Useful mulch is five to eight centimetres deep and stands clear of the trunk. A volcano mound around the collar is a mistake. Judge wood chip, straw and gravel against Iran's wind and heat.
Mulch is one of the cheapest jobs there is for holding moisture in the soil, evening out the surface temperature and reducing weeds at the foot of the tree. That same job, done wrongly, rots the collar, shelters rodents and suffocates the roots. In Iran, where summer evaporation is high and irrigation is often drip or irregular flooding, the temptation of the more mulch the better is common. The usual result is a mound of wood or manure around the trunk — what is called volcano mulching. On orchard and landscape visits, Farin Kesht Taban first digs the collar out from under that mound; many cases of yellowing and collar fungus started with that mound, not with a nutrient shortage.
Five to eight centimetres deep, and the trunk must be visible
The effective layer for most Iranian orchards and urban trees is between five and eight centimetres. Thinner than five centimetres, and the wind of Isfahan and Yazd moves it quickly and leaves the soil bare. Thicker than ten centimetres, especially if it stays permanently wet, reduces oxygen to the surface roots and brings saprophytic fungi forward. Spread the mulch as a doughnut: leave about ten to fifteen centimetres of clear space around the collar so the trunk bark stays dry and inspectable. Cover the surface roots and the canopy shade rather than building a tight ring around the trunk. If the lawn comes right up to the tree, first pull the lawn back from the canopy shade; lawn competition and bad mulch do not cancel each other out.
Why volcano mulching is wrong
A mound of mulch pressed against the trunk keeps the bark damp and soft. In that state the collar tissue cracks or rots, and the entry route for pathogens stays open. Rodents nest under the mound and in winter girdle the bark. On top of that, adventitious roots grow inside the mulch itself, and the tree becomes dependent on an unstable layer. When the mound settles or rots away, those roots die and the tree is suddenly under stress. The correction is simple: remove the mound until the natural flare of the collar is visible, keep the wounds dry, and spread the mulch again with a gap. Spraying the collar does not take the place of that correction.
- Keep a clear gap of ten to fifteen centimetres from the trunk.
- Hold the depth at five to eight centimetres and each year top up only what has settled.
- Never press fresh manure against the trunk as mulch.
- Water under the mulch and onto the soil, not onto a mound of wood that sheds the water away from the collar.
Wood chip, straw and gravel in Iranian conditions
Hardwood chip, or a horticultural mix, provided it carries no chemical colouring and no diseased wood, is the best general mulch: it decays slowly, suppresses weeds and gradually adds organic matter. Avoid large quantities of fresh walnut wood on sensitive saplings, and do not bring painted joinery timber into the orchard. Straw is cheap and available and has a use in vegetable gardens and nurseries, but the wind carries it away, it brings weed seed and sometimes stored-product pests with it, and in the hot humid south it may attract termites; keep the layer thinner and renew it each season. Gravel and pebble give a clean look in low-water urban beds and around evergreens in hot dry areas and reduce evaporation, but under Iran's sun the soil beneath heats up, it adds no organic matter, and spread over compacted clay it makes infiltration worse. Use gravel over improved soil and in a limited small ring, not as a substitute for an organic matter programme in a fruit orchard.
Good mulch is like clothing: it does not cover the trunk, it covers the soil.
Design drip irrigation and mulch together
If a dripper disappears under a thick mass of wood, you notice a blockage too late, and salt gathers at the wet edge under the mulch without your seeing it. Spread the mulch so that the run of the line and the position of the drippers stay inspectable. Under flood irrigation, the mulch must not channel the flow towards the trunk. In urban landscaping with foot traffic, settle a light mulch with a little water in the first weeks so the wind does not carry it into the gutter. Do the annual renewal in autumn or late winter; in summer, a very fresh, thick layer produces uneven temperatures and sometimes surface fungus.
- Find the collar and remove any mound pressed against the trunk.
- Pull weeds and lawn back from the canopy shade.
- Spread suitable chip five to eight centimetres deep, clear of the trunk.
- Place the drippers outside the blind layer and measure the depth again after the first irrigation.
Very fresh chip, full of white sapwood, temporarily ties up nitrogen at the soil surface as it decomposes. On a weak sapling, put down a thinner layer or use slightly aged chip. In pistachio and pomegranate orchards where the irrigation is saline, organic mulch reduces evaporation and slows the rise of salt to the surface; gravel alone does not do that. Every spring, measure the depth with a ruler rather than by eye. If the layer is down to three centimetres, top up only that; do not pour another eight centimetres over last year's mulch, or by the third year you have built a hidden mound. Healthy fallen leaves from the same orchard, if there is no branch disease, are themselves a light autumn mulch and should not all be burned.
Correct mulch in an Iranian orchard does three things: it holds water in the soil, it protects the roots from the burning surface sun, and it reduces weeds without harming the collar. Wrong mulch reverses all three. Five to eight centimetres, a gap from the trunk, and a choice of material based on wind, termites, salinity and irrigation method — that is the whole instruction. If you have an old tree or a newly planted sapling, ask Farkesht to look at the collar and the soil texture before you buy a lorry-load of wood; sometimes the old mound has to come off first and the new mulch second. A tree does not need a layer of wood against its bark — it needs cool, moist soil in the shade of its canopy.
