Microclimate

What a microclimate is

A weather station measures the air at head height above short, open grass. Plants live somewhere else: at the foot of a wall, in a hollow, on a slope that faces the sun, under the crown of an old tree. Each of these places has its own climate, warmer or colder, wetter or drier, calmer or windier than the figures for the region. That small climate is the microclimate. The German climatologist Rudolf Geiger described the layer of air near the ground in 1927 in a book that is still a standard work, and his central finding holds for every garden: the closer to the ground, the greater the differences over short distances.

For a designer this is good news. The regional climate cannot be changed, but microclimates can be found, used and to some degree made. A sheltered, sun-facing corner can carry plants that would fail in the open a few metres away. On the scale of permanence, climate stands at the top as the least changeable element; microclimate is the part of it that the designer can work with.

What shapes a microclimate

• Slope and aspect. A slope facing the sun receives more energy than flat ground, and much more than a slope facing away. In the northern hemisphere south and west slopes are the warm ones, in the southern hemisphere north and west.
• Cold air. On calm, clear nights the ground radiates heat away, and the air just above it cools and becomes heavier. On any slope it flows downhill and gathers in hollows, valley floors and behind barriers across the slope: frost pockets. A band in the middle of a slope, above the pooled cold air, often stays milder.
• Stones and walls. Rock and masonry store the day's heat and release it at night. A sun-facing wall can keep frost off a fruit tree and help it ripen. The ground beneath a larger stone stays cool and moist, which is why the page on rock mulch recommends stones as a cover in dry places.
• Water. Water warms and cools slowly. Ponds and larger water bodies soften night frosts on their banks, and their surface reflects light onto nearby plants. Sepp Holzer uses ponds on his mountain farm partly for this reason.
• Wind and shelter. Wind dries leaves and soil and cools exposed plants. According to the USDA National Agroforestry Center, a windbreak slows the wind most strongly at two to five times its height on the downwind side, and noticeably to about ten times. Porous plantings work better than solid walls, which push the wind over the top and create gusts behind them.
• Plants themselves. Every crown casts shade, catches dew and keeps the ground beneath it cooler and moister. In dry climates this is how young trees establish under nurse trees and shrubs, the effect described on the page on ecological facilitation.

How to read and use microclimates, step by step

1. Walk the land at the extremes. Go out on the coldest morning of the year and on the hottest afternoon. Where does frost or dew lie longest, where does the ground steam first, where is the air still and where does it pull?
2. Watch the signs. Where does snow melt first, where do the first flowers open, where does the grass dry out earliest in summer? Plants that come up on their own, the indicator plants, tell the same story.
3. Measure a little. A few cheap minimum and maximum thermometers, set at the same height in different places, show within a single season how far the corners of a site differ.
4. Draw the findings on a map. Warm spots, frost pockets, wind gaps and moist channels belong on an overlay of the base map, next to the sectors of sun and wind.
5. Place, then shape. Put tender plants in the warm, sheltered spots and hardy ones in the exposed places. Only then shape: a hedge against the drying wind, a gap to drain cold air, a stone heap or a pond where heat or moisture is needed. The herb spiral shows on a few square metres how much a small change of form achieves.

An example from the world: La Geria on Lanzarote

On the Canary Island of Lanzarote, the eruptions of the eighteenth century buried farmland under a thick layer of volcanic ash. In the area of La Geria, winegrowers responded with a landscape of microclimates. According to the Bodegas El Grifo winery, they dig hollows roughly two to four metres wide into the ash and plant a vine at the bottom of each. A low semicircular stone wall, the zoco, guards each hollow against the constant trade wind. The ash layer holds moisture and protects the roots from heat. Together they give each vine a climate in which it can grow on an island with very little rain. The page on rock mulch describes the ash cover of the Canaries, the enarenado, in more detail.

Common mistakes

• Trusting a distant weather station. A station in a town higher up a valley, or on an open plain, can describe a site quite wrongly. Local measurements and observation come first.
• Building a frost dam. A solid hedge or wall across a slope stops the cold air and makes a frost pocket above it. Leave gaps.
• Forgetting the summer. In Mediterranean climates the problem is often heat and drought, not cold. Warm stones around young plants on a hot slope can add stress rather than relieve it; shade and ground cover matter more there.
• Planting tender trees in the lowest spot. Hollows collect water, which tempts. They also collect cold air.

Further in the TerraWiki

• Herb spiral: many microclimates packed into a few square metres.
• Rock mulch and mulch: soil covers that change the climate of the soil surface.
• Nurse trees and pioneer plants: plants that create shelter for those that follow.
• Use edges and value the marginal: edges between sun and shade, wet and dry, multiply microclimates.
• Sectors, zones and the design process: where microclimates enter a plan.
• Observe and interact: the principle behind reading them.

Microclimate at Sa Medada

Sa Medada is a slope in Jerzu, in Ogliastra on the east side of Sardinia. It is frost-free, with mild winters and a dry season from roughly May to September. Climate figures from the town of Jerzu, which lies higher up in the mountains, are no guide to it; this is the first lesson of microclimate in practice.

Within the land, too, conditions differ. The first tree, a service tree, was planted in 2024 on the very hot southern flank. The LiDAR wetness map shows a comparatively dry main area and moister channels along the edges, and macchia grows on the steeper slopes. Banana, mango, avocado, cherimoya, papaya, moringa and citrus are being trialled in the microclimate, and how they fare shows what the small climates of the slope can carry.