WiGard wrote:

...Last summer I worked in a nursery which had hoop greenhouses. Their floor
consisted of pea gravel over weed barrier. I doubt whether they had sand
beneath the weed barrier. I still saw weeds...


Greenhouses have weeds. No matter what you put under foot, you are going
to be working with growing media. You will spill some, some will wash
out of the pots/cells, organic material (leaves, cuttings) will collect
in inaccessable places and compost. All these things support weed
growth. The only thing you can do is remain vigilant and try to address
weed problems early, before they become established.

Weed barriers prevent leaves from coming up through. They do not prevent
roots from going down through them. If you get a weed growing on top of
a weed barrier, it will put roots through it, after which it becomes
very difficult to deal with, since removing all the roots damages the
weed barrier. Removing only some of the roots allows the weed to
re-grow. Get the weed before it puts down a significant root structure.

Use of Glyphosate in an active greenhouse is difficult since you have to
avoid drift. If you have to use it, it's best to use it when there are
no plants there. Greenhouses do accumulate crud. You should schedule
some time during the year to empty the house and give a thorough
cleaning.

There is a rule of thumb that seeds will only germinate if they are
buried less than seven times their maximum dimension. (This applies to
the field, where temperature fluctuations are large and may not really
apply to greenhouses). Since you are not disturbing the soil in the
greenhouse, you may get an initial flush of weeds from sand as the upper
layer of seeds germinates, after which you will only get weed seeds that
are brought in from outside (unless your indoor weeds go to seed).

I use a propane torch to kill weeds in the field to form stale seedbeds
for direct seeding things like carrots. This is difficult inside the
greenhouse if you have something like a woven poly ground cover floor
(although you might be able to wet the floor first and minimize damage,
I wouldn't count on it). With pavers, you might get away with it.
However, using a torch inside a greenhouse presents other problems such
as avoiding flammable coverings and carbon monoxide if the greenhouse
isn't well ventilated. Glyphosate might be easier.