"Anthony Ward" wrote in message

I have a plot at an allotment that is under siege from couch grass,
mares
tail etc. I want to create some kind of barrier around it that will
stop the
invaders and prevent the edges from eroding. I have some old joists
of about
12"x4" that I could sink into the ground although presumably they
will rot
over the years.

I have used old wooden railway sleepers to edge the beds of my veg
garden and they have been there 10 years so far with no sign of
rotting. I think they would be about 8"x4" so smaller than your
timbers, but similarly old and hardened. I've not been able to keep
couch out of my veg bed in one spot and although I hate using any
chemicals I do paint herbicide on by hand in the one spot it comes in.
That seems to knock it for a full year but each year it returns. I
can live with that but YMMV.