When the Rain Forgets to Fall

As I write this, the Dordogne is baking.

The grass has abandoned any pretence of being green, the stone walls are radiating heat like storage heaters, and our normally enthusiastic collection of chickens has retreated into the shade to reconsider its life choices. Even Duggy, who generally considers charging about at full speed to be appropriate in all weather conditions, has slowed to something approaching a dignified walk.

Temperatures have been nudging towards 40°C (and over it for a while), and there has been precious little meaningful rain. A thunderstorm may make an appearance, but summer thunderstorms are unreliable creatures: spectacular, noisy and sometimes capable of depositing most of their rain on the neighbouring field.

For those of us growing truffle trees, all this raises an obvious question:

What does drought mean for the truffles?

The reassuring answer is that a spell of hot, dry weather does not automatically mean that there will be no truffles. Truffle seasons are shaped by the weather over many months, by the soil beneath each tree and by the remarkable ability of both trees and fungi to adapt to changing conditions.

Nevertheless, summer water matters, so let us have a look at what may be happening beneath our feet.

A rather demanding underground guest

The Périgord black truffle, Tuber melanosporum, spends its entire life beneath the soil. It lives in partnership with the roots of its host tree—in our case, predominantly holm oaks—and forms an extraordinary network of fine fungal threads called mycelium.

The tree provides the fungus with sugars produced through photosynthesis. In return, the fungus helps the tree obtain water and minerals from the soil. It is a remarkably successful partnership, and one which has evolved to cope with the naturally hot, dry summers of the Mediterranean region.

The tiny beginnings of the truffle fruiting bodies are thought to develop from late spring onwards. They then remain underground throughout the summer and autumn, gradually growing and maturing before the winter harvest.

This means that June, July, August and September are not quiet months in the truffière, however peaceful it may appear above ground. Beneath the surface, the year’s potential truffles are attempting to establish themselves and grow.

For that, they need water.

Not a swamp. Truffles dislike heavy, waterlogged soil and require plenty of air around them. But they do need enough moisture to prevent the young fruiting bodies from drying out before they have had the chance to become the wonderfully aromatic black treasures we are all hoping for.

What the science tells us

Truffle growers have always talked about the importance of summer storms. Science now gives considerable support to this traditional wisdom.

Research using truffle-production records from Spain has found that soil moisture during the summer months—particularly from June to September—is closely connected with the following winter’s harvest. Moisture during October and November is also important, as the truffles enter their final stages of growth and ripening.

Studies of Mediterranean truffle harvests have similarly found that very hot summers and low rainfall can reduce production.

The difficult French summer of 2022 provided a particularly dramatic example. Following severe and repeated droughts in 2021 and 2022, some truffle-growing regions reported substantial reductions in the following harvest.

However, it is important not to turn this into a simple rule.

A dry July does not automatically mean a poor winter harvest. Soil depth, shade, localised storms, the age of the trees, the type of soil and rainfall later in the season can all make an enormous difference.

Two neighbouring truffières can receive almost the same weather and produce very different results. Even within one field, a truffle beneath a shaded tree on deeper soil may experience quite different conditions from one growing only a short distance away.

Rainfall totals alone do not tell the whole story either. A sudden deluge onto hard, sun-baked ground may rush away down a slope without penetrating deeply. What matters most is the moisture that remains available within the soil around the roots.

Truffles, it seems, are every bit as particular underground as they are on the dinner table.

Fortunately, we planted tough trees

There is considerable reassurance standing in our fields.

Most of our adopted truffle trees are holm oaks, Quercus ilex. These evergreen Mediterranean oaks are not merely able to tolerate hot, dry summers; they have evolved to live with them.

Their tough, leathery leaves help to reduce water loss, while their roots explore the soil in search of whatever moisture remains available. Compared with many other oak species, holm oaks are particularly well equipped to cope with drought.

They can slow their growth during very dry periods, conserve water and then respond again when conditions improve. As the trees mature, their expanding root systems can also reach a greater volume of soil, making established trees generally more resilient than very young ones.

This does not mean that either the trees or their truffles are entirely drought-proof. A holm oak may survive conditions that are still difficult for a developing truffle. The tree’s first priority, quite reasonably, is keeping itself alive.

Nevertheless, the choice of holm oak gives our truffière a valuable degree of resilience. They are exactly the sort of trees one would choose for a landscape where long, warm summers are normal and occasional droughts are part of life.

Our limestone soil is another advantage. It drains freely, allowing oxygen to remain around the roots, but its cracks, stones and deeper pockets of earth can also preserve moisture after the surface has dried.

Above ground, the soil may appear dusty and lifeless. A few centimetres below, conditions can be surprisingly different.

So, should we be worried?

Concerned and observant, certainly. Despairing, no.

Truffle growing has never been an entirely predictable business. A hot and dry beginning to summer may be followed by thunderstorms in August, steady rain in September or a mild, damp autumn. Each of these can alter the prospects for developing truffles.

One difficult season does not damage the long-term value of an adopted tree either. The adoption is part of a much longer story: the growth of the oak, the development of its roots and the gradual establishment of the fungal colony beneath it.

Every year the tree becomes larger, stronger and better able to explore the soil. Every growing season adds another chapter, whether the eventual harvest is generous, modest or determined to remain mysterious.

For now, the truffière looks quiet. The holm oaks stand calmly in the heat, their grey-green leaves giving very little away. Somewhere beneath them, fungal threads are exchanging water and minerals for sugars, roots are searching through cool pockets of limestone soil and—with luck—a few tiny truffles are stubbornly growing.

We will continue caring for the trees, observing the soil and watching the skies.

And while we would certainly welcome some rain, the truffière has not given up simply because summer has arrived.

Holm oaks are patient trees.

Truffles are patient fungi.

And fortunately, truffle growers learn to be patient too.

Although a gentle night of rain would still be very much appreciated.

Preferably tonight.

And preferably for rather longer than seven minutes.