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Column: Controlling algae in ponds

Hanbidge on Horticulture: The simplest method to control algae is to limit light and nutrients.
water lily (Medium)
Having plants cover a good part of the surface of the pond will aid in reducing algae growth.

Water in a landscape makes the ordinary – extraordinary. Somehow water finishes off the lovely ambiance we have in our outdoor living spaces. It attracts birds, butterflies and all sorts of other critters as well. However, this time of year the crystal-clear ponds are often more like a rendition of pea soup. Read on to learn more about controlling algae in your ponds.

Algae are primitive plants that do not have the regular plant parts like leaves, stems or roots. They reproduce by spores, cell division and fragmentation and just like land plants they need light, water, nutrients, carbon dioxide and oxygen in order to live. Knowing this is key to controlling algae as it is usually light and nutrients that will limit the growth.

The simplest method to control algae is to limit light and nutrients. Having plants cover a good part of the surface of the pond will aid in reducing algae growth. Limiting the amount of nutrients in the water will also help. Avoid using fertilizer around the pond as it is a great source of nutrients for the algae. It is important to have a balance between the size of the pond, the plant material and the number of fish. Do not overfeed fish as this is one of the fasted ways to increase algae growth. Rain, pollen and falling foliage will also create a surplus of nutrients that is ideal for increasing algae growth.

There are a number of forms of algae you may experience. Suspended or planktonic algae are very common and can quickly turn your clear pond into pea soup. Algae feed on the nutrients in the water and reproduce rapidly. When the nutrient level decreases, so will the algae population and the water will clear. When large amounts of algae die, it depletes the oxygen supply in the water which can also potentially kill fish or other animal life in the pond. It is also this anaerobic (without oxygen) water which can be rather foul smelling.

Bacteria play an important role in keeping your pond in balance. They actually feed on nutrients that are in the water caused by decomposing leaves and leftover fish food. They will also help to decrease the sediment often found in the bottom of the pond and will also help to eliminate odours.

As in most aspects of life – just a little balance is the answer. Ensure that floating plants like duck weed or hyacinths cover 50 to 70 per cent of the pond surface. Submerged plants or oxygenators will help keep the water smelling fresh. These plants will also help remove dissolved nutrients giving the algae less available food. If you have fish in your pond and you feed those fish, it is sometimes as simple as limiting the amount of food you feed as this is a great additive for algae.

I have a large pond which has a waterfall and a pool skimmer to help ensure any debris is removed from the water prior to rotting thus reducing the amount of "food" for algae. However, when there is a lot of vegetative material, the pond quickly takes on a green hue. Unfortunately, the only place in my landscape where I could dig in a large pond is beneath a beautiful and old but self-pruning willow tree. Needless to say in spring, when it flowers, the pollen and catkins are instant algae food, thus requiring extra care when my tree is shedding.

As always there are a number of chemical solutions on the market that state your pond will become pristine if you purchase and use their product, but keep in mind that any of these “quick fixes” are not long-term solutions. Ensure you read the labels carefully and follow the manufacturer directions.

Once you have an algae bloom, it takes a couple of weeks to get that ecosystem back in balance. Patience is required in order to achieve a good natural balance.

Happy summer!

Hanbidge is the Lead Horticulturist with Orchid Horticulture. Find us at; by email at; on facebook @orchidhort and on instagram at #orchidhort.

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