It’s the scourge of backyard water gardeners everywhere: variously called “green water,” “blanket weed” or “pond scum,” it’s known to scientists as thallophytes and to less-emotional water gardeners as green algae.
Green algae can grow in any environment where there is light and water: in mud puddles, dirt, even in your bathtub. It shouldn’t shock you then that algae can virtually take over a water garden. Algae spores can come in your pond by wind, rain, or on fish and water plants you place into the pond. In extreme cases, algae can bloom into “pea soup,” completely obscuring everything else, including fish and water plants.
Can algae be defeated by backyard gardeners? No – and it shouldn’t be, at least not completely. That’s because algae is an essential part of the natural food chain in all water gardens. Algae keeps the pond “in balance,” helping to maintain a healthy environment for fish and other pond creatures. But there’s another reason why you shouldn’t try to eradicate green algae completely – it’s impossible.
The reason? Backyard ponds have an unnaturally high ratio of fish and plants per gallon of water. While we may believe our water gardens are a “little bit of nature”, they are very unnatural when it comes to biology and chemistry because of this imbalance of flora and fauna. This produces an ideal growing environment for green algae.
So what’s a backyard water gardener to do when it comes to staring down algae?
1. Avoid too many fish in your water garden; more fish mean more poop which breaks down and provides nutrients like phosphates and nitrates for algae to eat.
2. Plant a good amount of aquatic plants in your pond; they consume the same nutrients as algae. You should try to cover at least 50% of your pond with floating plants like lilies, hyacinths and submerged plants like Cabomba.
3. Snails and tadpoles consume pond scum-type algae. Try a few Japanese trapdoor snails and tadpoles.
Sometimes all efforts of “natural” methods don’t seem to work, or you’ve run out of time and patience. In that case, an algaecide can be your answer. Biological filtration can also help; it’s designed to remove harmful ammonia released by fish, decomposing plants and algae, and uneaten fish food.
To conclude: the best way to control algae problems naturally is to minimize the amount of phosphate and nitrate in the water. You can limit these nutrients by limiting the number of fish, limiting the amount you feed them, and adding plenty of aquatic plants to the pond.