advice on tree pit guards / curbside planters

[mattkime]

I've built a couple of curbside planters here in brooklyn. i've discovered that people in brooklyn don't really realize they shouldn't trample over them. a small fence should get the point across. but what kind of fence? i'm looking for -

- durable
- simple (nice but not fussy)
- cheap

someone nearby used bent rebar which i liked but my girlfriend doesn't like the idea and she's usually right about these things.

here's an example -



Ideas on how to do it on the cheap?