Is it safe to use galvanized containers (in this case a large
galvanized tub like the kind used for watering livestock) in which to
grow root vegetables, herbs, etc? I’ve seen some sites sell these for this
use, but there are also some postings referring to the potential for ill
effects of anti-rust coatings.
You may want to consider both what goes into the galvanizing process, and what could be leaching out of containers as the coating wears down over time.
Here is information on the process of galvanizing from the American
Galvanizers Association. Excerpt:
“What are the steps in the galvanizing process?”
- Pre-inspection – where the fabricated structural steel is viewed to
ensure it has, if necessary, the proper venting and draining holes,
bracing, and overall design characteristics necessary to yield a quality
galvanized coating - Cleaning – steel is immersed in a caustic solution to remove organic
material such as grease and dirt, followed by dipping in an acid bath
(hydrochloric or sulfuric) to remove mill scale and rust, and
finally lowered into a bath of flux that promotes zinc & steel reaction
and retards further oxidation of the steel… (steel will not react with
zinc unless it is perfectly clean). - Galvanizing – the clean steel is lowered into a kettle containing 850
F molten zinc where the steel and zinc metallurgically react to form
three zinc-iron intermetallic layers and one pure zinc layer
Based on the above, one concern would be whether the zinc would be
harmful. Zinc is one of many nutrients needed by plants, but I couldn’t
hazard a guess as to what effect the zinc from the coated steel would
have, if any, or whether the galvanizing process involves other
substances.
The book The Edible Container Garden by Michael Guerra (Fireside, 2000)
says the following:
“Galvanized buckets are increasingly popular but don’t use them for
ericaceous or acid composts.” (This would be a compost which is
lime-free. Usually soils in the Pacific Northwest tend to be acidic. I
don’t know from your message what part of the country you live in, but
this might be something to consider as well).
Another issue is that the metal containers will probably heat up quickly,
meaning that your plants might need more attentive watering.
Los Angeles County Cooperative Extension offers the following information, in an
article on trace elements and urban gardens. Excerpt:
“Cadmium is a contaminant of many manufactured products containing zinc.
Any zinc plating or galvanizing operations and galvanized metal
containers sometimes used in horticulture and gardening operations are
potential sources of cadmium.”
I certainly don’t think you want to grow vegetables in a cadmium-laced
container. In situations where there is any doubt about safety, I would
recommend growing ornamental plants in the tubs, and growing edibles in
untreated wood or clay pots.