AusBonsai

I've been growing poinciana seedlings found in the gutter a couple of months ago. I have numerous seedlings and have planted three very close together in a pot with the Idea that I will get a thicker trunk more quickly. I know that most ficus trees will fuse very well, but does anyone know if a tree like the poinciana will work in a similar way? How long would they need to be tied together as in the pictures? I would appreciate any ideas and advice on this on this.

Not certain of the answer to first Q but most trees will unite if they are held together for long enough.
How long to leave them is easy. Until the trunks have fused.
You may find you'll need to untie the string and put another lot on at some stage because the trunks will fatten up and start to bulge over the ties.

interesting---wondering if you would get ,in the same time frame, the same thickness trunk as a single rather than three fused smaller trunks

I've been trying both fusing and fast growing with trident maples. I'm convinced I can grow a single trunk just as large in the same time frame.
Fused trunks can, however, give really great texture to the new trunk, twisted and gnarled looking as the individual trunks join together. Best ones I think have been made with the trunks twisted in a spiral rather than straight up like these in the picture. Plaited trunks were popular once but most of the knobbly trunks that resulted did not really look good.
As with all techniques there is no guarantee of success. Bulges and swellings around the ties, one or more trunks die off and leave gaps, one or more not fusing properly or leaving gaps, Fused trunk with no taper or reverse taper. there are a lot of things that can go wrong so plenty of chances to get a substandard bonsai from this technique

Thanks for your replies. I'll take it all on board and keep a close watch. I will try the Idea of the twisted/intertwined trunks. I've already separated one to be a low branch.
All the seedlings are growing very fast, so I'll post some updates in a couple of months.

I noted the low branch you have left. From my experience, watch out that it does not get too thick. Outside trunks have more root run so they grow quicker and often end up larger than some of the other trunks. If that happens to be the one you've left as a branch you'll have a branch that's thicker than the trunk Just another possible problem I have experienced when doing this

The poincianas must be in fight or flight mode because of the way the seasons are messed up, my friend gave me a dozen seedlings that came up in her yard.

I put 9 into one bonsai pot, one into a 4" plastic pot and another in a1" plastic pot. The one in the 4" pot I also sat in a terracotta pot full of potting mix and left it out in the very strong summer sun, each afternoon it was wilting, but it picked up with some water. It's now around 60cm high. The 9 in the bonsai pot are all growing well, but are still only 20cm high, and the one in the tiny pot has barely grown at all.

Definitely worth a try to wrap a few together. If you can get a hold of some grafting tape it's a little softer on the trunks, and being wider doesn't seem to cut in so easily.

I've wrapped 4 or 5 trident whips I got from shibui last winter, they're merging nicely. I also joined three prunus mume seedlings together. So far no slater problems! Fingers crossed for some interesting trunks!

/sigh, double post

If you can get a hold of some grafting tape it's a little softer on the trunks, and being wider doesn't seem to cut in so easily.

I was going to suggest this. Any sort of tape holds the trunks together well - the wider the better - but there is always the top where the tape stops. That's the place I have found that it bulges over the edges of the tape/ties

I'm actually using raffia bought at a craft shop to bind the three seedlings together. Raffia is a bit wider than string and very easy to cut. Cheap too.