Hello - tuberous trailing or pendula begonias can get big, usually I put one plant (tuber) into a 10 or 12" pot or hanging basket, and by the middle of the summer it has completely filled out the container and has fully covered the sides.
I looked at the 'easy fill' baskets on line and honestly they look like just another gimmic to me, probably way overpriced, and they aren't going to make your plants grow any better or make a nicer hanging basket than by using a 'regular' hanging basket. I would choose a mossed hanging basket for begonias if you want to plant things into the sides of the basket, with one - ONE! begonia tuber on the top. I am a purist and would grow only tuberous begonias alone in a pot not mixed with other types of flowers.

Here is a picture of one of my Antonelli Begonias grown from seed in a 10" pot - there is only ONE PLANT in this pot!

Thanks for the reply. That plant looks great from one tuber. Did you start the plant off in the greenhouse before moving outdoors ? And, if you don't mind me asking, what is the growing medium exactly ? Thanks

Thanks for the compliment. The begonia in the picture was planted in mid March under fluorescent lights in my basement, once the tuber had new pink sprouts it was planted, I have found that if the tubers are not sprouted before they are planted and watered they tend to rot, so don't rush things and only get them planted and growing when they have shoots starting on them. The tubers were first put into 4" pots, shifted once into a larger 6" pot as the plant grew larger, and then put into their final 10" pot. The begonias are put into the greenhouse in mid April and I know that not everyone has a greenhouse to work in, but you can get the same results in a very sunny window or under fluorescent lights. Begonias like a lot of sun, give them as much as you dare without burning them - full sun in the morning or late afternoon or both, but shade them from mid day (11 am to about 3 pm) during the summer. Dappled sunshine that would be under an open (not too dense) tree all day long works too.
I grow my tuberous begonias in Pro Mix BX, you can use any good potting soil that is light in texture and has a good amount of perlite so it drains well. The plants get a timed release fertilizer like osmocote, and also fertilized with half strength liquid feed (Miracle Gro, etc.,) almost every time I water during the summer. Make sure the soil gets a chance to dry before you water your plants, and don't ever water begonias when they are hot and dry with the sun shining on them - they don't handle that very well - sit them in the shade and let them cool off for an hour or so before you give them water.
Hope this is helpful to you, now go grow some of the nicest baskets you've ever had this summer! Good luck with your begonias.

I'll certainly try to ! Many thanks for follow up, much appreciated.

Jebfarm - you grew the plant in the picture from seed?

How how long did it take to get to that size?

I am trying tuberous begonias from seed this year for the first time. I have lost about half of them in the early stages, but have about 25 of them at the 2-3 leaf stage right now. Its taken them about 8 weeks to get this big and they are still tiny.

Yes, the begonia in the picture was grown from seed by me a few years ago, that was its first year from a seedling started early January in full flower by July. I start saved tubers from my favorite begonias that were grown from seed in March or April, whenever they start new sprouts on the tubers and want to get growing. First year seedlings can get really big if they are grown well. I think larger older tubers get tired and don't produce as nice of a finished plant as a younger smaller tuber. I start my tuberous begonia seed in early January in my basement under fluorescent lights that are on for 14 hours a day until mid April, then the plants are given natural daylight and daylength in the greenhouse.
Lets see - the seed is sown in January and plants will begin blooming for me sometime in late May or early June, so that is five months total growing time from seed to blooming mature plants. The plants start to flower in May, and some of the smaller seedlings will bloom for the first time later through the summer.
Do you remember the picture I posted of my begonia seedlings that were covered up with saran wrap? They have been transplanted into a 72 cell and have been growing strongly and now need to be transplanted into a 4" pot sometime soon! Here is a picture of my seedlings this morning - the leaves on these seedlings are all 2" or more across which is about where they should be at this time of year. I would like to add that I have been growing tuberous begonias from seed for the last 20 years or so and think I've got it figured out - I am happy to share any information that might help you succeed with your plants. Enjoy your begonias!

This post was edited by jebfarm on Mon, Mar 11, 13 at 17:02

Wow - they look great. They are much, much bigger than mine!

Mine were started mid Jan, the ones of mine that seem to be healthy still only have 2-3 leaves smaller than the size of a dime. Many of them sprouted, grew one tiny little leaf and then just sat there for a month before expiring.

I have been keeping mine on the top shelf of my lighting set-up which is the warmest spot I have. Its where I germinate most of my seeds. I took the dome off the tray a few weeks ago because the algae growth was making me nervous.

At the rate they are growing I was worried that I wouldnt get blooms till August, but maybe they will speed up.