Jacko, my big pots take so long to water that it's coming out the bottom of the pot, so gave mine some lovely well rotted horse poo and watered that in, and then I will add more later on before winter gets a full grip on us, and hope they put out some decent new cain's next year
but they dry out so fast Jacko, hence when is enough water enough I can only guess when it comes out the bottom of the pot, my Vivax is planted out in the ground and it to is loosing some of it's old leafs, it's about 3yrs old now with two new decent size Cains of about 8-9ft tall with huge side branches of at least 3ft long
thats the trouble with pots.as long as you give them a good soaking when you water them they should be ok.if the pots are drying out quickly it could be full of roots and rhizomes already or be in a windy exposed location that dries the pot out quick.your vivax will take off now its in the ground.i predict 13ft to 14ft next season for you