How to Naturally Control Pond Algae

You Can Control Pond Algae The Pond Plant Girl

Q: We have a ¼ acre pond (approx 5-6 ft deep in the center). After the heavy rains we had about a month ago, our pond filled up to the top, but we had complete coverage of green algae on the surface. 2 weeks ago we burned a bunch of tree limbs right on the edge of the pond bank and the rains from last weekend flowed through those ashes into the pond. We didn’t see any algae after that… does the ash have an effect on the algae or will it just return? How much should we add? Will it harm catfish or other fish? I want to get rid of the algae, but would prefer to do it naturally, safely and inexpensively as we want to restock the pond with fish. Can you provide tips, thoughts and ideas for us?

A: Wood Ash: Wood ash is heavy in potassium, and normally increases algae growth, however nitrogen and phosphorus are usually more limiting factors for algae growth. A couple of ways that potassium could control algae growth are: 1) An overdose of potassium could temporarily poison the algae, although in the long run it would should make the problem worse; 2) Potassium is used by some aquatic plants to promote growth. The idea is for the other plants to out-compete algae. If there are other aquatic plants in the pond, particularly submerged plants, this might explain the algae control.

Fertilizers: If fertilizer get’s into the pond from rain runoff or if there are heavily fertilized plants in the pond, this can also increase algae growth. Water draining into the pond from a nutrient source (such as a cow pasture, fertilized crops, or lawn) can also be the cause.

Algae Season: Fall and Winter is the time of year when algae growth declines due to cooler temperature and daylight.

Pond Sediment: Large amounts of accumulated organic sediment on the pond bottom can promote algae growth.

Location: It is difficult to know what is primarily responsible for algae growth in large ponds. Different parts of the country have different algae issues. Examining the water source for the pond is a good start. In parts of Iowa and Kansas, well water often contains so many nutrients that it will kill fish unless treated. Heavy levels of metals in the water will also kill plants.

There are several natural (ecologically friendly) means of controlling algae, but they are not cost-free, and this is especially true for larger ponds:

• Using a plant bog filter is best, but this requires a bog area about 10% the size of the pond with the appropriate plants and gravel, and recirculating water from the main pond through the plant filter at least twice daily for large ponds (more often for smaller ponds).

• Planting water lilies in the pond, with a 50 to 70 percent surface coverage, will help shade the pond. This lowers the water temperature as well as blocking some light, and helps control algae. It will not be complete control, and is best used in combination with one of the other methods, but it will help a lot, and help the other methods be even more successful. They would want to pick out large growing cultivars that still bloom well (the blooms are just for pleasure, not algae control).

• Adding submerged aquatic plants, such as anacharis or hornwort, will help outcompete the algae for light and nutrients. However, anacharis and hornwort will take over a pond and can become a menace. Dwarf Sagittaria is another good choice. It will grow like a short grass on the bottom of the pond, rooting into the soil or gravel, and will not get very tall. It will absorb nutrients and will compete with the algae. Unless a lot are purchased, it will be several years before they will have a significant effect in a pond that size. I recommend planting them at least a few months before koi are introduced. Koi will dig them all up and eat them if they are not well established. If established, they will still be a part of the koi’s diet (which is very nutritious for the koi), but the plants should grow faster than they are consumed.

• Many people have had good results with a bacterial product such as Ecological Labs P/L. It comes in a gallon quantity and needs to be put in weekly until the algae is under control; then monthly for maintenance.

• Barley straw has been used effectively, although the degree of control is probably less than using MicrobeLift P/L.

• Using a color dye is very effective. Color dyes are plant based and safe for animals and people. The colors used are blue or black. Many people prefer blue, because it is the color they think of around water (tropical beaches, swimming pools, etc), but black is actually a much more natural color for freshwater ponds. Black blocks light more, and does not break down as fast. Mud bottom ponds absorb the dyes faster than liner bottom ponds, but in a deeper pond, dyes will last long enough to make a difference. Large ponds need dye on a monthly basis. The black color probably won’t be noticeable unless you are swimming in it. Fish will still be visible when they come up to the surface (it would take a lot of black dye to make fish invisible at the surface). If you have waterlilies, an advantage of the black dye is that it creates a highly reflective water surface, which is great for viewing or photographing water lilies. (Denver Botanic Gardens and Longwood Gardens, among other botanic gardens, use black dyes in their ponds). We sell the blue or black dye as a liquid in various sizes, although only the gallon would be cost-effective. We are also going to be importing the black dye from China as a powder, which is by far the most cost-effective way to use it in a large pond. We use the black powder in our own production ponds for algae control, as well as control of submerged aquatic weeds.

• While it is not quite as eco-friendly as the other methods mentioned, using copper for algae control is fairly nontoxic except for invertebrates and amphibians. While simple copper is more toxic and goes out of solution quickly, chelated copper releases slowly, and much less of it is used, making it the best copper treatment (especially for trout, which are more copper sensitive than most fish). We sell a double chelated copper (F-30 algae Control, by Diversified Waterscapes), which is better than single chelated copper, because double chelated works well in both low and high pH water conditions.

• There are also several granular peroxide-based treatments, which I consider to be natural in how they approach algae control. We carry ones by BioSafe, Ecological Labs, and Winston. While there are large sizes of these used for large ponds, I do not consider them to be as cost effective as other methods for large ponds, and they are a short term solution, as the nutrients from the dead algae stay in the pond and will probably eventually be used to grow a new “crop” of algae.

Reference: Oregon Aquatics