Blanket weed, a cane and a bale of barley straw

14 June 2025 · 3 min read

The pond went green in late April, a thick pea-soup green that hid the bottom completely, and then in the second half of May the soup cleared and was replaced by blanket weed: long bright green filaments in ropes and mats, wrapped round the oxygenating weed, floating up in the sun and sinking at night. By the first week of June I could have lifted a bucketful. New ponds do this, I had read, because the water is full of nutrients and the plants that would use them are not yet established. Knowing that did not make it look any better.

Three things I did. First, the cane: a bamboo cane pushed into a mat of weed and twirled like spaghetti on a fork lifts out an alarming amount, which I leave on the edge overnight for anything in it to crawl back and then compost. Ten minutes every two or three days keeps the surface clear, though it does nothing about the cause. Second, a small bale of barley straw, about the size of two fists, in a net bag weighted with a stone and floated near the surface. The theory is that it releases something as it rots that discourages new algae. It went in on the 20th of May and I cannot say for certain it has done anything, but the regrowth after each twirl does seem slower in June than it was in May. It is cheap and harmless and I will leave it until it sinks.

Third, and I think most important, I stopped topping up from the tap. The pond loses about 2 centimetres a week to evaporation in warm weather and I had been replacing it with the hose, which here is hard water with plenty of nitrate in it. Since mid-May the top-ups come from the water butt only, and if the butt is empty the level is allowed to drop. The marginal plants have also taken off, the iris and the water mint especially, and they are presumably eating the same nutrients the weed wants.

The water is now clear to the bottom for the first time since April. The blanket weed is still there in the shallows but thin and manageable. The tadpoles, I notice, graze on it, and so I am leaving a patch for them.