Ask a gardener what their biggest gardening problem is and they will probably tell you it's weeds. The same is true of water gardeners, said Jeff Rugg, U of I Extension horticulture educator.
"The weed is called algae," said Rugg. "There are thousands of different kinds of algae, but they are grouped into two kinds based on their type of growth.
"The free-floating algae cells that tint the water green are called green-water or pea-soup algae. There can be 30,000 cells floating in a single teaspoon full, so just think of how many there are in the pond."
Algae that grow together in long strands stuck to surfaces in the pond are called hair or string algae.
Just like the weeds in the dry garden can be controlled in several ways, algae can be controlled using natural and chemical methods. "Algae grows in a pond because there is water, sunlight and nutrients," he explained. "Take away any one of the three and algae can't grow. Okay, taking away the water means we don't have a pond, so that's out. Taking away the sunlight is pretty hard to do and prevents us from growing other plants, so all that is left is to take away are the nutrients."
What are the nutrients, and where do they come from? Principally, we are dealing with nitrogen, and phosphorus, although the algae also need carbon, hydrogen, oxygen and many other nutrients. The nitrogen often comes from fish waste, and the phosphorus usually comes from fertilizer tablets for the potted plants. Even without fish and plants, a pond can have algae, so what do we do?
"The key is to have as many nutrients removed by other organisms as possible," he explained. "Bacteria growing on filter material, rocks and gravel and the pond walls will help and so will plants.
"Research is being done on how the bacteria accomplish the job, but one thing we do know is that they work. From small backyard ponds to large golf course ponds, bacteria products have cleared the water of green water algae and reduced string algae to an acceptable level in every climate type. More bacteria growing on more surfaces means less algae, so add large filter pads and circulate the water from the pond through the filters as often as possible. Filters that include an ultra violet light will help reduce the number of green-water algae cells, but not string algae cells."
Floating plants hang their roots in water to take out nutrients, so they compete with the algae. Plants like lilies that have leaves floating on the surface reduce the amount of sunlight into the water, but ponds almost completely covered in lily leaves can still have green water algae.
"Potted plants do not compete with the algae as they have a supply of nutrients in the potting soil and fertilizer tablets," said Rugg. "Some of these plants can be taken out of their pots, the roots washed clean of mud and then replanted into a rock flower pot.
"Make a campfire ring of large rocks on a shelf and fill it with gravel. Plant the cleaned-up plants in the ring and now the plant must take its nutrients from the pond water, thus competing with the algae for food."
Just like in the flower garden bed, the more good plants you have, the fewer weeds there will be. In the pond, a mix of floating plants, underwater plants, surface leave plants and plants growing in gravel, the fewer algae there will be.
The next method works on string algae in warm water ponds.
"Algae-eating fish can be added to eat the string algae," he said. "These tropical fish from the pet store can't survive pond water below the 60s, so they only work during the summer for northern ponds, even though the algae may start growing months earlier when the water temperature was still in the upper 30s."
If you have a water feature without plants, the chemical weed control approach is appropriate. Bacterial products may still be useful, if you want a natural approach. There are algae-killing chemicals that are fish safe and won't harm the beneficial plants and bacteria, such as AlgaeFix from Aquarium Pharmaceuticals.
"If you do not want any plants, then you have a water feature like a swimming pool or fountain and it is maintained the same way with chemical weed controls," said Rugg. "Just like in a swimming pool, no matter how often the chemical sterilizers are added, the algae does come back, so the weed control chemicals will need to be reapplied too."