This article has been checked for horticultural accuracy by Oliver Parsons.
What is blanket weed?
Blanket weed is a common type of algae found in garden ponds, especially in spring and early summer. Masses of fine, hair-like, bright green strands form cotton-wool-like clumps in the water. Blanket weed is most vigorous in the early part of the growing season when rising temperatures, increasing day length, and a high level of available nutrients all boost growth. Later in the year, aquatic plants growing in and around the pond tend to out-perform blanket weed, so it usually becomes less of an issue. During winter, blanket weed usually disappears from view as growth slows and stops.
Is blanket weed a problem?
While small amounts of blanket weed are not a cause for concern, particularly early in the year, an abundance of weed may cause problems by inhibiting plant growth, and affecting fish health by reducing oxygen levels in the water. Large amounts of blanket weed make the pond look unsightly. However, some wildlife such as newts and toads use blanket weed to lay their eggs, and it provides cover for their young. Young tadpoles eat blanket weed and other algae, so it provides plenty of food for developing larvae.
Blanket weed is not bad for dogs. It's sometimes confused with blue-green algae or cyanobacteria, which forms toxic ‘blooms’ and can be fatal if ingested. Blue-green algae appears as green, blue-green, or green-brown foam, scum, or clumps, in lakes, ponds, reservoirs, and waterways.
What causes blanket weed?
Blanket weed thrives in water that is high in nutrients, which may come from one or more sources such as tap water, garden fertiliser or manures, fish waste or food, and organic debris such as leaves.
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How to prevent blanket weed
- Tap water is rich in minerals, so avoid using this to fill or top up a pond where possible and use collected rainwater instead
- Nutrients from manures and fertilisers can be washed in to a pond by heavy rain. Ideally design and build a pond to avoid this, and do not apply these close to the pond
- Ponds with fish are more likely to have blanket weed problems as fish faeces add nutrients, as does surplus fish food. Ponds without fish are most wildlife-friendly and easiest to care for. To keep fish with the minimum of problems, ensure the pond is not over-stocked – choose fish types that have the least impact, and take care not to over-feed them
- Organic matter falling into the water such as leaves and plant debris will rot down and release nutrients. Regularly removing the leaves can help prevent a build-up of them
- When planting pond plants, use aquatic planting soil or sieved garden topsoil from ground that hasn’t been manured or top dressed. By growing more plants there will be fewer available nutrients to the algae
Using plants to prevent blanket weed
Creating a natural balance of planting will reduce and prevent blanket weed, because there will be fewer nutrients for the algae to grow. This ‘holistic’ approach is the best treatment for blanket weed in terms of long-term resilience and easy maintenance. Aquatic and marginal plants shade the water’s surface, preventing sunlight entering the water and boosting blanket weed growth, while the plant’s roots take up nutrients from the pond and effectively ‘starve out’ the algae. Aim to cover at least a third of the pond’s surface with floating foliage of water lilies and other deep water aquatics. Submerged oxygenating plants are also efficient at taking up nutrients. Be aware that in spring, blanket weed and other algae are likely to proliferate for a short period of time before plants start growing strongly. Bear in mind that this is natural and a source of food for tadpoles.
Some types of pond snail, notably ramshorn snails, eat blanket weed, algae, and other debris, helping to keep the water clean. Snails usually appear naturally over time or buy from a reputable supplier.
Treating blanket weed in ponds
A wide range of products is available to treat blanket weed but most only tackle the symptom rather than the root cause and will need repeated applications. Natural, environmentally friendly products are the safest choice for wildlife and fish.
While there are some anecdotal reports of using salt to combat blanket weed, this is highly likely to cause damage to pond plants, fish, and wildlife, and is not recommended.
An electric filter and/or an ultraviolet clarifier may be effective and these can be installed if there is an electrical supply suitable for outdoor use.
Products to treat blanket weed
Some products are specifically targeted at blanket weed, others indirectly tackle the problem by reducing nutrients in the water or by blocking sunlight. Timing of repeat applications varies according to the product.
Barley straw – available in bags or pads, barley straw is a traditional method for tackling blanket weed and algae. As it breaks down it releases a chemical that inhibits algae growth
Pond Balance – reduces nutrients and alters the water chemistry to tackle blanket weed growth.
Pond Clean – reduces nitrogen and ammonia, which algae feeds on.
Pond Clear – reduces nutrient levels and speeds digestion of organic debris.
Pond dye – dyes the water dark and therefore reduces the amount of sunlight entering the pond water, which inhibits algal growth.
Should I remove blanket weed?
During spring, blanket weed is best left undisturbed for wildlife to breed and eat. Later in the season, if blanket weed is still present in large quantities, remove it by twirling round a cane, using a net, or a plastic-tined rake. Pile weed by the side of the pond for a couple of days so creatures can make their way back to the water.