Blanket weed (Spirogyra adnate) is one of the most prolific aquatic weeds known to pond-keepers in the U.K. Blanket weed is an algae, which is very successful at colonising an environment under favourable conditions such as sunlight. If left unchecked this algae can cause serious oxygen depletion in ponds and possible stress to larger fish that find it an obstruction. To control the proliferation of blanket weed, you need to reduce one of their 3 key requirements; sunlight, nutrients, and a warm temperatures. Koi fish keepers will want their ponds to be as warm as possible to stimulate the health and growth of their fish, therefore look at reducing sunlight and dissolved nutrients. Sunlight penetration can be reduced by providing shade over your pond to reduce blanket weed photosynthesis. A unique, long-term control of blanket weed is to filter out the sun’s rays by adding vegetable dyes to the pond water. The best advise offered to pond keepers is to aim to adopt the strategy of prevention is better than cure. If you continue to treat blanketweed (which is a symptom of an unbalanced ecosystem in the pond) then it will always return, once the treatment has worn off. Instead, try to determine the factors that are the problem in your own pond. If you think it is high levels of nutrients, test for nitrates and phosphates. Algae will readily absorb nitrates and phosphates to satisfy their need for nitrogen and phosphorous as they grow. These are readily available in tap water or indirectly through fish metabolism. If sunlight is a factor, reduce this by ensuring there is shade cast over your pond in the warm summer months. Please refer to previous blog post on adding aquatic plants to your pond to bring a balance, thereby reducing blanket weed from inhabiting your pond.