Bloomin' Algae

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Oh, my aching muscles...

Algae blooms are common in spring. istock photo

By Kristin Mehus-Roe

It’s springtime in the Northwest and green is everywhere—in the trees, the grass, and even in your pond. As the weather warms, algae bloom as lavishly as cherry blossom trees, if not quite so prettily.

Green trees and green grass are one thing—a green pond is quite another. Or is it? If you’re a pool or spa owner, you’re used to fighting algae. If you’re a pond owner, algae are your friends, kind of.

Algae are normal in ponds. The water in a healthy pond should be green, not clear, like in a swimming pool or spa. The velvety algae that grow on the stones in your pond and dance suspended in the water are healthy in moderate amounts —they produce oxygen during the day; give your fish, and the tiny animals your fish eat, something to nibble on; give fish a soft area in which to spawn; and they’re pretty.

“You are going to have some algae in the pond,” says Mark Harp, also known as Mark the Pond Guy, owner of The Pond Store in Sumner, Washington. “It’s normal, it’s healthy, it’s natural.” However, he cautions, you also don’t want algae so thick “you can walk on it.”

Types of Algae

The two types of algae that you are likely find in your pond are planktonic algae (usually called green algae, free-floating algae, or suspended algae), and green filamentous algae, usually called string or hair algae.

Green algae are the algae that make your pond green and in moderate amounts are quite good for your pond. Green algae produce oxygen and they provide your fish and other critters with snacks. Green algae, however, become a problem during blooms. At this time they may become frothy and even turn a putrid yellowish color. Particular forms of planktonic algae, such as blue-green algae, can be toxic for some animals, including dogs and cats.

String or hair algae are the gloopy, gloppy algae that can sometimes grow out of control. String algae usually cause the most problems for pond owners. They bloom early, grow aggressively, and cover ponds with their stringy mats.

Cause and Prevention

Algae blooms are triggered by sunlight and access to nutrients. That’s why you will commonly see algae blooms in the spring and summer.

Blocking sunlight from the pond can help minimize algae blooms. You can prevent light from reaching the bottom of the pond by making your pond at least three feet deep. If you have koi, your pond should be at least this deep regardless, to prevent predation. You can also help keep algae from receiving light by using plants to shield the pond from the sun. Floating plants, such as water hyacinths and water lettuce, and submerged plants (which bloom out of the water), such as water lilies and lotus, should cover at least 50 percent of the pond’s surface. Some pond owners also use tarps or arbors over their ponds to reduce light.

To keep excess nutrients out of your pond, install the pond in a location where fertilizer will not wash into the water during heavy rains or with regular watering. (Be aware of where water flows from your neighbors’ property as well.) Build ponds with a raised lip so runoff will not go directly into the pond. If your pond is near a structure, make sure gutter downspouts are directed away from the pond. Do not build your pond under a tree. Algae thrive on leaf litter, which sinks to the bottom, decays, and turns to organic matter.

Five Steps to a Healthy Pond

According to Mark the Pond Guy and other pond experts, there are five other things a pond needs to avoid excessive amounts of algae: a mechanical and biological filter, rocks and gravel, plants, fish, and added bacteria.

A mechanical filter, or skimmer, collects all that fish waste and dead plant matter that decay and become the nutrients algae thrive on. Be sure to clean out the filter regularly or the nutrients will leach back into the pond water. A biological filter, which is ideally used in conjunction with a waterfall (biological filters that are fitted into a waterfall are called bio-falls), adds beneficial bacteria to the pond water. Those beneficial bacteria consume the extra nutrients in the water, leaving less for the algae to feast on, essentially starving out the algae. Mark describes the biological filter as “the hotel for natural bacteria to live in.” Waterfalls assist the bacteria by oxygenating the water. While beneficial bacteria need oxygenated, flowing water, algae prefer stagnant water. (If you don’t use a waterfall, at least use a water pump or spitter in your pond to aerate the water.)

According to Mark, rocks and gravel should be added to the pond not only for beauty but as another hotel for natural bacteria—they provide additional surface space for the bacteria to live on.

Plants and fish do the same thing that beneficial bacteria do; they consume the extra nutrients, leaving less for the algae. Be careful when feeding your fish, however. Overfed fish will not assist in eating algae and uneaten fish food will sink to the bottom and decay, becoming algae food. You can also add other critters to your pond, such as tadpoles and snails – they’ll eat the extra nutrients and the algae itself.

You can increase the amount of bacteria in your pond by adding a store-bought nitrifying bacteria product for ponds. Mark adds it to his clients’ ponds once a week.

“If you don’t have one of these things, you will not have a complete ecosystem,” says Mark.

Giving Algae the Boot

If you do, despite your preventive measures, end up with a particularly bad algae bloom, there are some ways to reduce it. Do not use algaecides in your pond. Algaecides will kill algae but they’ll also kill your koi and your plants.

Instead, add 35-percent hydrogen peroxide to your pond. This is obtainable at any pond supply store. Hydrogen peroxide kills most but not all of the algae in your pond, clearing up blooms.

Although hydrogen peroxide has been shown to be safe when used in recommended amounts (talk to your pond supply store), some pond owners prefer to go the organic route by using logs of barley straw. Placed in the pond water, barley straw releases hydrogen peroxide as it decays.

Don’t get carried away when it comes to battling algae. While excess algae can cloud the water, foul filters, and choke out other plants, shocking the algae from the pond can be even more problematic: a sudden die off of algae will cause a plummet in oxygen in your pond and a dramatic increase in ammonia, possibly killing even your prize koi.

And ultimately, a pond, unlike a pool, is a living environment. A balanced and healthy pond ecosystem is made up of all its elements, including the algae.

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