Skip to content
Terra Humana Foundation Logo
Terra Humana
ENDEIT
Soil and life

Cover Crop

Terra Humana editorial teamLast updated: 2026-10-126 min read

An old olive grove on a slope in spring, the ground between the trees fully covered with grasses, clover and wildflowers

In brief

A permanent or seasonal plant cover between trees or rows instead of open soil; it slows runoff and erosion and feeds soil life and beneficial insects.

What is a cover crop?

A cover crop, in the sense used here, is a plant cover kept between trees or rows instead of open soil. In orchards and olive groves it replaces the old practice of keeping the ground bare by repeated tilling or herbicides. The cover can be sown, typically with cereals such as barley, with wild grasses, or with legumes such as clovers, medics and vetch. Or it can be the spontaneous local vegetation that is simply mown instead of destroyed. In Italian the practice is called inerbimento, and it is widely promoted in Mediterranean olive growing and viticulture.

In a Mediterranean climate a cover follows the rhythm of the rain. It germinates with the first autumn rains, grows through winter and spring, and is cut, grazed or rolled before summer. The residue stays on the ground, so the cover becomes mulch for the dry months. In this sense a cover crop is mulch that grows itself.

Mulch that grows itselfLearning panel: a cover crop is mulch that grows itself, a plant cover between trees instead of bare ground. In a Mediterranean climate it follows the rhythm of the rain: it germinates with the first autumn rains, grows through winter and spring, is cut, grazed or rolled before summer, and the cut material stays on the ground as mulch through the summer. In a three-year trial in an olive orchard, tilled soil lost 46.7 tonnes per hectare and year, soil under a sown grass cover 6.5 tonnes, and soil under a sown mix of ten species 7.9 tonnes. The more diverse covers carried more plant species and more predatory arthropods, the natural enemies of orchard pests. The water question: a cover left growing too long draws on the moisture the trees need for flowering and fruit set. The answer is to manage it: cut it before the soil dries out; the year decides the moment, not the calendar. It is cut when water demands it, not to look tidy. TERRAWIKI · LEARNING PANEL Mulch that grows itself A plant cover between trees instead of bare ground. 1FOLLOWING THE RHYTHM OF THE RAIN Autumn germinates withthe first rains Winter, spring grows whilerain falls Before summer cut, grazedor rolled Summer the cut staysas mulch 2SOIL LOSS IN AN OLIVE ORCHARD tonnes per hectare and year, three-year trialtilled 46.7grass cover 6.5mix of ten species 7.9 The more diverse covers carried more plant speciesand more predatory arthropods, the naturalenemies of orchard pests. 3THE WATER QUESTION Left too long draws on the moisturethe trees need forflowering andfruit set Managed cut before the soildries out; the yeardecides the moment,not the calendar It is cut when water demands it,not to look tidy. Basis: Gómez et al., Environ. Sci. Pollut. Res. 25 (2018) · TerraWiki: Cover Crop

What the research shows

The strongest argument for a cover is erosion. Bare, tilled soil on a slope is exposed to every heavy rain, and in the Mediterranean the heaviest rains often fall in autumn, on soil that has just come out of a dry summer. In a three-year trial in an olive orchard, published in 2018 by José Alfonso Gómez and colleagues, tilled soil lost 46.7 tonnes of soil per hectare and year. Under a sown grass cover the loss fell to 6.5 tonnes, under a sown mix of ten species to 7.9 tonnes. The more diverse covers also carried more plant species and more predatory arthropods, the natural enemies of orchard pests.

A cover also feeds the soil. A meta-analysis of Mediterranean olive orchards, vineyards and almond orchards by Vicente-Vicente and colleagues found that soils under plant cover consistently held more organic carbon than tilled soils. The gain was slower than with organic amendments such as compost or olive pomace, which is a reminder that a cover is the foundation and not the whole building. Roots, cut residue and undisturbed soil life work together; humus grows where all three are present over years.

The question of water

In a dry climate every plant between the trees is also a competitor for water. A cover that keeps growing into late spring draws on the same soil moisture the trees need for flowering and fruit set, and in a dry year that can cost yield. This is the main reason growers hesitate.

The answer is not to give up the cover, but to manage it. The cover grows while rain is plentiful and is cut in spring before the soil begins to dry out. When that moment comes is decided by the year, not by the calendar: after a dry winter it comes earlier. Strips directly under the tree crowns can be kept short or mulched, while the cover grows in the open alleys. Annual species that complete their cycle by early summer, such as subterranean clover, have the advantage that they withdraw on their own and leave a straw layer behind.

How Terra Humana understands it

For Terra Humana a cover crop is the basic measure wherever trees stand on a slope. It is the cheapest protection against erosion, it needs no imported material, and it turns the winter rain into soil life instead of runoff. Where we can, we start with the vegetation that is already there and add sown species where the soil is bare. Legumes in the mix are nitrogen-fixing plants and bring nitrogen into the system, and on terraces a cover holds the soil on the steps.

What we do not copy is the logic of a lawn. A cover between trees is allowed to flower and set seed, because that is where beneficial insects find food and where next year's cover comes from. It is cut when water demands it, not to look tidy. Where animals are available, grazing can take the place of mowing; timing matters just as much then, so that the soil is not trampled and the cover is not eaten down to the roots.

The valley of Sa Medada near Jerzu, Sardinia, with olive grove and macchia

Sa Medada

What this looks like at Sa Medada

Sa Medada has old terraces and an old olive grove, on a site with a dry season from May to September. Project Eden begins with pioneer plants, ground cover and holding water. Between the olives and on the terrace steps, a living cover is the plainest way to let winter rain soak in where it falls instead of running down the slope.

More about Project Eden

Frequently asked

What is a cover crop in an orchard?

A cover crop in an orchard is vegetation that is deliberately kept between the trees instead of leaving the soil bare by tilling or spraying. It can be sown, typically with grasses such as barley or with legumes such as clovers, medics or vetch, or it can be the spontaneous local flora, simply mown instead of removed. In Mediterranean orchards the cover grows with the winter rains and is cut or grazed in spring. Its cut residue then stays on the ground through the summer as mulch.

How much erosion does a cover crop prevent?

A great deal, according to field trials in olive orchards. In a three-year trial published in 2018 by José Alfonso Gómez and colleagues, conventionally tilled soil lost 46.7 tonnes per hectare and year, while a sown grass cover reduced the loss to 6.5 tonnes and a sown mix of ten species to 7.9 tonnes. The covers with more species also carried more plant diversity and more predatory arthropods that feed on pests. Results always depend on slope, soil and rainfall, but the order of magnitude is consistent across studies.

Does a cover crop compete with trees for water?

Yes, and in a dry climate that is the central question. A cover that keeps growing into late spring draws on the same soil water the trees need for flowering and fruit set. The answer is timing: the cover grows while rain is plentiful and is mown, grazed or rolled in spring before the soil starts to dry. In a dry year it is cut earlier. Managed this way, the cover protects the soil in the rainy season and becomes a protective straw layer in the dry one.

Should the cover be sown or left to grow spontaneously?

Both work. Spontaneous vegetation is free, adapted to the site and often rich in species, but on exhausted soil it may be sparse or dominated by a few plants. Sown covers establish more reliably and can be steered, for instance with legumes that add nitrogen. Field research suggests that diverse covers control erosion about as well as single-species ones while supporting more life. A practical path is to sow where the soil is bare and let the local flora take over where it already grows.

How is a cover crop different from green manure?

The two overlap but answer different questions. A cover crop is about protection: keeping the ground covered permanently or every year between trees, mainly against erosion and heat. A green manure is about feeding the soil: a crop sown specifically for its biomass and nutrients and cut at the right moment. A cover between olives can include green manure species, and a green manure, once cut, protects the soil like any other mulch.

Large comfrey with purple flowers at the foot of a young fruit treeNext in this themeDynamic AccumulatorA deep-rooted plant that draws minerals up from the subsoil and returns them to the surface through its leaves, feeding neighbouring plants.

Related terms

A lifted layer of straw and wood-chip mulch with moist dark soil underneath, next to a dry, cracked pathMulchA layer of organic or mineral material on the soil surface that cuts evaporation, keeps the ground cooler, stops erosion and feeds soil life.Young fruit trees, each in a circle of straw mulch, with flowering white clover and medics in betweenLiving MulchA low, living plant cover that grows alongside the main crop and protects the soil; in the Mediterranean it greens in winter and dries to straw in summer.A flowering green manure of faba beans, vetch and oats between young fruit treesGreen ManurePlants grown on purpose to feed the soil: often a mix of legumes and grasses, cut at flowering and left on the ground as biomass and nitrogen.Hands holding dark, crumbly soil with fine rootsHumusThe dark, stable fraction of organic matter in soil, the part that holds water and nutrients and gives a living soil its colour, smell and structure.Terraced slope with olive trees on every levelTerracingCutting a slope into level steps so that water soaks in instead of running off, and soil stays on the hillside instead of leaving it.Flowering fava beans between old olive trees in springNitrogen-Fixing PlantsPlants that host bacteria or actinobacteria able to convert atmospheric nitrogen into a form plants can use, fertilising the soil around them.

Mentioned in

See this in practice

Project Eden →

Sources

  • Gómez et al., Soil erosion control, plant diversity, and arthropod communities under heterogeneous cover crops in an olive orchard, Environmental Science and Pollution Research 25 (2018)
  • Vicente-Vicente et al., Soil carbon sequestration rates under Mediterranean woody crops using recommended management practices: a meta-analysis, Agriculture, Ecosystems & Environment 235 (2016)
  • Andy Clark (ed.), Managing Cover Crops Profitably, 3rd edition, SARE (2007)
  • FAO, Conservation Agriculture: principles of permanent soil cover

Last updated: 2026-10-12