On 02/05/2021 11:03, Martin Brown wrote:
The only common UK plant I can think of that seriously causes problems
is walnut which really does have a potent anti competition compound in
its roots called juglone.

Garlic mustard is reportedly a problem in the US because of its chemical
warfare against other plants (or rather against symbiotic fungi);
presumably European ecosystem have adapted to cope. (In the US garlic
mustard can take over forest floors; here, in my experience, it is a
plant of woodland edges, including hedgerows.)

--
SRH