Controlling Algae in Garden Ponds: Natural and Artificial Methods

Coping with Algae

Aquatic algae species fall into two practical groups: free-floating/suspended (minute, single-celled, cause pea-soup green water) and filamentous (silkweed, blanketweed, mermaid’s hair). Rarely a sign of an unhealthy pool, algae can still cause distress, ruining the visual effect.

Suspended Algae

Green water is most prevalent in spring, but may occur anytime if the pool is too nutrient-rich. At the start of the growing season, even a balanced pool may have a temporary algal bloom as water warms. Algae proliferate until higher plants—submerged aquatics, waterlilies, floating foliage—grow actively and provide surface shade. Once all aquatics are established and growing freely, nutrients are mopped up and the bloom disappears as quickly as it arrived.

Be patient in spring; wait for nature. With a suitable plant balance, all will be well. Resist emptying and refilling—this may temporarily alleviate, but fresh tap water is loaded with mineral salts that take longer to absorb, especially as higher plants are disturbed and checked. Never change green pond water if other aquatic life is present. Just be patient.

A flocculating product can temporarily sink algae out of suspension, allowing light to reach submerged plants and accelerating their growth.

Artificial Control of Algae

Every pond owner encounters green water. Algaecides kill green water algae, but dead algae must be removed or it decomposes, harming fish and plants. Alternatively, a flocculating product clumps single-celled algae, making them heavier to sink, letting light reach submerged plants. Repeat treatments may be needed as flocculants don’t prevent reformation.

For ongoing green water, install an ultraviolet (UV) filter: a fluorescent tube emitting UV light (enclosed in dark tube for eye safety). Water passes around the UV tube; UV kills algae. Use with a mechanical filter to collect dead algae.

Filamentous algae (blanketweed) require different methods. Algaecides available, but dead algae must be removed to prevent pollution. Pond balancing products adjust nutrient balance—slower but increase nutrients for desirable plants while restricting algae. After any treatment, remove dead filamentous algae to prevent decomposition; small broken pieces are safe to leave.

Filamentous Algae

Blanketweed: thick, mat-forming, coarse, fibrous, tangles around plants. Forms large floating mats; lower parts decompose into smelly brown/black masses. Often grows lustily in otherwise clear water; rarely in green water.

Silkweed: slimy, clings to submerged aquatics and waterlily stalks. Very dark green. Unlike blanketweed, slips through fingers, harder to remove.

Mermaid’s hair: hairy, pale green, clings to baskets and pool walls. Quite innocuous, can give a natural look. Unless it invades waterlily stems or tangles submerged aquatics, not a serious problem.

Natural Control

The ideal method is biological: create a balance preventing free algae development. Sufficient submerged plants to utilize major plant foods, plus enough surface shade from floating foliage to reduce light to a level only submerged plants tolerate, yields clear water naturally.

This theory works well, though occasional setbacks occur. A pond is a natural, evolving environment—changes happen, constant monitoring needed. Simply adding a fresh container of aquatic plants in compost (adding nutrients) can tip the balance.

Natural control is best. Right plant balance is key, linked with sensible fish introduction. Over-population of fish quickly tips balance back to algae, especially green water types. If fish become a major interest and you want to steadily increase stock, consider abandoning natural algal control in favor of a filter.