperflorens type) in a trough on my outside window sill. A fungal infection started on the plants between the plant itself and the pain of glass, as air could clearly not move here. I had to spray it lots of times with fungicide to keep it healthy through to the autumn. I am mixing my plantings with Bacopa,- lovely stuff. Sounds like either John Innes no 2 by itself (like T and M say) or just multipurpose by itself can work OK. I am making a 1 part to 2 parts mixture this year and hoping for the best. I think fairly congested (closely packed) begonia plantings a possibly best only if in a breezy spot. But in the more wind-sheltered corners of the garden they may be better planted with a little further in between. I am putting so much into my begonia display this year. It should be my biggest and most beautiful ever. (I am expecting the judges from the 'Your Gardens' Bracknell In Bloom Competition in two weeks time, but they won't have bigged-up fully by then).

Not a single bud yet on mine. I'd agree that a little space in between might be a good idea if you have had fungal infections before.Funnily enough, I've used a lot of Bacopa this year!

Good luck with the competition!