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Elisabeth C. Miller Library

Keyword: Irrigation water quality

Rain barrel care and maintenance

I have two white plastic rain barrels that catch the runoff from the roof. I use the water to irrigate my lawn, Rhododendrons, and Azaleas. How do I keep the pondscum from building up in the barrels? Can I use bleach in the barrels, or will that hurt the plants? I am thinking about 1 tablespoon per 55 gallon barrel.

One of the benefits of harvesting rainwater is that it should be relatively free of things like chlorine (found in bleach), and therefore not harmful to your plants or to to anything downstream of your garden. If you can avoid algae build-up by locating the barrels so they are not in full sun, that would help. Rain barrels in full sun and barrels which are a light color are more susceptible to algae growth. You might consider painting the exterior of the barrels a dark color.

I realize that the amount of bleach you are thinking of using is small and, in fact, some resources suggest removing algae with a dilution of bleach (one site said a teaspoon per 20 gallons, another recommended considerably more). The bleach solution should be used as infrequently as possible. You can empty the water into a household drain, or at least allow the bleach solution to dissipate for some time before using water in the barrels on the garden. The following links discuss rain barrel maintenance.
The following excerpt is from the City of Winston-Salem’s Rain barrel FAQ brochure:

“Algae need sunlight to grow. A dark-colored rain barrel will exclude the sunlight; paint clear barrels or cover to prevent growth.”
Lebanon, PA County Conservation District: Rain Barrels
Excerpt:
“The water in my rain barrel has developed a green scum on top — how do I get rid of it without harming my plants?”
“That green scum is probably algae. Algae grows almost in any water with sunlight
and is not harmful. To eliminate it, put one or two capfuls of bleach in the water
(not in your empty tank). Although that small amount of chlorine won’t be harmful,
let the water sit for a few days before you use it on plants. When the barrel is
empty, turn it over and use a scrub brush to clean it out.”
Here is information from Wisconsin Horticulture on rain barrel care and maintenance

Excerpt:
“Growth of algae may also be a problem if rain barrels are placed in direct sunlight. If algae become a problem, empty the barrel and then wash the barrel with a dilute bleach solution: cup of bleach per one gallon* [my note: this sounds like far too much–perhaps they mean 3/4 teaspoon per gallon, and even this seems high] of water. Rinse the barrel well after bleaching and dispose of the bleach water in a household drain.”

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using fluoridated water in the garden

Are there any adverse effects from using fluoridated water in the garden?

I do know that some indoor plants do better with water that is not fluoridated. There are conflicting opinions on the effects of fluoride on human health and the environment (including plants). According to this 2004 article entitled Water fluoridation and the environment by Howard Pollick in the International Journal of Occupational Health (reprinted by the Centers for Disease Control), the fluoride level in residential water (as opposed to industrial runoff) seldom rises above a level of concern for plants.

For an alternate viewpoint, see the Fluoride Action Network’s website.

harvesting rooftop runoff water for vegetable gardens

I need to replace my roof, and I am thinking of installing asphalt or composite shingles because they’re what I can afford. I planned to capture rain water to irrigate my vegetable garden, but I’m concerned about toxicity. Is runoff from the shingled roof likely to be toxic?

 

I would not recommend using reclaimed rainwater from an asphalt shingle roof for any edible crop. Asphalt is petroleum-based. The runoff might be acceptable for ornamental plants, but the fact that asphalt or composite shingles tend to shed tiny particles means that those particles would be introduced to the soil around your crops.

An article from North Carolina State University Extension discusses “Water Quality of Rooftop Runoff.” It doesn’t specifically mention asphalt, but I don’t think it would be wise to use the reclaimed water on food crops.

Green Living Journal (now archived) has an article about roofing materials, and discusses asphalt shingles as well as alternatives.

The National Gardening Association site has a report, no longer available online, that describes rainwater harvesting.
Excerpt:
“Water from the rain barrel is, of course, not potable, but some experts also raise concern about possible contaminants from rooftops that can make the water unsuitable for edible gardens. According to an article in Landscape Architecture magazine, asphalt shingles and other porous or rough roofing materials can hold particulates such as bird droppings and other debris, as well as heavy metals from the air, which then wash into the rain barrel. Wood shingles that are chemically treated to resist rot and algae can leach the chemicals into the rainwater running off the roof. Zinc strips that prevent moss build-up can also be problematic. Some large-scale rainwater collection systems are even designed to allow for the first flush of water off the roof — which carries the majority of the questionable substances — to be diverted.

“Other people dispute these risks and say washing your garden produce is all that’s needed. It’s a judgment call. I tend to research things to death so I think it would be interesting to have some of my rooftop runoff tested at the health department.”

There is a very detailed and technical document entitled “Roofing Materials Assessment” from Washington State Department of Ecology (2013-2014 study results) if you wish to pursue the topic further.

If installing slate, clay tiles, untreated wood shingles, or a green roof is prohibitively expensive, the best solution might be to landscape the garden in such a way that you can reclaim runoff from the roof for non-edible plants.