Leucaena

What is leucaena?

Leucaena is a tree of extremes. Few plants fix as much nitrogen, few resprout so willingly after cutting, few produce so much fodder. And few have spread so far beyond the places where they were planted. Both sides belong to its portrait, and anyone who works with it should know both.

Leucaena leucocephala is a legume from Mexico and Central America. In the twentieth century it was planted throughout the tropics as a fodder tree, for firewood, as shade over coffee and cocoa and in hedgerows for alley cropping. Its fine, feathery leaves feed cattle and soil alike. Among nitrogen-fixing plants it is one of the most productive, and among biomass plants one of the most vigorous.

Origin and growth

The species has three subspecies, from a heavily branched shrub to a tree with a clear trunk. It reaches 3 to 15 metres, rarely 20, and roots deeply. The leaves are twice pinnate with many small leaflets that fold up in heat, cold or drought. The white flowers stand in round heads; the flat, brown pods, about 10 to 20 centimetres long, hold many hard seeds. Leucaena is self-fertile and, where moisture allows, flowers and fruits all year round.

Requirements

Leucaena is a tropical tree. It grows best at mean annual temperatures of 25 to 30 °C and copes with 650 to 3000 mm of rain and dry seasons of six to seven months. It is sensitive to cold: light frost makes it drop its leaves, heavy frost kills everything above ground, though the trees often resprout the following summer. It does best on calcareous, well-drained soils up to pH 8 and fails on acid, waterlogged soils or soils poor in phosphorus and calcium. It wants full sun.

Role in the soil: nitrogen, biomass and mulch

Under suitable conditions its abundant root nodules fix a great deal of nitrogen; reviews give up to a few hundred kilograms per hectare and year, field measurements are often lower. The condition is the right partner. Leucaena is choosy about its rhizobia: in Australia, where it is widely grown for fodder, native soil bacteria are considered unlikely to fix effectively with it, and inoculation with a specific strain is standard. A glasshouse study found no strain that worked equally well with all leucaena lines.

As a biomass plant it is outstanding. A cut stump sprouts many new shoots, and where water is not limiting it produces large amounts of leafy fodder. Its leaves have one particular property: they break down very quickly. They release their nitrogen in a rapid, short pulse, which is why leucaena was one of the first species used for green manure in alley cropping. As mulch for covering soil and suppressing weeds, however, they are of little value; that requires coarser material or a slower litter, such as that of Inga.

In the food forest

In a food forest leucaena occupies the shrub and tree layer, kept at the height the planting needs. In syntropic agriculture it is a classic support tree: planted densely, cut back several times a year, its prunings laid around the fruit trees. As a nurse tree it gives young, sun-sensitive trees light shade; as a pioneer it roots into compacted subsoil and opens it for water. Its place in the succession lies in the early and middle years: it prepares the ground and is thinned out as the long-lived trees take over.

In the Mediterranean climate

Leucaena has arrived in southern Europe. In Sicily two varieties grow that researchers from Palermo examined as a fodder resource for Mediterranean agroforestry, with nutrient-rich, low-mimosine foliage. A review in Animals recommends it for the marginal inland areas of the Mediterranean countries. In the Canary Islands, by contrast, it has gone feral on several islands; Spain lists it there in its national catalogue of invasive species, and on Tenerife control work took place in 2017 and has continued since 2019.

This shows what matters in practice. Leucaena is invasive where it is warm, frost-free and open all year. In regions with frost and long summer drought its potential is considered lower, but not zero. It is not on the EU list. Anyone who plants it should cut it before the pods ripen, remove seedlings and keep it away from natural habitats.

A note on fodder: the leaves contain mimosine, which is harmful to horses, pigs and rabbits. Ruminants tolerate it when their rumen bacteria can break it down; the database recommends no more than a fifth of the ration.

Propagation and care

Leucaena is easily raised from seed; the hard seed coat is softened with hot water before sowing. Seed stays viable for many years, which is exactly why ripe pods should not remain on the tree. Pruning in the rainy season, from autumn to spring, keeps the trees low and supplies mulch. In summer every bit of shade is welcome.

How Terra Humana understands it

Terra Humana uses vigorous, non-EU-listed species such as leucaena deliberately, because their strength can set a tired soil in motion. That is not a justification but a stance, and it comes with an obligation: leucaena at Sa Medada is meant to be guided, cut back before it sets much seed and replaced step by step once the trees it serves have grown. Whether it fixes nitrogen without the bacterial partner it usually needs is one of the questions the land will answer.