How Hampstead Heath is controlling the rapid spread of knotweed

Azolla Weevil

Azolla Weevil - Credit: Archant

City Corporation of London’s conservation team explain why they are taking biological solutions to control north London’s most rampant greenery.

For centuries, British flora and fauna have been subject to additional species joining their ranks, either as deliberate introductions or more increasingly by accident. The Victorians were particularly keen on introductions of plants to adorn formal gardens and grander landscapes.

Two of most problematic offenders found on the Heath today bearing testament to this legacy are Japanese knotweed and Himalayan balsam. Japanese knotweed was introduced to the UK in 1825 and has spread for decades, much to the detriment of existing ecosystems. A more novel and environmental approach other than using herbicides to control these plants is the use of biological agents. A helpful insect which eats the plant’s leaves has recently been introduced. Known as a psyllid, its full scientific name is Aphalara itadori and it was released in various secret locations four years ago. The trial is so far proving successful, with the insect surviving British winters, breeding and eating its fill of knotweed. It remains to be seen if and when it gets to the Heath.

Himalayan balsam is a pretty but problematic annual related to the Busy Lizzy summer bedding plant. Like Japanese knotweed, it successfully competes with native plant species for space, light, nutrients and pollinators, and excludes other plant growth, thereby reducing native biodiversity. It also creates bare earth areas after dying back at the end of the year, which can cause erosion. This plant has also been subjected to a new biological control: a rust fungus. Found in the plant?s natural home of Asia following extensive searches for a suitable pathogen, it was new to science when first discovered, and has been assessed for its safety in a quarantined environment. As with Japanese knotweed only time will tell how effective this will be.

Occasionally in late summer and autumn ponds can turn an unusual bright red colour because of an introduced aquatic weed called South American water fern or Azolla filiculoides. which was introduced by the aquarist trade and can smother water bodies and exclude light, causing the death of submerged plants and a reduction of oxygen in the water. The plant can also fragment and spread if disturbed, making physical control problematic. One solution is the use of a tiny North American weevil called Stenopelmus rufinasus. Originally from the Americas, it is now commercially bred for release onto water bodies with this problem. The weevil has been present in the UK for as long as the fern, so is now deemed safe to release into the environment, and it’s reassuring to have it in our arsenal should we need it.