they'll sure rust... but i dont think it'll harm the plants... they actually need some iron once in a while to synthetize (is that the way to spell it?) the chlorophil (is that the correct spelling?) they need to photossynthetize their food...
but if you want to, i guess you could buy that protective paint we use prior to the real colour in gates and such... i dont think it'll harm the plants... but it'll just slow the rusting process down... it'll eventually rust and come to pieces...
Bohrier