Skip to content
CoffeeHQ

Science

The coffee plant: roots, flowering and fruit

How a coffee tree roots, what sets off its flowering, how it is pollinated and the stages a cherry passes through between blossom and harvest.

In short

A coffee cherry takes many months to make. The tree flowers after rain breaks a dry spell, the fruit sits almost unchanged for weeks, swells quickly, fills with dry matter while it is still green, and only then colours. Most of what a grower can and cannot control about a harvest is decided somewhere along that sequence.

This page follows the plant from root to ripe fruit. It leans on one published review of coffee physiology, and gives that review's figures as its figures: nearly all of them come from field studies in Brazil and Kenya.

A tree from under other trees

Sourced

The review describes the natural habitat of coffee species as the understorey of African tropical forest: forms of robusta in equatorial lowland forest from Guinea to Uganda, and wild arabica confined to highland forest in south-western Ethiopia. That origin explains a good deal of how the plant behaves on a farm, including its tolerance of shade and its shallow roots.

How high wild arabica grows is given differently by different authors. The physiology review cites 1,600 to 2,800 metres; a study of wild arabica led from Kew, citing other work, gives 950 to 1,950 metres. The two are not measuring quite the same thing: the first is given for the highland forests of south-western Ethiopia, the second for a native range that also takes in outlying populations in South Sudan and northern Kenya, and it adds that 1,200 metres is the most frequent lower limit. Neither figure describes where coffee is farmed.

Roots that stay near the surface

Sourced

According to the studies the review gathers, the main part of an arabica tree's root system sits in roughly the top 30 centimetres of soil, in a circle about a metre and a half across around the trunk, and robusta's pattern appears similar. The review calls the root system highly plastic: its depth and spread change with the tree's age, the variety, the soil, how closely trees are planted and how the ground is managed.

A shallow root system is one reason the timing of rain matters so much to this crop, and one reason the weeks without rain do too.

Why a dry spell comes before the blossom

Sourced

Flower buds form, then stop. The review describes a quiescent phase that coincides with a dry spell of two to four months, and says that spell is important in stimulating flowering in both species. Once buds have reached what it calls the ripe-to-flower stage they respond to a return of water, usually the first rains after the dry period, which growers call blossom showers.

Not every bud is ready on the same day, because buds were started at different times. Where sporadic light rain falls late in bud development, the review says, that is believed to be one of the reasons a tree flowers several times in a season. It gives south-central Brazil as an example: blossoming between August and November in two to four or more synchronised flushes. Where rain is abundant all year, flowering and therefore harvest are scattered, and the review links that to low yields.

Several flowerings mean fruit of several ages on one branch. That single fact is behind most of what is difficult about picking coffee.

One species pollinates itself; the others cannot

Sourced

Arabica is self-compatible: a flower can be fertilised by pollen from the same plant. The review states that most, if not all, other coffee species are self-sterile, robusta among them, which is why a robusta variety is planted as a mixture of compatible clones.

Self-compatible does not mean insects make no difference. The review reports an older experiment in which arabica bushes caged with honey bees set about the same number of fruit at first but gave half as much again in mature berries. A field experiment on thirty farms in Costa Rica, published in 2022, kept birds, bees or both away from arabica branches. Its authors estimate that excluding birds alone would cut yield by 13.5 per cent, bees alone by 24.5 per cent and both by 24.7 per cent, and report that fruit set, fruit weight and its uniformity were all highest where both could visit. That is one experiment in one country, on farms between about 670 and 1,110 metres.

Five stages between blossom and ripe cherry

Sourced

The stages and week counts are the review's, drawn from earlier studies of arabica. They describe a sequence, not a timetable for any one farm.

  1. Pinhead

    The first six to ten weeks after blossoming, in which the fruit hardly grows. It is not dormant: growth is by cell division, not expansion.

  2. Rapid swelling

    About ten weeks in which the fruit gains size and fresh weight quickly. The review says the expansion of the seed's outer covering in this stage sets the maximum size of the bean.

  3. Slow growth

    Roughly two weeks in which the fruit has reached its final size but still holds little dry matter.

  4. Filling

    From about the seventeenth to the twenty-eighth week, dry matter is laid down, mainly in the seeds. The seeds reach their final dry weight while the fruit is still green.

  5. Ripening

    The changes are now mostly in the skin and pulp, which grow and turn red or yellow. The review says ripening can spread over about ten weeks, from roughly the twenty-fourth to the thirty-fourth week after blossoming.

A sequence taken from a published review of arabica physiology. The week counts come from older field studies and differ with climate, species and variety; the review notes, for instance, intervals from about three months in one wild species to over a year in another.

From flower to seed: what is built, and in what order

Sourced

The five stages above describe the fruit from outside. This is the same period from inside the seed, from two reviews of 2006, one of seed physiology and one of coffee carbohydrates, and a 2018 study of seed development. It is a sequence, and the only timings given are theirs.

  1. Fertilisation

    A fertilised flower's ovary becomes the fruit, which normally holds two seeds. In arabica the pollen can be the plant's own.

  2. A temporary tissue fills the fruit

    The first expansion of the fruit is the growth of the perisperm, a tissue belonging to the mother plant. A 2018 study of seed development says this growth determines the final size of the seed. At this stage the sugars are glucose and fructose, and the carbohydrate review locates them in the perisperm.

  3. The endosperm takes its place

    The endosperm, the tissue that becomes the bean, grows into the space the perisperm occupied. What remains of the perisperm is the thin layer later called silver skin. In the endosperm sucrose is the main sugar from the start.

  4. The walls are built

    Once the endosperm has stopped growing, oil begins to accumulate and the tissue hardens, which the 2018 study attributes to massive deposition of galactomannans in the cell walls. Wall material ends up as about half the seed's dry weight. Pectin, about a fifth of the youngest endosperm, falls to a few per cent.

  5. Maturation

    The seed becomes able to survive partial drying. In the 2018 study, in which arabica seeds took about 260 days to develop, most of that tolerance was gained between 150 and 190 days after flowering. Observations at a Brazilian gene bank put maturity at 210 to 250 days in arabica and 300 to 350 in robusta.

  6. A living seed in a hard coat

    The innermost layer of the fruit wall hardens around the seed. Dried, it is the parchment. The seed inside is alive when it is picked and, as the fermentation guide records, for some time into processing.

A model assembled from two reviews, which draw on anatomical studies going back to the 1930s, and one field study. The day counts are one gene bank's observations and differ with climate; liberica and excelsa were recorded there at about 360 days.

A seed that cannot be dried and banked

Sourced

The seeds of most crops can be dried hard, frozen and kept for decades. Seed scientists call these orthodox. At the other extreme are seeds that die if they dry at all, called recalcitrant: cocoa and rubber are examples. Coffee is in between. The 2006 review of coffee seed physiology uses the category introduced for this case, intermediate: seeds that tolerate some drying but do not survive complete drying, or drying combined with cold.

The tolerance differs by species. The review reports the lowest water content to which seeds can be dried without damage as about 0.09 grams of water per gram of dry matter in arabica, 0.10 to 0.12 in robusta, and 0.24 in Coffea liberica, which therefore tolerates much less drying than the other two. It describes a narrow window of moisture in which some arabica seed lots survived a month at 20 degrees below zero, and adds that the protocol cannot be applied to all arabica varieties.

Two consequences reach beyond the laboratory. The first is conservation. Because the seed stores badly, the review says, coffee's genetic resources are kept as living trees in field collections, where they are exposed to weather, disease and the cost of upkeep. The guide to coffee genetics explains why that matters for a crop with so little diversity. The second concerns green coffee. The moisture at which coffee is traded is close to the lower limit the living seed tolerates. And a 2018 study of seed development found that the tolerance is acquired on the tree, mostly between 150 and 190 days after flowering in seeds that took about 260 days to mature: at 150 days most seeds could already germinate but none survived drying, and the damage was to the endosperm, not the embryo. Neither source draws a conclusion about traded coffee from this, and none is drawn here.

Germination: a slow push through a hard wall

Sourced

Coffee germinates slowly and unevenly, and the review treats this as a practical problem: it makes uniform seedlings hard to raise. In the field, arabica seedlings begin to emerge from the soil 50 to 60 days after sowing in the warmer part of the year, and the review cites an older report of up to 90 days when it is cooler.

Much of the delay is mechanical. The parchment drastically inhibits germination and is generally removed in germination tests. Under laboratory conditions the review gives, 30 degrees Celsius in the dark, the root tip begins to emerge around the fifth or sixth day and half the seeds have done so by the tenth. Before it can, the hard endosperm in front of the root tip has to give way. The review describes this as happening in two steps, each marked by a fall in the force needed to puncture the tissue and by rising activity of the enzymes that break down mannan and cellulose, the same polymers that make up the seed's thick walls.

Plant hormones regulate it in a way that surprised the people who studied it. Abscisic acid, the hormone that generally holds seeds back, blocks the second step and not the first. Gibberellin, which promotes germination in most seeds, is needed by the coffee seed and made by it, yet adding more of it inhibits germination; the review reports a proposed explanation, a build-up of the sugar mannose released from the walls, and presents it as a proposal.

After germination the seed leaves absorb what is left of the endosperm and turn green. What happens next is the nursery's business, and the next section follows one manual's account of it.

From seed to a plant in the field: the nursery in one manual

Sourced

The stages are those of a growers' manual the FAO published for arabica in Laos in 2005, the one open account of a coffee nursery CoffeeHQ could read in full. Its timings are for that country's cool season and its own seed, and are given to show the order and the scale of things. They are one manual's working figures, not a schedule.

  1. The seed is parchment, and it is fresh

    What is sown is coffee still in its parchment, not the hulled green bean. The manual says seed loses its viability within about three months unless it is kept cool and moist, and that it should not be dried below about 10 per cent moisture, which fits the seed physiology described above: this is a seed that dies if it dries too far.

  2. A seedbed under shade

    Seed goes flat side down into a bed of loose soil and sand under a roof giving about half shade. The manual stresses that heavy soil alone is too compact, and that germination is so dependent on soil temperature that growers there cover the beds to keep them warm.

  3. Root first, then the seed lifted out of the ground

    The young root appears three to four weeks after sowing. Some three weeks later the stem below the seed leaves straightens and carries the seed, still in its parchment, above the soil. Growers call this the matchstick stage.

  4. Two round seed leaves

    The parchment falls away and two rounded seed leaves open, unlike any later leaf on the tree. The manual puts this four to six weeks after sowing at the earliest and says shoots may take 30 to 50 days to show in cool soil. The first pair of true leaves follows, and the seed leaves, having fed the plant, die.

  5. Moved into a bag while the taproot is still short

    At the matchstick or seed-leaf stage, two to three months after sowing, each seedling is lifted and set in a bag of fresh soil mix. The manual's concern here is the taproot. A seedling whose taproot is bent, which it calls a J root, is discarded, and so is one with few root hairs; seedlings that have grown past this stage are not used either, because, the manual says, they are too slow to grow on.

  6. Months in the bag

    The plants stay under shade, are kept moist and not wet, and are moved apart as they grow so that they do not draw up tall and weak. The first side branch appears when a plant has several pairs of leaves. The nursery diseases the manual names, a rot that fells seedlings in patches and a leaf spot, it attributes to reused soil, too much water, too much shade and crowding.

  7. Hardened, then planted out in the rains

    Over the last two months the shade is taken away by degrees so that the plants meet full sun before they leave. The manual plants out at six to eight pairs of leaves, on a cloudy day in the wet season, into ground prepared and shaded a year ahead, and again rejects any plant with a twisted or pot-bound root.

A sequence from one country's manual. The same manual gives sowing densities, a soil mix, feeding and sprays for its own conditions; none is repeated here, and none would transfer unchanged. What does transfer is the order, and the two things the manual returns to at every stage: a seed that cannot be stored dry, and a taproot that must not be bent.

What the weather decides while the fruit is growing

Sourced

A good share of the fruit never reaches harvest. The review describes three periods of fruit drop, mostly within the first three months after blossoming, and says the first, which follows failed fertilisation, seems to be unavoidable.

Bean size is set early. Citing work at Ruiru in Kenya, the review says potential bean size is strongly determined by rain falling ten to seventeen weeks after blossoming, when the berry is expanding: fruit that expands in wet weather is larger and is later filled with larger beans. After that the shell around the seed hardens and the size is fixed.

Why a heavy crop is often followed by a light one

Sourced

A tree carrying a large crop spends heavily on its fruit. The review describes branch die-back in overbearing trees, links it to drought, heat and strong sun as well as to the load itself, and says die-back may accentuate the alternation of heavy and light years because a tree can need two or three years to recover. It notes that the pattern is weaker in robusta as grown in Brazil, where stems are pruned and renewed on a cycle.

The size of that swing in one country's national crop is described in the Brazil entry of the atlas.

What this page does not cover

CoffeeHQ explanation

How long a tree takes to bear, and how long it stays productive, are not described, because no source on them was read. The nursery is described from one manual written for one country, and no research study of nursery practice was read: grafting, cuttings and plants raised from tissue culture are named in that manual and not explained here. The week counts for the fruit are for arabica; for robusta only the total time to a mature seed was read.

What to do next

Related