Some people, as we know, talk through their hats. Plants, by contrast, talk through their leaves, which can act as a mouthpiece by indicating the condition of the entire organism. It repays every gardener to learn their language – as leaves of all kinds are now cascading to the ground in great quantities, often having changed colour quite remarkably before doing so.
This colour change is brought about in two ways. First, by cessation of the production of green chlorophyll, revealing colours that it previously masked. And second, by the production of new pigments, for reasons that are not altogether clear, although it may have something to do with the deposition of waste substances.
Some leaves are coloured all the time: the smoke bush Cotinus coggygria “Royal Purple” springs to mind, because I can see one catching the autumn sun as I write. Its red pigments are constantly present, masking the chlorophyll. Other leaves lack a green colour in parts: this is the commonest cause of the so-called variegated effect, much beloved of some gardeners. Pale yellow or white variegation arises when chlorophyll is absent from some of the leaf cells. As chlorophyll is essential for the production of nutrients by photosynthesis, and nutrients are equated with vigour, variegated plants are generally feebler than their wholly green counterparts. This is why, when a fully green shoot arises on a variegated plant, it should be cut out before it takes over the entire organism.
[See also: As Shakespeare reminds us, the goodness of manures and composts is often misunderstood]
Leaf size may also be related to vigour, and when leaves appear consistently smaller than normal, this is a sign of stress or nutrient shortage. It is, however, in relation to shortages of specific nutrients that leaves really are at their most vocal and most clever: leaf symptoms are highly accurate indicators of deficiencies. Some require experience to spot, but the most common and clear-cut example is a shortage of iron. As iron is an essential part of the manufacture of chlorophyll, iron-deficient plants may show some paleness (or “chlorosis”, to give it its proper name), the leaf vein cells staying green while all those around become yellowed. Fortunately, this is the nutrient deficiency most readily remedied – simply apply iron in an organic form called “sequestered iron”.
I am always fascinated by the way one can often tell – simply by looking at the leaves – the type of environment a plant requires (and, therefore, where it should be placed in the garden). The highly indented leaf of a woodland fern, for example, provides a large surface area for light capture in a poorly lit environment, while its smooth surface and thinness, which would encourage water loss, are indicative of a moist atmosphere in which such loss would present no problem. If this kind of plant is placed in direct sunlight, the fragile leaf tissues will almost certainly be scorched and the plant suffer as its photosynthetic abilities are impaired.
Conversely, the reason so many alpine plants are difficult to grow in gardens is that they have densely hairy leaves, adapted to minimise water loss in the drying mountain-top conditions of high sunlight and strong winds. Those same hairy leaves in a lowland garden will trap moisture and encourage fungal decay.
Yes, leaves do speak very fluently, if only you can learn to understand their language.
Next week: Nina Caplan on drink
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This article appears in the 14 Oct 2020 issue of the New Statesman, Can Joe Biden save America?