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.”