Here is a pic of the plants above the benches.

If it were me, I'd hate to be spraying Roundup repeatedly inside the greenhouse. It looks like you have drain rock or gravel under the benches. I'd rake that out, lay down a thick fabric weed barrier, and put the rock back on top.

Kudzu, thanks for the post.

There is weed barrier fabric under the crushed rock. The weeds hitched a ride in with some plants and got started growing in the rock.

We've tried pulling it but it is very difficult to get to because the benches have wire fencing on each side used to hang mounted plants. Also, there are misters mounted under the bench (to help regulate humidity) so I can't move the benches.

I'm hoping that someone knows of some herbicide that can be safely used as long as no spray gets on the orchids. I've done a lot of research and gotten conflicting information concerning the risk from evaporation/volatilisation.

Thanks,

Joe

Joe-
I'm not a toxicologist, but I do have a background in environmental health and safety. I tend to be conservative about repeated spraying of herbicides in enclosed spaces which are regularly occupied by humans...even if it is something with a supposedly short activity life, like Roundup. As far as I'm concerned, there is no "safe" herbicidal spray, just some that are less toxic than others.

I don't mean to be unhelpful, but I think you are going to have a continuing problem, and will have to worry about spraying regularly, if you don't make some physical changes. I can see how access is a problem, but can you unfasten one part of the wire screen so you can at least crawl under the table? What if you cover the area with black plastic for a few weeks until you kill the weeds, or use Roundup once, and then put down weed barrier over what you have down there and don't add any more rock on top of it?

If you still want to try to control this chemically, below is a link that discusses the pros and cons of several chemicals for use inside greenhouses. Good luck with whatever approach you decide.

Here is a link that might be useful: Greenhouse Weed Control

Kudzu,

Thanks for all the help and advice. I can see your point about exposure to toxins from repeated spraying. Definitely something to take seriously. These weeds have been out there for a long time. We were able to keep them under control just by pulling a little at a time. We were hoping that one good application would get it back under control.

The black plastic may be a good option. I have heard of using that in gardens but never considered using it in the greenhouse. Thanks for the idea.

Maybe we just need to get a couple of pet rabits! :-)

Seriously though, thanks for the advice, ideas, and links.

Joe

Ah, the black plastic is an excellent idea Kudzu! Especially with the heat of the greenhouse, that should really cook things nicely. I will have to remember that next time I need to get rid of weeds. Last year I resorted to applying roundup to some dandelions that had taken hold in a place that was impossible to reach. I applied the roundup with a paintbrush to minimize drift, and it did the job. But I really dislike the idea of using any chemicals in the greenhouse or veggie gardens. I just don't want that stuff near my food supply!