Where coffee grows, and why

The climate each species is reported to do best in

Sourced

These are optima quoted in one review from older authors, not limits and not recommendations. The review notes that how much rain a crop needs depends on the soil, humidity, cloud and how the farm is managed, and that selected arabica varieties under intensive management give satisfactory yields at averages of 24 to 25 °C in north-eastern Brazil.

What goes wrong at either end

Sourced

On the warm side, the review says that above about 23 °C fruit develops and ripens faster, often with a loss of quality, and that high temperature at blossoming, especially with a long dry season, may cause flowers to abort. On the cold side it says growth is largely depressed where the annual mean is below 17 to 18 °C, and that frost, even occasional frost, may strongly limit whether the crop pays.

Robusta, it adds, is much less adaptable to low temperatures than arabica. That is the plainest reason the two species are farmed in different places.

Altitude is how a tropical farm finds a cooler climate

CoffeeHQ explanation

Air cools with height. Near the equator, the temperatures in the table above are found well up a mountain; further from it, the same temperatures occur lower down, and frost becomes the risk at the top of the range instead of heat at the bottom. That is why an altitude that is ordinary in one country would be too cold for coffee in another, and why a height in metres cannot be compared between origins without their latitude.

There is evidence that altitude is associated with quality, and it should be read as that. A systematic review of 73 studies of coffee quality found eighteen that looked at altitude. Thirteen of those reported on how the coffee tasted, and twelve of the thirteen found better sensory scores at the higher sites, which the review calls one of its two most consistent results. One of the studies it describes compared Ethiopian coffee from about 1,950 to 2,010 metres with coffee from about 1,600 to 1,680, a difference in mean temperature of about three degrees, and attributed the result to slower ripening and a longer period of fruit filling. The same review reports that results varied between studies, that the evidence on the chemistry of the bean was mixed, and that the direction of temperature's effect on quality is not established: of nine studies of temperature, four found better sensory scores at higher temperatures and three found worse.

So the mechanism the evidence points to is temperature and the pace of ripening, not height itself. A high farm that is hot for other reasons, or a low one far from the equator that is cool, does not fit a rule written in metres. The atlas entries state an altitude only where a source for that origin gives one.

Rain: the total matters less than the calendar

Sourced

The dry spell is part of the requirement, not a shortfall. The review says a lack of a dry period can limit coffee growing in lowland tropical regions, and that rain all year round is often responsible for a scattered harvest and low yields. Where the year has one clear dry season, flowering is concentrated and so is the harvest; where it has two, or none, the picking is spread out. The harvest calendars in the atlas entries are the visible result.

Humidity and wind

Sourced

The two species differ here too. The review says robusta grows successfully in air close to saturation, while arabica needs a less humid atmosphere, which it compares to that of the Ethiopian highlands.

Wind is a stress in its own right. Where plantations are exposed, the review says, yields are usually lower: wind reduces leaf area, damages leaves and buds and makes developing flowers and fruit drop, and hot wind raises the amount of water the trees need. Where strong wind is frequent, it suggests windbreaks or shelter trees.

Light

Sourced

The second consistent result in the systematic review of quality studies, from nineteen studies of light and shade, was that more light exposure goes with lower sensory scores; it words the same finding elsewhere as shade improving them up to certain thresholds. Coffee grown in the open is not thereby worse, since management, variety and processing differ between the farms compared, and a separate review of shaded coffee reports a study in southern Colombia, at about 1,440 to 1,630 metres, in which shade lowered cup quality, probably because the shaded air there was cooler than the crop does best in. It is one reason shade is discussed as a matter of quality as well as of yield.

No altitude is given here, and why

CoffeeHQ explanation

It gives no altitude that is good for coffee, because none exists independent of latitude. It says nothing about soil chemistry, acidity or drainage: no source on coffee soils was read, beyond the review's remark that how much rain a crop needs depends on how well the soil holds water. And it does not describe any one country's climate; those are in the atlas entries.

What to do next

• The coffee plant — Why the dry spell matters: how flowering is triggered and what follows it.
• Coffee and climate change — What published models project for the land that suits arabica, and what they leave out.
• Shade, spacing and water on a coffee farm — How growers change the conditions a tree experiences without moving the farm.
• The world coffee atlas — Each country's climate and harvest as its own sources describe them.