Duckweed can become a real problem if you have a pond in your garden. This small, cress-like weed can ‘carpet’ the surface of the water and choke off oxygen to other aquatic life, so it’s important to stop it taking hold. Here’s all you need to know.
What exactly is duckweed?
Duckweed is an aquatic perennial in the form of small, sound leaves with a shirt root underneath, which float on the surface of water and multiply until they cover it. They thrive in nutrient-rich waters, which makes them a particular problem for garden ponds – especially those that are left relatively untouched.
It can get into your pond on visiting water birds, or attached to plants, and if left unchecked can completely cover areas of slow moving or still water, causing it to stagnate. It can double in size every two or three days during summer if conditions are right.
What can I do about duckweed?
It’s impossible to totally banish duckweed, but it’s important to try to control it as much as possible before it becomes a serious problem.
For smaller ponds, regular raking or fishing out of the weed using a net is essential - any weed removed can be composted. A water feature or fountain to keep the surface of the pond moving should help inhibit duckweed, too.
It’s also important to prevent leaf build-up at the bottom of your pond, as duckweed will feed on the mulch. Netting placed across the pond during autumn and winter can help with this; otherwise try to remove any leaves in the pond with a net as often as possible.
Keeping part of your pond in shade will also help slow duckweed growth, so try adding plants on its south side. Waterlilies and other plants with floating leaves can also help prevent its spread.
Koi, grass carp and goldfish will all feed on duckweed, so consider having fish in your pond if you don’t already.
Can I use weedkiller on duckweed?
Aquatic weedkillers are not domestically available in this country, but there are some non-chemical treatments that can be used in a garden pond. These contain a bacterial culture that removes nutrients from the water to discourage the future growth of duckweed.