While I like the carpet idea and use one as a temporary 'driveway', please do not use 'used' carpet as it may (and probably does) contain all kinds of cleaning chemicals, and possibly high amounts of baking soda which can mess with your pH, unsanitary flood (septic overflow) residue, flea eggs, or could be from an old meth lab.

Stop by a job site where they are installing new carpet and ask for all the remnants, or make prior arrangement with a carpet installation company ; even new carpeting may contain unwanted chemicals such as a stain-repellents but is better than the unknown items in the used carpets.

Edited for clarification.

Most of what I use comes from the neighborhood, and I doubt they have a meth lab in their living room or bed rooms.

As for unwanted chemicals, new carpet is the worst with out gassing from manufacturing chemicals than old. I read that some production methods use formaldehyde and people that are very sensitive to it can be sickened from even low levels of vapor.

When I installed my wood floors, I saved the carpet to use for pathways, although I've yet to use it. Never thought of planting "thru" it.
Do you put the mesh side up or toward the dirt?

I know the only thing in my 35 y/o carpet is dirt!!!!!

Great idea
I have used my old carpets for years to smother weeds (total organic here)
and to put pots on to keep weeds from growing between them.

I put the mesh side down. If weeds should germinate on the carpet, I find it easy to just scrape them loose with a hoe.

EEK Greene - you raise a good point!
I've moved on to cardboard covered with chips.

But what I used was the original carpet donated to me from a shelter volunteer whose father had leftover NEW carpet from an installation job when I first fostered my house bunny rabbit. Promised to be FRESH AND NEW because bunnies can't handle chemicals either. It was in the bottom of her cage for 3-4 years, then went outside.