Why You Shouldn’t Use Rain Barrels On Edible Gardens

By Julie Dieguez,
Schoolyard Habitat Specialist - The Schoolyard Habitat Partnership

Recently a question was posed to me regarding the use of rain barrels on edible plants, such as vegetable gardens.   It was only after several years of working with rain barrels that I first heard the warning advising against using rain barrels to water edible gardens/plants. It is the "first flush" of concentrated pollutants funneling down from the shingles that is of most concern. After some research, I came across a very informative web site containing advice on this topic from several environmental & health agencies and experts.

It is a pretty solid recommendation across the board that you should always wash vegetables and other edibles thoroughly regardless of gardening & watering methods. There is some discussion about exactly how concerned one should be about using rain barrels to water vegetables and other plants. It appears that the type of roofing you have is the main concern as this excerpt from the Minnesota Dept. of Health (MDH) explains:

  "If your roof is made of wood shingles or shakes that have been treated with any chemical (usually chromated copper arsenate--CCA) to make them resistant to rot and moss, lichen and algae growth, don't water your plants from a rain barrel. Water collected from copper roofs or copper gutters also should not be used. Zinc (galvanized metal) anti-moss strips--usually mounted at the roof peak--also produce toxic chemicals you don't want in your garden. Don't use rain barrels if you have these strips (you may want to remove them), or if you have had your roof treated with moss-, lichen or algae-killing chemicals within the last several years. Note that nowadays there are asphalt shingles on the market which have zinc particles imbedded in the surface. Check your shingle specifications if you have recently re-roofed. General practice is to avoid watering vegetables and other edible plants, such as herbs you plan to use in cooking, with rain barrel water collected from asphalt-shingle roofs. These kinds of roofs may leach various complex hydrocarbon compounds, so most people avoid using water from asphalt-shingle roofs or flat tar roofs on plants meant for human consumption." 

Furthermore, there is a concern about air pollutants that are deposited on roofs and then concentrated in the first flush of rain water that fills the rain barrels (remember that for each 1000 square feet of roof in a one inch rain event, 632 gallons of water runs off  (fact courtesy of Arlington Echo Outdoor Education Center). The MDH notes: 

". . . The contaminants that you could be worried about are the heavy metals and polyaromatic hydrocarbons from asphalt shingles and other contaminants that may deposit onto roofs from air . It appears that contaminants that rainwater washes off of shingles may be a significant source of surface water contamination. The contaminants that are washing off of roofs include zinc, lead, chromium, arsenic, and polyaromatic hydrocarbons. It is similar to what you might collect off of a parking lot. It is possible to find data on the amount (concentrations) of chemicals in rainwater from asphalt roofs. However, I was not able to find information on whether or not the levels were high enough to accumulate in garden plants intended for consumption."  

Personally, I take location into consideration as well. For example, I live in an area that has some particularly notorious air pollution and we are located five miles from an airport, directly under a flight path. Taking that into consideration, while I do use rain barrels, I will not be using them to water edibles (and I will be washing edibles very thoroughly regardless).

The consensus is essentially that in the end it's a judgment call, but considering the likelihood you will be collecting concentrations of something nasty, it may be wise to err on the side of caution and not water edible plants using rain barrels. However, this in itself is an argument for the use of rain barrels for everything but edible gardens since it is an excellent way to slow that water down and help it infiltrate through the ground to filter out some of those nasty pollutants before they hit ground water, rivers, streams and the Chesapeake Bay.



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