Thanks for the comments. They are reasonably robust. One weak point would be the connection to the base, but the pins sort that. For larger pieces, the stems are a bit wider and I use longer lengths of fairly stiff wire instead.
For those trees with clump foliage, the other vulnerable area would be the foliage, so I think I'll spray them with some Matt varnish or lacquer to help keep it in place. The stem and 'branches' are not too weak. I haven't used Seagrass, but I see comments about it and I think it's much more delicate.
Incidentally, if you want some it might be worth looking in your local park. The flowers have finished the display and they will probably be removed soon. When I last looked in our local park, there were a fair number of them. You should ask before taking them but I think they would agree to clipping a few seed heads.