Algae presents one of the greatest potential problems for the gardener with a pond. There are many different species of algae, but from a practical pond management point of view they can be conveniently divided into two groups: suspended and filamentous. Suspended algae are mostly primitive minute single-celled species that cause a green algal bloom in the water, turning it like pea soup. Filamentous algae includes species variously referred to as Spirogyra, Silkweed, Blanketweed and Mermaids Hair.

Although not an indication of an unhealthy pond, algae can cause the water gardener considerable distress, totally ruining the overall visual effect. Suspended algae that create an unpleasant bloom are usually seen during the spring, especially in temperate districts where the growth of higher forms of plant life during the winter has become virtually non-existent. There is a sudden greening of the water as it warms up. The primitive unicellular algae appearing almost immediately and continuing to proliferate until the higher and more evolutionarily developed plants take over.

Once growth starts and the submerged plants begin to utilize the nutrients in the water, thereby competing with the more primitive algae, they rapidly go into decline and the water becomes clear. However, at any time during the year a green algal bloom may appear if the natural balance of the pond is disturbed. In new ponds, where the aquatic plants have had little time to establish, suspended algae can be particularly troublesome. It is important to be patient and to allow them to become established. Providing that the formula for creating a balance of plants and fish has been adhered to, the pond will eventually clear.

Whatever happens resist the temptation to empty the pond of water and refill with fresh. This will temporarily alleviate the problem, but within a few days the water will return to its green state. Fresh tap water is rich in mineral salts, and so to some extent the water will have been fertilized. Never change pond water just because it is green if the plant balance in the pond is correct. Just be patient. However, a flocculating product which will temporarily sink the algae out of suspension and allow light to pass through the water, thereby accelerating the growth of submerged plants, is very useful.

Filamentous algae can be very persistent and troublesome. In the most severe cases, where the algae forms large floating mats or colonies, the lower parts of the algal mass begins to decompose through lack of light and creates an unpleasant smelling brown or black mess. One of the most annoying aspects of blanketweed is that it often grows freely in a pond that otherwise has completely clear water. Filamentous algae rarely accompanies suspended algae in the same pond.

When looking at the overall control of algae by natural means, it is the submerged aquatics that provide much of the solution. Their main task is to mop up nutrients in the water and to release oxygen during the day to sustain fish and other aquatic life. By removing excess nutrients from the water, they deprive green water-discolouring algae of the opportunity of becoming established. When planting a garden pond allow one bunch of submerged plants for every square foot of water surface area.

Green algae-laden water can also be controlled by the amount of light that is permitted to fall into the pond. Algae, like most other aquatic plants, must have full uninterrupted sunlight in order to prosper. Planting trees and shrubs to shade the pond is ill-advised as this will prevent the desirable plants like waterlilies and marginal aquatics from growing properly and will have an adverse effect upon their floral display. The solution is to provide shade on the surface of the water by means of floating plants and waterlily pads. These should not cover the entire pond, for cutting out the light completely beneath the water will cause the submerged plants to perish. No more than one third of the surface area of open water should be covered with foliage if green suspended algae is to be controlled successfully and submerged plants permitted to grow freely.