How an Andean tuber reached European gardens
Potatoes had been grown in the Andes for thousands of years - in hundreds of local varieties. The Spanish brought them to Europe in the second half of the 16th century, but not as food: the first plants ended up in monastery and botanical gardens, where they were seen as curiosities with pretty flowers.
Then things stalled. Potatoes are nightshades, related to belladonna; their green berries are inedible, while the edible part grew underground. But there was a simpler reason: the first European plantings yielded almost nothing.
Tubers form when the days begin to shorten. In the Andes, day length is roughly even year-round, so the signal comes at the right time. In northern Europe, summer daylight lasts much longer, and the plant put out foliage instead of tubers, delaying its harvest until October frosts. It took decades of selecting plants that form tubers under long days; later, varieties from southern Chile helped, where summer days are closer to Europe's.
Why kings promoted potatoes
Tubers have 2 qualities that matter in wartime: they lie underground, so they cannot be burned with the field or trampled by cavalry. And they produce more food from the same area than grain. So potatoes were pushed from above, by decree.
In the mid-18th century, the Prussian king ordered people to plant them under threat of punishment. In France, pharmacist Antoine Parmentier took up the cause after encountering potatoes as a prisoner; the story of a field guarded by day and left unguarded at night is often told, but there is little evidence for it. In Russia, instructions for growing "earth apples" were sent to the provinces in 1765, and potato riots swept through state villages in the 1840s.
About a quarter of Old World population growth from 1700-1900 - the estimated contribution of potatoes
Cheap calories change demographics. That quarter is an economic model, not a measurement, but the scale is clear: food that reliably grows on poor soil raises the limit of how many people a village can feed.
The downside: one variety for everyone
Only a tiny fraction of Andean diversity reached Europe, and for centuries fields were planted with nearly identical plants. Ireland went furthest: for its poorest tenant farmers, potatoes were not a side dish but the basis of their diet.
The table of Ireland's poorest tenant farmers before 1845: potatoes every day of the week
In 1845, late blight arrived in Europe - a disease that makes tubers blacken right in the ground. Where potatoes were one of several dishes, it meant a bad year. In Ireland, where they were food itself, famine began: about a million people died, and about another million left for overseas. Potatoes were not the problem; the problem was betting on one crop and almost one variety.
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Sources
- Economic estimate of potatoes' contribution to Old World population growth (2011), research on potato photoperiodism, and historiography of the Irish famine of 1845-1849