Bohemian Knotweed in the Tame Catchment

Over the summer of 2023, a riverside survey of invasive species of plant was commissioned by Greater Manchester Combined Authority as part of Natural Course.

The survey is a detailed snapshot of the invasive non-native species (INNS) issues on the River Tame catchment and how issues may arise downstream on the River Mersey and beyond. The Tame is a major tributary of the Mersey in Stockport and any invasive issues recorded on the Tame, will indirectly impact the greater Mersey catchment.

One interesting result from the survey was the discovery of Bohemian Knotweed (a hybrid invasive species).

We caught up with Mike Beard, Natural Course Project Officer to tell us more about the significance of this find.

What is Bohemian Knotweed?

Bohemian Knotweed (Fallopia x bohemica) is an increasingly common hybrid between the invasive plants Japanese Knotweed (Fallopia japonica var japonica) and Giant Knotweed (Fallopia sachalinensis).  It has the potential to spread even faster than its already troublesome parent species.

Bohemian Knotweed has features that are intermediate between the two – the leaves are larger than Japanese Knotweed, but not as large as Giant Knotweed.  Giant Knotweed and Bohemian Knotweed leaves have a lobed edge either side of the stem, whereas Japanese Knotweed has a straight edge.  Japanese Knotweed lacks the threads underneath the leaves, while Bohemian Knotweed has tiny spikes that are thicker at the base.

Where did the survey find it?

The survey team searched 104 km of river within the Tame Catchment in Greater Manchester (assisted by additional data and local knowledge provided by volunteers from the Mersey Rivers Trust).  They found eleven instances of Bohemian Knotweed at three locations:

  • The River Tame at Reddish Vale in Tameside and Stockport
  • The River Tame at Jet Amber Fields in Tameside
  • Diggle Brook in Oldham

What are we doing about it?

Compared to other Knotweeds found in the UK, Bohemian Knotweed grows in a wider range of habitats, grows and spreads more quickly, and is more difficult to control.  Also, it can produce fertile seed with Giant Knotweed, or another Bohemian Knotweed, and can also successfully pollenate the already widespread Japanese Knotweed.

The important new information that our survey provides will be invaluable when devising the best strategy for the use of the resources available to tackle invasive plants in the River Tame Catchment.  Now that we are aware of the new threat posed by Bohemian Knotweed, we can raise it up in the list of priorities.  It has yet to gain a firm foothold on our rivers, so hopefully control measures can be put in place before it becomes too difficult to remove.

Can I read the survey results?

Yes! You can read the full report here

Bohemian Knotweed leaf with a mobile phone for size context

Bohemian knotweed

Main photo credit – Groundwork Greater Manchester