In this article, we look at coffee and the coffee plant from a botanical perspective. What is the plant’s structure, and what roles do the roots, leaves, and fruit play? And how do we describe them correctly in botany?
Everyone knows the feeling that freshly ground coffee can trigger. The mere thought of it is enough to whet your appetite for coffee. It is not without reason that grinding coffee fresh is so helpful, as the aromas dissipate within a very short time.
We put the ground coffee into a portafilter, a coffee filter, or a French press, pour or press hot water over it, and enjoy. But what exactly are we drinking when we drink coffee?
We are drinking the dried, roasted, ground, and brewed seeds of a fruit from the Coffea genus.
In short, we simply call it coffee – but there are huge differences between genera, species, and varieties; there are many species we cannot even drink, and various varieties do not smell or taste like the familiar beverage we simply call “coffee”.
100% Arabica - old school and meaningless
Coffee packaging is still labeled with “100% Arabica,” which firstly does not do justice to the complexity of the product and secondly offers no added informational value. Furthermore, distinguishing solely by species is no longer contemporary. The coffee plant is subject to climate change just like other organisms, and we should therefore expand our vocabulary in order to understand and describe the coffee of the future.
To ensure we can continue to drink coffee at all in the future, despite rising temperatures and unpredictable weather patterns, more research into the further development of the coffee plant is required—and this includes an in-depth examination of the most diverse varieties and species.
For years, researchers have been intensively working on bringing new hybrids from the lab into the field and scaling them. The reduced distinction between Arabica and non-Arabica (e.g., Robusta) is far too simplistic.
Botanical taxonomy, i.e., the classification of the coffee plant into categories, is not self-explanatory. These are reasons enough to break down this important fundamental topic.
Coffee Botany
At a lunch with agronomists, I once asked whether the coffee plant is a tree or a shrub. A funny fundamental discussion about definitions ignited, with almost everyone agreeing in the end:
The coffee plant is a tree. Because trees usually have only one main trunk protruding from the ground. Bushes, on the other hand, have multiple wooden stems that can also be pruned individually.
Coffee trees in the agroforestry system in Marcala, Honduras.
From seed to tree
The coffee tree originates from the seed of a fruit. The coffee seed is the actual coffee bean, which can be extracted from a depulped coffee cherry. This seed should be sown relatively freshly with a moisture content of over 50%.
The seeds are usually sown in a substrate of sand and loose soil, well-watered, and brought to germination. After one month, the hypocotyl has already formed—the lowest section of the shoot axis between the still-fine roots and the cotyledons. The pergamino (parchment) still covers the cotyledons before they are shed shortly after.
After a good three months, the seedlings are ready to be replanted into individual plastic bags with soil. At a height of approx. 40cm and with a well-developed root system, they are brought out into the field and planted after about 1 year.
From tree to tree
It generally takes three years from the germinating seed to the first small harvest. Hybrids, however—plants resulting from a cross between different species—can produce fruit after just two years. This acceleration of the growth process brings more efficiency to the planning of a coffee farm.
A coffee tree can bear fruit for several decades if it is well cared for. However, production capacity gradually decreases after about 20 years. The tree trunks increasingly become woody, and the fruit yield declines. In addition, trees in advanced age can become more susceptible to diseases.
Wintgens writes in his 2004 classic that a coffee plant in production is hardly older than 30 years. In conversations with trading houses and producers, however, it emerged that their plants were hardly older than 20 years, and that 30 years might have been correct “some time ago.” The climatic challenges facing coffee plants have intensified, and they are subject to massive change.
On this farm in Santa Barbara, Honduras, temperatures reach up to 33 degrees. This is too high for coffee production. The focus on agroforestry is becoming increasingly important.
It turned out much more in conversations with producers that more than 90% of their tree stock is younger than 15 years. The Norcafé cooperative in Peru encourages its members to replace plants after 15 years. The trees would then invest more and more in wood growth and less in fruit production; the yield gradually decreases. The tree would still produce fruit for another 15 years, but not in an efficient ratio.
The root system of the coffee tree
The coffee plant is a perennial plant. Without a well-developed root system, the plant could not guarantee its supply of nutrients and water and, ultimately, would not reliably produce fruit. A good root system is a basic requirement for tree health.
Wintgens mentions that the taproot can reach up to 1m deep into the soil. Loose soil allows the root system of a single coffee plant to penetrate up to 15m3 of soil with its roots. (Wintgens, p. 7)
The task of the roots is clear: they absorb water and transport it through the plant. At the same time, water serves as a solvent that transports gas and minerals into the cells and organs. The root system functions as a storage for carbohydrates and produces the plant's own growth hormones.
A wide variety of factors influence the shape and growth of a root system: the species and varieties, the amount of fruit, the resilience of the above-ground part, attacks by fungi or diseases, the distance to the next plant, soil conditions and health, and the water content in the soil.
Just because we don't see the roots doesn't mean they shouldn't be in our consciousness. The development of the root system in the coffee plant, and other trees, is impressive—and shows quite accurately what has happened above ground, or is currently happening. Root systems are something like an EKG and a library at the same time.
The root types of the coffee plant
The taproot
In its full stage of development, the coffee tree has a taproot that penetrates vertically up to a good half meter into the soil. This is the largest root and clearly shows why we call it a tree. If this taproot is not planted straight at a young age, it can become crooked and hardened, and the tree will not grow as planned.
Lateral roots and fine roots
The lateral roots (axial roots) are the roots that penetrate deepest into the soil and develop up to 3m in all directions. The lateral roots run parallel to the soil, remain superficial, and, depending on planting density, come into contact with neighboring roots. The fine roots or hair roots are of varying lengths and are distributed along the lateral roots. They are primarily responsible for providing minerals to the plant.
Coffee Farm Training, Nicaragua. The agronomist Oscar shows a massive taproot.
Leaves
As with other trees, we can tell which coffee species or variety we are dealing with by looking at the leaves. The size, shape, thickness, and curvatures are all indicators that help us analyze the plant.
I remember well when Don Oscar, an agronomist from Costa Rica, said during the 2019 Coffee Farm Training in Nicaragua:
“Leaves are not just there for photosynthesis. The leaves talk to us. The coffee tree needs leaves so it can communicate with us. They show us how the coffee tree is doing right now.”
Indeed—those who look closely at leaves will see more and more details.
According to Wintgens, a mature coffee tree has between 22 and 45 m2 of leaves. Leaves are fully grown after approx. 35 days before they fall off at about 10 months, and the tree produces new ones.
Wintgens also writes that a coffee tree evaporates approx. 6g of water per square decimeter daily. If we assume that a coffee tree has approx. 35 m2 of leaves, that would be 3500dm2, times 6g of water = 21l of water.
According to Wintgens, an average coffee tree therefore evaporates approx. 20l per day.
And here we see clearly how important the soil's water storage capacity is for coffee production. Canephora is generally artificially irrigated, as they are significantly more susceptible to moisture stress and require regulated irrigation times for the flowers to develop as hoped.
From bloom to fruit
Coffee fruits are called coffee cherries—however, unlike the cherry we know, the pulp is not eaten and the seed discarded, but the other way around. In a cherry, there are generally two seeds facing each other—these are the later coffee beans.
A flower shortly before opening (right) and approx. six-month-old coffee cherries in Nicaragua.
On a trip to Costa Rica, a producer once told me that he still had to visit his “kids.” “There are so many by now”—I didn't want to be indiscreet and ask for the exact number. It soon turned out, however, that he meant his coffee plants with “niños.” The analogies go even further, as most Arabica plants take about nine months from flowering to the production of ripe coffee cherries, while Canephoras take up to 11 months.
The coffee plant is actually dependent on stable weather patterns in order to develop well. But these weather patterns have been shifting for years, which requires a great deal of flexibility from producers annually. In the best case, it would be dry after the harvest so that the coffee tree feels stressed: the bud would thus interrupt its rest period because it is forced to survive.
During the dry phase, the plant significantly throttles its metabolism. It constricts the capillaries and prepares for the first rain. When this comes, the plant is ready to absorb water quickly, and cell division in the bud takes place just 3 to 4 days later.
Depending on the amount of precipitation, more or fewer buds are activated—multiple short and staggered rain showers result in less uniform ripening and ultimately lead to the coffee cherries having to be harvested at different times. In short: unstable weather patterns lead to more picking labor.
Coffee types, coffee varieties & hybrids.
The majority of coffee drunk is still of the Arabica type. For several years, the ratio has been leveling off at 40/60. 40% of the coffee grown is Canephora (colloquially Robusta), 60% is Arabica. At the same time, however, more and more coffee is being drunk, so the proportions of both types are growing simultaneously, but remain in the 40/60 ratio so far.
However, rising temperatures and rapidly changing weather patterns will accelerate the increase in the Canephora share, as well as that of hybrids: a special form of crossbreeding in which two genetically distinct lines are crossed.
The “Marsellesa” hybrid, for example, which we grow a lot in Nicaragua, is a cross between the “Villa Sarchi” and “Timor Hybrid 823/2” varieties. These are two varieties that would hardly come into contact in the field in a “natural” way. Marsellesa was developed by CIRAD, the French agricultural research center, and tested in various Central American countries for more than ten years before the hybrid came onto the market.
At World Coffee Research, you will find the most common cultivated coffee varieties cataloged and classified by origin.
While Arabica and Robusta have been the most widespread terms in the coffee sector so far, this is likely to change in the coming years. Coffee production is massively affected by climate change, which, among other things, continues to accelerate research into new hybrids and varieties.
To classify the new terminology, we have collected all common ones and recorded them taxonomically: the classification into systematic categories. The first person to botanically classify Arabica coffee was the Swede Carl von Linné. He laid the foundation for modern botanical and zoological taxonomy.
Before Linné created the fundamental classification system for plants in his 1753 Species Plantarum, coffee was first classified somewhat by Jussieu in 1713:
Jasminum arabicum, laurio folio, cujus femen apudnos coffee deciur (Arab Jasmine, with laurel type leaves, the beans of which we call coffee) (Wintgens 2009)
If we take Linné's system of plant classification and apply the coffee terms known to us, coffee taxonomy looks like this.
Coffee taxonomy
Download poster as PDF
Coffee TaxonomyWhat happens next?
In the next articles, I will write about the differences between the most widely grown species, Arabica and Canephora, about their future viability, and what research contributes to this.
