One of the solutions that has been proposed from research in continental Europe is sweeping up leaf litter from under the tree in the winter. The moth pupae overwinter in the leaf litter, so disposing of it should reduce the number of moths emerging in the spring to attack the trees.
From time to time we've had queries about leaf litter under horse chestnut trees, specifically whether it is a problem and, if so, what should be done about it.
Clearing leaf litter away
It was for this reason that we invited people to record the vegetation under their trees in 2011-2013 when recording the amount of damage that the moths had done. The final analysis is close to completion, but a look at the preliminary results suggests that the presence of mown lawn under horse chestnut trees (a measure that leaf litter would not have been present over the winter) is related to a reduced amount of damage recorded for any particular location and time.
In other words - the data from this project strongly suggests that clearing leaf litter is worth it to reduce damage to your tree!
What to do with the cleared leaves
So, if clearing leaf litter is worth it, then what should be done about it? Firstly, it is worth noting that general leaf litter is a fantastic ecological product that allows the recycling of nutrients and provides shelter and food sources for a wide range of animals, plants, fungi and all other sorts of microscopic creatures. But, the horse-chestnut leaf-miner overwinters in the leaf litter and emerges in the spring to attack the leaves of horse chestnut trees.
Nigel Straw and colleagues at Forest Research produced a report showing that to effectively kill the moth pupae it is neccesary either to:
- Cover the leaves in a layer of soil to stop the adult moths emerging. To quote from the Forest Research website, you will need to "cover the leaves with a 10cm layer of soil or 15-30cm layer of other plant material, and leave the heaps undisturbed until the end of May".
- Compost the leaves at a high temperature. (The easiest way is via a commercial composting scheme, e.g. through your local council.)
Once the leaf litter is well rotted (so at least a year old, once all the moths have died) then it will be safe to use as normal compost.