No gardener should ignore the rich harvest of fallen leaves just now. Research is showing that after rotting down, the resulting leaf-mould not only improves soil structure but also contains nutrients.
Despite its immeasurable value to gardeners, you can’t buy leaf-mould, you need to make your own.
The best approach is to construct two square boxes next to each other, with either wooden or wire netting sides. Simply collect fallen leaves by using a rotary mower, leaf blower or tine rake. Pile the leaves into one box and leave to rot down over 18 months.
Use the second box for the following year. Leaves must be kept moist, so leave open to the rain. When harvesting, you’ll find a top, dry layer; scrape it into the other box and dig out the good stuff.
You can also use large plastic bags. Use a fork to pierce holes in the side and bottom of bags and again leave the top open.
The resulting well rotted leaf-mould undoubtedly improves the ground’s quality, acts as a brilliant mulch on beds and containers and, critically, contains very few weed seeds.
But there is much more nutritional value than has previously been realised. Deciduous trees admittedly do reabsorb nutrients from leaves in autumn. The reducing light levels we’re seeing just now trigger the process.
Trees start by removing up to 95% of the green pigment, chlorophyll, leaving the red, yellow and orange pigments. This incidentally explains why we’re enjoying such a wonderful autumn display.
Trees then remove other nutrients, nitrogen, phosphorus and potassium, safely storing them in their trunks and, to a lesser extent, in twigs and roots.
Around 10-30% of calcium and magnesium is removed. While this process is ending, a line of small corky cells grows at the edge of leaves, the abscission zone, and the leaves then fall.
The amount of nutrient left in leaves varies largely according to species and the ground’s fertility. A tree doesn’t need to expend energy in re-absorption when the soil is fertile and can easily supply most of its needs in spring. But some always remains, even in poor ground.
As we know only too well in Scotland, autumn gales often strip trees of leaves prematurely, so although we miss their glorious colours, they will be more nutritious.
Leaves are even more valuable for gardeners than has often been thought.
Plant of the week
The leaves of Viburnum opulus, Guelder rose, turn to pink and claret in autumn making a striking display. An added bonus are the translucent red berries, if the blackbirds haven’t eaten them first. These berries are very acidic and mildly toxic to humans so are better left for the birds.
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