Johnson Grass

Rapid response required – Johnson grass in central London

London is unique in regards to invasive non-native species as new and previously unseen species often establish first in our urban landscape. One such species, Johnson grass (Sorghum halepense), is already known as a highly invasive species in other locations throughout the world.

Johnson grass has become naturalised in areas of the United States of America. It currently costs the US millions of dollars annually due to its ability to contaminate crops and reduce overall yield of pastures. It is also know to produce toxic amounts of cyanide if growing under stressful conditions such as extreme cold and frosts, leading to loss of stock.

While this might not appear to be a concern for Great Britain there is still a responsibility to manage such as species due to its known track record in other countries. Treatment of this population was highlighted as necessary within London for several key reasons:

  • It is able to spread by both seed and rhizomes, meaning it is has a increased chance of spread
  • It will grow in a range of habitats, which means it has an increase possible chance of success when it spreads from the current location.
  • It is able to easily create monocultures meaning that it can cover an entire area and outcompete other species not allowing them to grow.
  • It is a major economic and environmental concern in agriculturally dependant countries as it is able to infest and contaminate grain and sorghum crops, with reports of up to 50% yield losses.
  • Management is made more difficult as it is able to quickly develop tolerance to herbicides and create new ecotypes in response to new habitat conditions.
  • Individual plants can become poisonous if stressed.
  • The species removal from London is a precautionary measure due to the economical and environmental damage it is known to cause throughout the world.

When the population was first recorded it only covered a distance of approximately 50 meters and consists of only around 60-70 individuals. This size of population meant that it was at a stage where it could be treated and removed quickly. Consent was received by the landowner and contractors have been visiting the site frequently since this time. Manual removal was found to be the most effective technique as both strimming and herbicide has been used previously by the land managers to little success. While manual removal is initially a more costly solution it meant that removal would be longer term and would not rely on the use of chemicals for success.

To date, several visits have been completed with a dramatic reduction in the population size recorded. The last visit in October 2014 found only several individuals remaining on site which were removed at that time. The site will be monitored for regeneration of any remaining seeds or vegative matter and slowly over time the frequency of monitoring visits to site will be reduced.

Overall the programme has been seen as a success as it has enabled the removal of a previously unrecorded invasive non-native species within London. Its shows that by being reported quickly and by responding quickly, species are able to be treated at a fraction of the price that they may cost if left to establish.