Elisabeth C. Miller Library logo Miller Library Home UW Botanic Gardens Home UW Botanic Gardens Home book graphic

3501 NE 41st Street, Seattle, WA 98195 | (206) 543 0415 | Open: | Library Schedule

Gardening Answers Knowledgebase

Search Results for: Algae | Search the catalog for: Algae

Plant Answer Line Question

Keywords: rain barrels, Irrigation water quality, Algae

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
"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
"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."

Date 2016-12-23
Link to this record only (permalink)

Plant Answer Line Question

Keywords: Algae

I wonder if you can tell me what this gelatinous greenish substance is that is spreading all over the gravel driveway at our house. It looks kind of like seaweed. We tried spraying pesticide on it, but it came back with a vengeance. I'm worried about slipping on it, and would like to know how to get rid of it.


What you are describing sounds a lot like Nostoc, also known as witches' butter, which is a type of cyanobacterium (blue-green algae). It is often found on surfaces which are made of some form of calcium carbonate rock (such as gravel, steps, and sidewalks). According to The Seaweed Site, nostoc is "common on limestone gravel near path edgings [...], particularly in autumn. It can suddenly appear after rain, seemingly from nowhere, on paths, on roofs and sometimes in poorly-growing lawns. This gave rise to an early belief that it was material from shooting stars that had fallen to earth, hence some of the English common names Fallen Star, Star Jelly and Witches' Butter."

It apparently causes trouble in plant nurseries, too, as this document entitled "Nasty Nostoc" describes. (Source: Heather Stoven and Jennifer Parke in Digger magazine, June 2014, Oregon State University.) Many of the treatments mentioned in the article are not appropriate for use on gravel, and with any treatment, care must be taken not to contribute to toxic runoff into stormdrains. Avoid using phosphorus fertilizer, as that can encourage development of nostoc. Try to keep water from pooling on the gravel (hard to do in a rainy Seattle fall season). If the algae is only on a small area, you may be able to shovel it up and dispose of it. If you can solarize the area during the hottest weeks of the summer, that may also be helpful (covering with clear plastic for two to four weeks).

Date 2017-04-14
Link to this record only (permalink)

Didn't find an answer to your question? Ask us directly!

Browse keywords

Search Again:

August 01 2017 12:36:01