Stacking in space and time

What stacking is for

A field of one crop uses one level of space for one season. Everything above and below that level, and everything before and after that season, stays empty or is taken by weeds. Stacking is the art of filling these gaps on purpose. Several plants share the same ground because each uses a different part of it: a different height, a different depth of soil, a different time of year, a different stage in the life of the system.

Bill Mollison named two forms of it, plant stacking and time stacking, and counted both among the small-scale intensive systems that let a small, well-tended area yield much. The model is the forest: light is caught by the crowns, filtered through the understorey and used once more near the ground, while roots reach water at different depths.

Stacking should not be confused with stacking functions, the rule that each element should do several jobs at once, which element analysis explores.

Stacking in space: the layers

The usual model of a temperate forest garden goes back to Robert Hart, who turned his orchard in Shropshire into a food forest in the 1980s. He described seven layers:

• Canopy: the tallest fruit and nut trees.
• Low trees: smaller fruit and nut trees, often on dwarfing rootstocks.
• Shrubs: berries and other fruiting bushes.
• Herbaceous layer: perennial vegetables and herbs.
• Ground cover: creeping plants that protect the soil, a form of living mulch.
• Root layer: tubers and roots below the surface.
• Vertical layer: climbers that use trees and supports.

Hart preferred shade-tolerant perennial vegetables to sun-loving annuals, since the latter fit poorly under trees. Martin Crawford, who developed the approach further in Devon, advises filling the layers gradually over several years rather than all at once.

Ernst Götsch orders his plantings differently. In descriptions of syntropic agriculture, the strata are defined less by height than by how much light a species needs at maturity, from emergent trees that want full sun to low plants that thrive in partial shade.

Stacking also happens below ground. Deep-rooted trees, shallow-rooted herbs and tubers draw water and nutrients from different horizons, and nitrogen-fixing plants feed their neighbours through roots and fallen leaves.

Stacking in time: from season to succession

Within a year, time stacking means sowing the next crop before the last one ends. Deep Green Permaculture gives the example of sowing broad beans in the cool season while tomatoes fade, so that the bed is never left bare.

Over years, time stacking becomes ecological succession guided by the gardener. Fast pioneer plants and nurse trees give shade, biomass and shelter early on, while slow, long-lived trees grow up among them. The pioneers are cut back step by step and their mass goes to the soil as mulch. As the page on food forests describes, the share of helpers is high at the start and the productive trees carry the stand at the end; the mass planted early is not wasted but consumed into shade and soil.

The longest form of time stacking is measured in decades. On the website for Holmgren's principles, Rowan Reid of the Bambra Agroforestry Farm in Australia describes trees he planted in 1987 on a patch of alluvial soil: poplars, to be harvested for veneer once they reach 55 centimetres in diameter, and black walnuts that are to keep growing for another forty years after that. One planting, two harvests, a generation apart.

How to plan a stacked planting

1. Start with the long-lived core. Choose the canopy and low trees first and space them by their size at maturity, not by their size in the pot.
2. Draw a timeline as well as a plan. For each species note when it yields, how long it lives and when it will be shaded out or cut back. A simple table with years along the top is enough.
3. Fill the gaps with temporary layers. Pioneers, ground cover, annual crops and short-lived shrubs occupy the space the trees will claim later.
4. Match each layer to its light. Sun-loving plants on the sunny edges and in young plantings, shade-tolerant ones under established crowns. Sectors and microclimate tell where the light falls.
5. Plan the cuts. Decide in advance which plants will be pruned or removed, and when, to make room for the next stage.

An example from the world: the Chagga home gardens

On the southern slopes of Mount Kilimanjaro in Tanzania, the Chagga have tended home gardens called kihamba for generations. FAO recognised the Shimbwe Juu Kihamba Agro-forestry Heritage Site as a Globally Important Agricultural Heritage System in 2011; the designated site covers 615 hectares. According to FAO, the gardens have four main layers: widely spaced trees on top that give shade, medicine, fodder, fruit, firewood and timber, and fix nitrogen; several varieties of banana beneath them; coffee below the bananas; and vegetables of many kinds, including climbers, at the lowest level. The system holds over 500 used plants, about 400 of them not cultivated but kept in place. Traditional canals bring water from the mountain forest. The kihamba is stacking at its most mature: not a design from a book, but a garden refined over a very long time.

Common mistakes

• More layers than water. In a dry climate, the number of layers is limited by water, not by ideas. Each plant drinks, and a crowded planting can suffer in a long summer.
• Trees too close together. Spacing by nursery size leads to a canopy that closes years too early and shades out everything below.
• Annual vegetables in deep shade. Most annual crops need full sun. Under trees, shade-tolerant perennials do better.
• Pioneers left too long. A pioneer that is not cut back on time becomes a competitor rather than a helper.

Further in the TerraWiki

• Food forest, plant guild and polyculture are stacking in practice.
• Syntropic agriculture and ecological succession explain stacking in time.
• Pioneer plants, nurse trees and nitrogen-fixing plants are the first floor of a stacked system; living mulch and cover crops are its ground.
• Element analysis, sectors and microclimate help decide which plant goes into which layer; the design process puts it all in order.
• Behind the technique: Mollison's design principles and use and value diversity.

Stacking at Sa Medada

Sa Medada is a slope in Jerzu, in Ogliastra on the east side of Sardinia, with mild winters and a dry season from about May to September. The growing areas of Project Eden are conceived as food forests: fruit and nut trees, supporting shrubs, herbs and ground cover brought together into layered communities, drawing on permaculture and syntropic succession. Bananas, mango, avocado, guava, cherimoya, papaya and citrus, among others, already grow in the open beside the old olive grove. Nitrogen fixers such as Tephrosia, the coral tree, Inga edulis and Leucaena are in the ground without inoculation.

Time stacking at Sa Medada follows the climate. As the page on syntropic agriculture explains, pruning happens when rainfall exceeds evaporation, which here means winter and spring, and in summer every bit of shade stays, because there it protects more than any layer of mulch. Project Eden begins with pioneer plants, ground cover and holding water: the first layers that make the later ones possible.