Mmmm - I can't really answer your main question except to say all Kilmarnock willows are grafted so presumably they don't form a trunk on their own roots. They are a weeping sport of the common Salix caprea. I don't think they'll be more vigorous, just more congested. But you'll find that out by experiment.

What's really bothering me is your statement that you 'trimmed it up nicely for the Spring'. Have you done this successfully before? My understanding was that this plant requires little or no pruning and that in fact pruning could ruin its shape. The thing is so small and dumpy I can't quite imagine what trimming it needed.

Here is a link that might be useful: Kilmarnock willow

Hi, thanks for the reply. It will be interesting to see what happens.

Yes we do trim it up every year (planted in 2005) as it starts to droop to the ground and we like to keep it a bit airier and off the ground if that makes sense. I've taken a picture so you can see what it looks like after the trim (sorry, no pics before the trim...but the branches I cut off varied between 10cm and 50cm). If left without trimming it just gets too out of control, and the branches underneath all die off and make the canopy a bit too thick for our liking. The majority of the trimming was done underneath...removing the dead branches (a lot of work!)