On Mon, 15 Feb 2010 20:04:10 -0000, Stuart Noble
wrote:

Thanks everybody. The plastic sheet idea doesn't sound promising then,
but I might try a damp course membrane weighted down with paving slabs.
I would guess there are always weaknesses in concrete and tarmac, and
maybe the uniformity of the polythene would make a difference. I'd
probably be doing this anyway to discourage normal weeds, so there's
nothing lost if it doesn't work.

Stuart, if as you said in your first post you are going to leave the
garden for a year, you really do need to do something about the knotweed
*before* you cover it.
If you don't, the infestation will be worse, much worse in a year's time.
Even if the shoots do not manage to pierce the membrane, the roots will
continue to multiply both directly under the garden and sideways beneath
the surounding concrete areas. Knotweed has even been known to bury under
house foundations and come up through the floor inside.
You should listen to the advice about applying a generous dose of
glyphosate.