Gardening guru PETER CUNDALL says you can find out virtually everything you need to know about the health of a plant simply by keeping a close eye on its foliage. Learning to read the signs can help prolong — and sometimes even save — the life of a plant.
You can learn plenty about a plant’s health by keeping a close eye on its foliage.
It pays to wander around the garden to study the foliage of those plants that have a vulnerability to common disorders or nutritional deficiencies.
Plants cannot talk, but they communicate through their leaves. Once we learn this visual language it enables us to respond to a range of distress signals by taking action to rectify problems.
Leaves are astonishing, living organs that do amazing things. They are highly versatile solar panels, able to convert the sun’s energy so plants can draw up nutrients and moisture from the soil.
Leaves also collect and direct rainwater to where it’s needed, while providing valuable shade around plants to reduce wasteful evaporation from the soil.
And for most plant lovers, leaves are the means by which plants display the extent of their health, and warn us when things start to go wrong.
When leaves flourish and show good, strong colours and firm textures, they are telling us how healthy they are and assuring us we are properly looking after the plants.
On the other hand, when things get out of balance, leaves become vital, early warning systems so we can provide remedies. Good gardening is based on understanding these foliage distress signals.
For example, when the soil around most soft-stemmed plants such as tomatoes starts to become too dry or excessively saturated, they immediately begin to droop. If we respond quickly by either applying water to the roots or digging drainage channels in time, most plants rapidly pick up again.
Other plants with thick, resinous or leathery leaves such as conifers, rhododendrons and camellias cannot provide the same quick indication of dry or wet soil.
Instead of wilting and drooping, these tough leaves gradually die from the tips or margins. Should these initial signals be ignored or misunderstood, it can become too late to do anything.
Oddly enough, waterlogged or bonedry soil produces almost the same kind of marginal leaf scorch.
Good gardening is based on understanding these foliage distress signals.
When acid-loving plants such as rhododendrons, azaleas, camellias, ericas, daphne and many Australian plants are mistakenly limed, or have highly alkaline stove ash dumped around them, adverse effects quickly show up.
This time, leaves describe their discomfort by losing colour. The youngest leaves at the tips of new growth begin to look very pale, while older leaves remain green.
Sometimes veins stand out bright green, while areas in between turn yellowish-white.
The trouble is, these so-called acidlovers are not efficient at taking up iron, a mineral that becomes locked in the soil by alkaline conditions. This is why pale new foliage is often described as “lime-induced iron deficiency’’.
While we can “sweeten’’ acidic soils by applying lime, it is difficult to make alkaline soils acidic.
So we can apply a special kind of chelated iron for acid-loving plants, which bypasses alkalinity to supply the iron needed. The mustard-coloured powder is cheap to buy and can be applied at any time of the year.
Other plants, especially leafy shrubs, citrus trees and vegetables such as brassicas and silverbeet, have a special need for nitrogen. If they don’t get enough they don’t grow because leaves remain small, pale and cannot efficiently harness the sun’s energy.
This is why the soil in which leaf vegetables are to be grown should receive large amounts of any kind of well-rotted poultry manure mixed with blood and bone deeply dug in before planting or sowing.
Citrus trees commonly display signs of nitrogen deficiency when leaves lose the natural, healthy green colour and gradually turn pale.
Such trees tend to remain stunted, drop leaves and often carry excessive blossom and lots of tiny, useless fruit that falls to the ground while still small.
Nitrogen deficiency is common in free-draining sandy soils or where citrus plants have become root-bound in containers. Foliar feeding the leaves and the soil around these suffering plants produces an almost instant cure.
I spray over a heavily diluted highnitrogen liquid fertiliser such as fish emulsion. It is wonderful to see citrus leaves dramatically return to a healthy colour within a couple of weeks.