I want to know how to prune a clematis. The clematis is called
"Clematis Henryi" that has large white flower and is blooming now in
my garden (zone-6, northern New Jersey). According to where I bought
it, this clematis is in group-2 that blooms in early summer (from last
year old growth) and fall (from this year new growth). Their on-line
instruction on pruning group-2 clematis tells me exactly "where" to
prune -- but it doesn't say "when".

I have searched this newsgroup, and I find this suggestion:

"Don't prune it except for deadwood, and do this in
late winter or early spring."

In other words, I should not prune out the living tissue. But I want
to prune it. The reason is that this clematis seems to grow up all the
way on the top of the trellis, and leave the lower 2/3 of the trellis
empty. I would like to encourage it to cover as much empty space in
the trellis as possible. If I prune it very hard to 1.5-ft near the
ground in late winter, I am hoping I may achieve this effect. But of
course I don't want to kill it or forgo any bloom in the process.

My questions a

- Can I prune it?

- Will I achieve the effect of covering the trellis if I prune it
hard?

- Does clematis growing on the top of the trellis have more to do
with lack of sun light and hard-pruning may not change its tendency?
The trellis is facing west against a wall and only get sun light from
noon to dusk.

Thanks in advance for any pruning tip or other suggestions.

Jay Chan