Support Plants for Temperate Climate Agroforestry - a few observations

I'm writing in mid-May, in those few days when — at last — the nights stop dropping below ten degrees and the garden lets out a long sigh of relief. Spring, around here, always arrives late and a little reluctantly, and by the time it does it already has one foot in summer. It feels like the right moment to take stock, to set down what the field has taught me — and today I'd like to do that on a subject that has been with me since my very first day here: support plants. Support plants are the species we include in an agroforestry system not for what we harvest from them, but for what they do for the system itself — producing biomass, shading the soil, suppressing weeds, hosting pollinators and predators, cycling nutrients, opening up compacted ground, stimulating ecological succession via strategic disturbance. They are the invisible infrastructure on which the productive species depend, and in a well-designed system they often outnumber them by a wide margin.

What follows are observations gathered over three years of trials (and errors), with no claim to completeness or authority. They are starting points more than conclusions, and I'd be glad if they opened up a conversation: on this subject I still have a great deal to learn.

Climate considerations

When I arrived, my eyes were fixed on the summer. I knew it would be hot — I expected the usual 35°C and above — and instead, in August, I found myself facing peaks of 40-45°C. That first year, not a drop of rain fell for two months straight.

In time, though, I came to understand that the real difficulty in our climate — typical of Mediterranean areas far from the sea — is not the summer but the winter. Here the thermometer reaches down to -9°C: only at night, only for a few days a year, but it does reach it. The springs are cold and long, and the nights stay reliably above 10°C only from mid-May onwards. And, above all, it rains a great deal in winter: around 700 mm between November and March, with very wet springs on top of that.

It's a climate with two faces, and that is exactly why it can't be easily copied. Much of the research on syntropic agroforestry comes from the tropics, where the temperature is always mild, rainfall follows a polarised but predictable rhythm, it never freezes, and it never reaches this degree of heat and drought. "Pure" Mediterranean climates — the coastal, maritime ones we usually picture — share our summers but have far milder winters, with little or no frost and often less rain. Taking a system designed elsewhere and dropping it here as it is, quite simply, does not work.

Which layer to start from

As anyone who's come to one of my courses knows — because I do bang on about it — there's one distinction that, in my experience, serves as a compass when designing support plant systems.

In Mediterranean climates it makes sense to focus the design on the species of the emergent layer. Without the shade of the upper storeys, little survives in the layers below; moisture is lost too, and without moisture, decomposition and humification grind to a halt, sometimes for very long stretches.

In colder climates, including temperate ones, the key is instead the low layer. It is this layer that smothers weeds, that keeps the soil covered — reducing erosion and compaction through the winter — that loosens the soil and shields it from the summer sun. And it does all this while producing plenty of biomass without stealing light from the productive layers — light that is already scarce in winter.

Our climate, however, asks for both at once. And this is where the challenge gets serious: a good support plant has to thrive with no inputs at all — that is exactly what we expect of it — but here it has to do so at -9°C and at +45°C, through two months of drought and under the winter rains, across cold springs and hot autumns. It's a different challenge, and probably a harder one, than the tropical or the Mediterranean.

In this article I'll focus on the low layer, the one I've come to know best year after year. The emergent layer deserves a discussion of its own, and I'll save it for another time.

Two very narrow windows

The first thing the field taught me is that, in this climate, many plants have surprisingly short windows in which to grow.

Species such as comfrey (Symphytum officinale), perennial kale (Brassica oleracea, ramosa group), chicory (Cichorium intybus), chard (Beta vulgaris var. cicla) and rhubarb (Rheum × hybridum) are at their best in autumn, but they restart very slowly in spring and suffer badly in the summer heat. They have just two windows in which to grow and build biomass — one in autumn, one in spring — and both are narrow.

At the opposite end, plants such as aloe (Aloe vera), New Zealand spinach (Tetragonia tetragonioides) and lemongrass (Cymbopogon citratus) would not withstand our cold, wet winters at all. Vetiver (Chrysopogon zizanioides, formerly Vetiveria zizanioides) is a slightly different case — it can survive the cold when dormant — but its useful growing season is so confined to the summer that, in practice, it belongs to the same group.

Finding species able to work right through the year — or at least to cover the gaps left by the others — has become the heart of my search.

Two paths

To find them, I followed two paths at once. On the one hand, I trialled dozens of species that, on paper, looked suited to our conditions. On the other, I began to watch the spontaneous vegetation closely, in and around the field.

Observing the local weedscape

When I consult for a new project, the first thing I do is read the landscape through its wild parts — the abandoned corners, the neglected verges, the marginal strips where nothing has been actively planted. It is the habit I brought to our own field too, the moment I arrived.

The weeds that dominate our field are, for the most part, plants I would never use as support species, because they are anything but well-mannered: couch grass (Elymus repens), bindweed (Convolvulus arvensis) and creeping thistle (Cirsium arvense). A few hundred metres away, however, I noticed a couple that were growing well and had real potential. Some later made their own way into the field: I observed them for a couple of years and, in the third, established them in the system for good. They are creeping cinquefoil (Potentilla reptans) and lemon balm (Melissa officinalis).

Watching, too, I had noticed in that first year how well the alliums (Allium spp.) grew in our vegetable beds. That is where the idea came from of trying chives (Allium schoenoprasum) as a low-layer support plant.

There's a lesson in all this worth more than any list of species: the wild plants are the best consultants we have. They tell us, free of charge and without ever being wrong, what that soil and that climate are truly willing to grow.

Introducing and trialling new species

On the side of the species introduced for trial, the selection took some years, but it produced interesting results.

The aromatic herbs — the great mainstays of Mediterranean systems — gave uneven results. Only sage (Salvia officinalis) really performed well. Lavender (Lavandula angustifolia) struggled; thyme (Thymus vulgaris) and rosemary (Salvia rosmarinus, formerly Rosmarinus officinalis) grow reasonably well, but they produce so little biomass that I chose not to introduce them on a large scale.

Among the many failures, a few plants proved their worth instead: horseradish (Armoracia rusticana), outstanding from early to late autumn; artichoke (Cynara cardunculus var. scolymus); Jerusalem artichoke (Helianthus tuberosus); and yarrow (Achillea millefolium).

The tireless workers of the low layer

Brought together — cinquefoil, lemon balm, chives, sage, horseradish, artichoke, Jerusalem artichoke and yarrow — and joined by field bean (Vicia faba var. minor), chard and chicory in autumn and spring, and by rhubarb in the shade of the trees, these species have become the tireless workers of the low layer in our system.

It is not a definitive list — in farming, no list ever is — but it is a solid starting point, the fruit of patient observation rather than theory.

And you — in your own context, what has worked well? I'd love to read about your experiences: it is by bringing many different fields together that, little by little, we learn to read our own.