AUTUMN LEAVES IT LATE

We had to wait for that blaze of glory this year – but now it’s come, why not try your hand at leaf mould, says Monty Don

As far as I can remember, autumn has never arrived this late. October passed with the leaves staying on the trees – and staying mostly green – right until the last week. I am sure we will increasingly get used to November, not the Octobers of my childhood, being the month of golden and fiery displays from our trees, shrubs and hedges.

Most leaves appear to turn yellow in autumn, but in fact this is just a revelation of their natural base colour, which is overlain by green pigment produced as a result of chlorophyll. When the chlorophyll fades in autumn, the yellow is then revealed. But to go from yellow to orange or red depends upon the weather as well as the ability of some trees – such as maples – to create sugars.

The greater the difference in temperature between day and night in late summer and early autumn, the more extreme the leaf colouration will be. Leaves convert starch to sugar but cold nights stop it moving that sugar from the leaf to the roots. It is this accumulation of sugar in the leaves that results in red pigmentation. Our milder, wetter summers, with less difference between day and night-time temperatures, reduce and delay this process.

The leaves fall when cells break down in the abscission layer between leaf stalk and twig. A corky scar forms over the wound that this causes, protecting the tree from infection. Some trees cannot form this scar tissue so they do not drop their dead leaves until the new ones are ready to push them off the following spring, which is why beech and hornbeam keep their russet leaves all winter. Once fallen, whatever their colour, all deciduous leaves are ready to be converted into leaf mould. I love leaf mould. It makes an ideal component in potting compost, is ideal for mulching all woodland plants and when added to any soil it helps improve its structure.

Unlike good compost, which needs turning regularly to spur the bacterial digestion which is the predominant action in its conversion from raw material to the final product, leaf mould is almost entirely made by fungal activity and does not need heat or oxygen for this to happen perfectly efficiently. The only really important factor in converting a fallen leaf to leaf mould within one season is moisture, as dry leaves take much longer to break down.

My own technique is to brush or rake the leaves into long rows and then mow them. Mowing leaves into small pieces makes them rot down faster and take far less storage space.

After being mown, they go into a large chicken-wire container and are kept wet. The latter is hardly a problem in this wet part of the world but in a dry year it means thoroughly hosing them layer by layer. I then do nothing at all and we have perfect leaf mould by the following October. We then empty the bay so it is ready for this year’s leaf harvest and put the leaf mould into reused compost bags ready for use throughout the coming year as part of our potting compost. It makes an excellent peat replacement as it is slightly acidic so is good for rhododendrons, camellias, sarracenias, blueberries and other ericaceous plants as well as being ideal for bulbs like lilies.

I appreciate that most gardens are not large enough to have a permanent large wire bay for leaf mould. In this case the answer is to put the leaves in a black bin bag, leaving the top turned but not tied. Make sure the leaves are really wet and punch a few holes in the side of the bag to drain excess water. They will rot down very well and can be stored behind a shed or tucked away in any corner to quietly convert from leaf to mould.

Monty with the fiery autumnal leaves of Rhododendron luteum