Andy writes:

As far as I know, anything "living" will compete with the plants
you want
for nutrients. And I saw on the Discovery Channel than many plants,
including grasses "exude" something that retards the growth of other
plants
around them, in order to enhance their own survival....

I will be watching this thread to see if anyone comes up with a
suggestion
for you, since it is an idea that has benefit to all of us, if
possible. But I
really don't think it's going to work out......

Good luck.