When your local reflecting pool or pond turns green with algae, don’t reach for chemicals – nature has better solutions

July 10, 2026

Here’s this week’s ‘Ask Mike’ feature, which should be interesting to anyone with a pool, pond or water feature.

by Eric Palkovacs, University of California, Santa Cruz

And we quote: “When the Lincoln Memorial Reflecting Pool turned green with algae just days after a 15-million-dollar restoration, the government scrambled for chemicals and expensive technical solutions to fix the iconic landmark.

“Trying to kill algae with chemicals is a common response when community ponds or other water features go green. But as a scientist who studies freshwater ecology, I can tell you there are better solutions that cost far less, last longer and carry less risk of harm to pets and wildlife.

“The algal bloom that turned the Reflecting Pool a vibrant green shouldn’t have been a surprise.

The pool is big, more than a third of a mile long, and around 165 feet wide. But it’s shallow, meaning it warms up quickly in the sun. When it was repainted “American flag blue” during the renovations in spring 2026, the new color darkened the pool, and darker colors absorb more heat.

On top of those conditions, the pool was refilled with water from the nutrient-rich tidal basin of the Potomac River. The combination of warm water and nutrients created prime conditions for algae to bloom, turning the water pea soup green. “Trying to chemically or mechanically remove algae can damage the structure of a water feature and may harm species in the water that could actually help solve the problem.

Also, chemical and mechanical solutions are only temporary fixes. When the Reflecting Pool is drained and filled again, there’s a good chance that the algae will bloom again.

Limnologists – scientists like me who study inland water bodies – have spent many decades learning why lakes and ponds turn green and how to clear them up.

Often, nutrient-rich waters fueled by fertilizer runoff from farm fields or sewage from cities are the sources that stimulate algal growth.

However, natural ponds host a grazing zooplankton that eats algae. For example, a type of zooplankton called Daphnia, known commonly as the water flea because of the way these tiny crustaceans swim, control algae by consuming it before it becomes a pea soup nuisance. Thus, a thriving Daphnia population can help maintain good water quality in a lake, pond or community water feature, even when nutrient levels spike.

In addition to being highly effective grazers, Daphnia have another superpower – they can evolve rapidly. Urban waterbodies are often harsh environments with a variety of challenges, including high temperatures, low levels of dissolved oxygen, and pollutants. Daphnia can adapt to tough conditions, making these creatures an ideal source of algae control in many urban ponds.

Rooted aquatic plants are also useful for algae control in ponds because they absorb nutrients. Thus, shallow ponds with thick beds of aquatic plants can often resist algal blooms when nutrient levels rise.

One downside to draining and refilling a pond or urban water feature to try to clean it is that doing so resets the aquatic ecosystem, erasing the signature of any past evolution that has taken place.

Imagine Daphnia in a shallow pond that experiences periodic heat waves throughout the summer. Through repeated exposure to high temperatures, natural selection favors heat-resistant genotypes that can thrive in an urban pond.

If a Daphnia population that evolved to tolerate warm temperatures and low oxygen levels is removed, the new population likely won’t be ready to handle those challenges. This evolutionarily naive new population will perform poorly in its new environment, reducing its effectiveness at controlling algal blooms.

As a result, traditional mechanical and chemical approaches may actually work against the goal of minimizing algae in ponds and other water features.

The hubbub over the Reflecting Pool holds a mirror up to assumptions about how to solve pressing environmental challenges. The idea of just engineering one’s way out of any environmental crisis has limits.

Understanding ecology and nature’s mechanisms of ecosystem resilience can achieve sustainable solutions that benefit both nature and people.”

This article was first published in The Conversation, a nonprofit, independent news organization bringing you facts and trustworthy analysis to help you make sense of our complex world. It was written by: Eric Palkovacs, University of California, Santa Cruz.

OK—so the big lesson here is not to use river water to fill a pool, pond or water feature; it’s a sure way to summon algae. If the Reflecting Pool had been filled with city water, the problem may have never occurred—although filtering the chemicals out of city tap water would be even more ideal for the little creatures that clean up the algae. And, of course, don’t use dark colors on the bottom, especially if the water feature is fairly shallow; plain white might be boring, but it won’t cause the water to warm up as much.

Now, I was going to leave it at that, but other than encouraging water fleas and not using chemicals, the article kind of leaves a little DIY hole in the story. Then I remember covering the use of barley straw to prevent such problems back when I was Editor-in-Chief of ORGANIC GARDENING magazine, so I started searching for the latest info on that technique and struck gold at the Penn State website:

{quote} “Excessive algae growth is one of the most common problems occurring in ponds in Pennsylvania. Traditional mechanical and chemical control methods are not always efficient or economical.

“In recent years, the use of barley straw has become more common as an alternative method for controlling excessive algae growth. This method has been extensively studied by Dr. Jonathan Newman at the Centre for Aquatic Plant Management in Great Britain. Here is a summary based on Dr. Newman's work and our own experiences in Pennsylvania.

Quote: “When applied at the proper time and rate, barley straw has been a very successful algae control technique in Pennsylvania ponds. Barley straw does not kill existing algae; it inhibits the new growth of algae. The exact mechanism is poorly understood, but it seems that barley straw, when exposed to sunlight and in the presence of oxygen, produces a chemical that inhibits algae growth.

Barley straw does not reduce the growth of aquatic plants. In fact, in some cases aquatic plant growth has increased after barley straw applications because algae are no longer present to compete with the aquatic plants.

Barley straw is most effective when applied early in the year prior to the appearance of algae (fall through early spring). When applied to cold water (less than 50°F), it may take six to eight weeks for the straw to begin producing the active chemicals that inhibit algae growth.

If the straw is applied to warmer water (above 70°F), it may become effective in as little as one to two weeks. In any case, barley straw remains effective for approximately six months after application. The most common application is about two to three bales per surface acre of pond (or about 10 to 25 grams of straw per square meter of pond area). The depth of water in the pond is not important. In ponds that are frequently muddy or those that have a history of heavy algae growth, two or three times this recommended dose may be required for the initial treatment.

The straw is most effective when applied loosely in cages or netting. It is best to anchor the straw packages to the bottom, but provide a float to keep the straw near the surface of the pond where sunlight and oxygen are more prevalent.

Apply the straw at several locations around the pond and especially near the water source if a spring or stream feeds the pond. In small garden ponds, onion bags or old nylon stockings can be used to hold the small amounts of straw needed.

Finding a local supplier of barley straw can sometimes be difficult. You might consult with private and government agencies that work with local farmers, such as farm supply companies, extension offices, and Conservation District offices, to determine if barley straw is locally available.

In addition, several suppliers are available online (just type "barley straw" into your favorite search engine).

By Bryan Swistock, Former Senior Extension Associate; Water Resources Coordinator, Pennsylvania State University.

A final note from Mike: A quick Internet search revealed many sources, both from mail order and locally.

Well, that sure was an interesting and informative look at the many ways to keep pools, ponds and water features nice and clear, now wasn’t it? As always, you can read the written version over at your leisure or your leisure by pasting www.youbetyourgarden.org into your cursor; where you’ll also find an audio link to the full show, which includes lots more interesting stuff. Or go to Gurney’s Seed and Nursery and scroll down to the You Bet Your Garden section.