animaux wrote:

On Tue, 02 Dec 2003 13:36:38 -0500, Trent-Lion opined:

In the GH I am restoring, I put some old paving bricks down to cover
the dirt floor before thinking about the potential weed problem. Is it
safe to use Roundup on the weeds that come up inside the GH, or should
I go to the trouble of removing all the bricks and putting down
weed-blocking cloth? Thanks.
T-L

Go through the trouble. Roundup does not prevent weeds from sprouting up. It's
a systemic pesticide which must make contact with the foliage, and in a
greenhouse the drift can vary and cause you to lose plants.



The best option would be to remove the bricks and any organic soil under
them, replacing it with gravel to bring the grade to the proper level.
This will not only remove the growth media and seed bank for the weeds,
but will minimize fungus gnats, which breed in the organic material,
whether or not there is a weed cloth over it.

Also, if there is really organic material under the paving bricks,
walking on it will eventually compress it to the point where it is
impermeable to water and you will have drainage issues. The gravel will
mitigate that (although you really only have to worry about it in the
walkways).

However, this solution is a lot of work and has a limited lifetime. You
will be occasionally spilling growing mix etc, adding to the organic
component of the subsoil, so eventually the fungus gnats will have a
place to reproduce. You can stretch this out by placing the weed block
cloth on the surface. This will allow you to clean the floor
periodically, minimizing the amount of organic material reaching the
subsoil. I have weed block cloth (the woven polypropylene stuff) that
has been in place for about 10 years now and it's still in good shape
despite having been walked on all that time.