08 August, 2013
Exotic plant species took up a lot of my time last week, particularly Japanese knotweed and rhododendron. I mentioned rhododendron at Mavisbank a couple of weeks ago. This time it was at Castle Campbell. Once a fashionably decorated stronghold of the powerful Campbell earls, the castle sits at the head of dramatic Dollar Glen.
A Site of Special Scientific Interest, the glen is owned and managed by the National Trust for Scotland. They are concerned about the spread of non-native rhododendron into the glen.
As I mentioned in an earlier blog, the problem with rhododendron is two-fold. It reduces the biodiversity of a site by forming blanket cover, which prevents anything else growing. And it is susceptible to the ‘sudden oak death’ pathogen, which can infect and kill other trees. We are therefore looking to remove these plants from Dollar Glen. We are sorry if this offends, but we have good reason to take this course of action.
Meanwhile at Dundonald Castle near Kilmarnock, a recent site visit revealed that Japanese knotweed was spreading into the property. There is no legal requirement to remove knotweed from private land, but it is an offence to allow it to spread into the wild.
Listed as one of the world’s 100 most invasive species, Japanese knotweed is extremely hardy and very bad for the environment. It can survive very low temperatures and its deep-growing roots are strong enough to damage stone and concrete. Like rhododendron it spreads aggressively, preventing other plants from growing.
Knotweed is remarkably difficult to get rid of. It reproduces from its rhizomes (creeping underground stems). You can dig them out, but unless you remove all traces of the rhizome the plant will re-grow from a small fragment. When the site for the London Olympics was being cleared it cost £70 million pounds to remove the knotweed. There are no natural enemies in this country, but experiments are being carried out to explore the feasiblity of introducing a natural pest from Japan.
For now, the best way to remove it is by using weedkiller, but even this will require many applications over many years. If your own garden should be stricken with an infestation, you would be best to enlist a professional gardening service, which will have access to extra-strong herbicides.