I'm designing a trouble to integrate into my trellis; the plants I'm considering a

Bougainvillea
Clematis Montana
Hydrangea anomala ssp. petiolaris.
Loncera japponica

The plants won't be directly bedded into the ground; the trellis I've designed will have a trough at the bottom, but I can't find any info on how deep I should make this container.

Could someone advise please? Any links to sites I can use as a reference would be greatly appreciated.

Thanks,

Will