How do I deal with blanketweed?

What the RHS says about blanketweed in a UK pond, why a UV clarifier often misses it, and how to remove it without emptying the pond.

Blanketweed is the hair-like green algae that sits on the liner, the plants and the surface. The RHS also calls it silkweed. The water between the strands can be perfectly clear, which is why a UV clarifier bought for "green water" disappoints people.

Why the UV misses it

A clarifier only treats water that flows past the lamp. Blanketweed is attached. It does not queue up to go through the hose. The RHS groups it with the algae you remove physically, and says a filter or UV is for algae and debris in the water. Use the UV for pea soup. Pull blanketweed out.

The RHS also says no home-garden weedkiller or chemical is approved for pond algae and blanket weed. Do not tip bleach, washing-up liquid or a "pond clear" product in because a video said so.

How the RHS says to remove it

Lift it with a net or a rake, or twirl it onto a rough cane or stick.
Do not take the whole mass in one ruthless afternoon if the pond is full of creatures. The wildlife guide says most animals prefer the planted maze, and that removing too much at once is the mistake.
Leave what you have pulled out on the bank so larger animals can crawl back. The summer care notes say overnight. The June notes say 48 hours before it goes on the compost. Rinsing the weed in a bucket of pond water helps the smaller ones. Then compost it, away from another pond.
Untangle dragonfly larvae and water beetles as you go. They look like bits of stick and leaf.

Slowing the next crop

Blanketweed likes sun and nutrients, the same diet as green water.

Shade a third to a half of the surface. The RHS June notes suggest waterlilies for that.
The construction guide suggests 60–75 cm of depth where you can, so the water warms more slowly. A very shallow pond in full sun is a blanketweed bed.
Feed fish less. Fish waste is fertiliser.
Top up with rainwater, not a hose, if the level has dropped. See topping up.
Do not put goldfish into a wildlife pond to "sort the weed out". The RHS says they make algae worse.

A new pond can grow a beard of blanketweed in its first summer and then settle as the plants take over. Pulling it out, and waiting, is a legitimate plan. If the pond is large enough that you cannot reach the middle safely, the RHS mentions hiring a contractor for aquatic weed control rather than wading in.

Will barley straw work?

You will see it recommended everywhere. The RHS page we checked does not give it as a method, and it does say that products vary and that no approved home weedkiller exists for this job. If you use a commercial preparation, you are following that product's label, not this site. Pulling the weed out is the step the RHS actually describes.

Can I leave it?

A thin growth in a wildlife pond is habitat. A mat that covers the whole surface blocks light and can crash the oxygen when it dies back. Lift the excess. You do not need a bare liner.

The filter is full of green slime. Is that blanketweed?

Slime on the foams is caught algae and debris, which is what a mechanical filter is for. Rinse the foams in pond water so you do not kill the bacteria with tap water. Strands you can twirl off a cane are blanketweed in the pond itself.