Saffron crocus

Year by year

• year 1

  First autumn — if the corm was big enough

  The first flowers come from the corm you bought, and only if it was big enough: corms of about 8 g and up flower, while smaller ones only make leaves. All that winter, each corm is used up and replaced by daughter corms, so by the end of year one the plant in the pot is no longer the corm you planted.

  see it on the strip ↑
• years 2–4

  The patch thickens

  These are the best years. Each corm splits into several, and given a warm summer and a sunny winter, every autumn brings more flowers than the last.

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• year 5+

  Crowded, and rising

  Eventually the clump is too crowded. The corms get smaller, flowering drops off, and because each new corm forms on top of the old one, the clump creeps toward the surface. The fix is to dig them up in summer, sort by size, and replant the big ones 15 cm deep, which takes the patch back to its best years.

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Getting started

arrives as dry corms in late summer — and a big corm (8 g and up) has this autumn's flowers already built inside it

• day 0–404–6 weeks · cool and moist

  Rooting in

  A corm planted in late summer does nothing you can see for weeks. It is making roots first, and the flower buds it formed during its summer sleep are already waiting inside it. Cooler nights and the first proper watering are the signal to start. Plant it 15 cm deep and pointed end up; in one trial 15 cm gave every corm a shoot and 20 cm gave the fewest.

  Nothing to see yet — the flowers are already packed inside.

  see it on the strip ↑
• day 40–270autumn to late spring

  The first winter

  Whether a new corm flowers in its first autumn depends on its size. In one study only corms of 8–11 g flowered, and lighter ones produced leaves only; the UVM saffron programme says corms over 8 cm round are worth planting but may skip flowering the first year. Either way it spends the winter in leaf, building the daughter corms that will flower next year.

  Small corms skip a year of flowers. Their leaves are building next year's corms.

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Typical timeline

• the plantMID-SEP – MID-OCT

  Waking

  The corm wakes when it gets cool and wet together. Under a pale sheath, a white shoot pushes up, carrying the flowers and leaves together. In the Khorasan saffron fields of Iran, farmers start the season with a single heavy watering of about 100 mm, and the field flowers 19–21 days later. In a pod the cue is the same: let the temperature fall toward 17 °C, then water properly for the first time since spring.

  Cool nights plus the first real watering: about three weeks to flowers.

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• the plantMID-OCT – APR

  Leaves all winter

  The flowers come up first, with the leaves just behind, and the leaves stay up all winter: narrow and grass-like. This is when the plant builds next year. The mother corm is used up completely and replaced by new daughter corms on top of it, and about 90 % of their weight comes from what the leaves make in the winter sun, not from the old corm. A dim winter means small daughter corms and few flowers the next autumn.

  The leaves look idle all winter. They're building next autumn's flowers.

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• the plantMAY

  Yellowing

  The leaves yellow and flop, earlier after a warm dry spring. Leave them alone until they are fully brown: they are still sending sugar down into the new corms.

  Yellow leaves in May mean it's going to sleep, not dying.

  see it on the strip ↑
• the plantJUN – MID-SEP

  Summer sleep

  There is nothing above the soil, but the corm is busy. This is when it forms next autumn's flower buds, and it needs warmth to do it: 23–27 °C for at least 50 days is ideal. At 9 °C no flowers form; at 30 °C they abort. Commercial growers in China and in Taketa, Japan, keep lifted corms in dark rooms for the whole summer and still get flowers. What the corm needs now is warmth and dry soil, not light.

  An empty pot in July, with the flowers forming inside the corm.

  see it on the strip ↑
• floweringMID-OCT – MID-NOV

  The threads

  One or two scented lilac flowers per corm, with darker veins, coming up with the first leaves or just ahead of them. The saffron is in the middle: the style splits into three deep-red branches, the stigmas, each 2.5 cm or more long and heavy enough to flop out over the edge of the flower. Those three threads are the spice. The yellow stamens beside them are not. The red is crocin, a carotenoid like the orange in carrots, but with sugars attached, which is why it dissolves in water instead of oil. Saffron's smell comes from safranal, which is barely present in a fresh flower and forms as the threads dry. In Vermont trials a planting kept flowering for five or six weeks, with most flowers in the second half of October. No seed pods follow. Saffron has three sets of chromosomes instead of two, it sets no seed, and every saffron plant alive was grown from a corm. People have gathered these flowers for a very long time. A fresco at Akrotiri on Santorini, painted around 1600 BC, shows girls picking crocus for a goddess, although the 2019 study that traced saffron to Attica argues those were probably still wild crocuses. At Knossos, Arthur Evans restored another fresco as a boy gathering saffron. A fragment of tail his team had missed later turned the boy into a blue monkey.

  Three red threads in each flower. That is the saffron.

  see it on the strip ↑
• undergroundJUN – AUG

  Next year's flowers

  Inside the sleeping corm, the flowers for next autumn are forming. This is the only part of the year that decides how many flowers there will be, and nothing shows above ground while it happens. It runs on heat alone: best at 23–27 °C, and it needs at least 50 days. Past about 150 days at that heat the buds start to abort, and a pot kept at 30 °C all summer makes no flowers at all. So a cool basement is as bad for saffron as a hot greenhouse.

  see it on the strip ↑
• undergroundDEC – APR

  Daughter corms

  Underground, the corm you planted is being used up and replaced. New corms swell on top of it, one from each bud that grew, and in spring the old one shrivels away. Because each new corm sits on top of the last, the whole clump rises about a corm's height each year, which is why beds are dug up and replanted every few years. Saffron cannot set seed, so these corms are the only way it reproduces, and every saffron plant is effectively the same plant: a 2009 study of 43 samples from 11 countries found no differences between them at any DNA marker it tested.

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Care milestones

• sowAUG – MID-SEP

  Plant the corms

  Plant firm, heavy corms, pointed end up, 15 cm deep and 7–10 cm apart, in soil that drains fast. That is roughly 6–12 corms per square foot, or a 30 cm pot. The UVM saffron programme recommends a pot at least 11–12 inches square. Keep the dry papery husk on; it protects the corm.

  see it on the strip ↑
• waterMID-SEP – EARLY OCT

  The first real water

  After a dry summer, one thorough soak is what starts the season, and it works best as the nights cool toward 17 °C. In Khorasan this single watering is timed so the flowers arrive three weeks later. In a pod, drop the temperature first and then water. Watering a hot pot just wakes the corm too early.

  see it on the strip ↑
• harvestMID-OCT – MID-NOV

  Pick in the morning

  Pick the flowers early in the morning, once the dew is off, while they are only just opening. A few hours of full sun on an open flower measurably fades its stigmas. Pinch out the three red stigmas the same day and leave the yellow stamens; they are not saffron. Dry them until they snap, below 12 % moisture, then store them airtight and dark.

  see it on the strip ↑
• waterNOV – APR

  Moist, never soggy

  While the leaves are up, keep the soil lightly moist, the way winter rain leaves it. Letting it dry out is costly: in irrigation trials, watering less than the crop needed produced smaller replacement corms. Standing water is worse, because it rots the corms. Use a free-draining mix and never leave the pot in a saucer of water.

  see it on the strip ↑
• watchDEC – FEB

  Frost in a small pot

  Saffron is tougher than it looks. In one frost study, potted plants survived down to −12 °C unharmed, and the leaves were damaged before the corms. UVM grows it outdoors as far north as USDA zone 4. The risk in a container is the soil freezing all the way through, so set pots on the ground, wrap their sides, and keep a pod's soil above freezing.

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• waterMAY

  Let it go dry

  Once the leaves start to yellow, cut back on watering and then stop completely. The plant is going dormant, and a dormant corm in wet soil is the most likely to rot. In its home climates, the summer drought does this job.

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• watchJUN – AUG

  Keep the sleep warm

  This is the most important instruction for growing saffron indoors. The flowers form inside the corm over the summer, so a pot left somewhere cool all summer will produce leaves and no flowers. Keep the soil dry and around 23–27 °C, and under 30 °C. Then in September let the temperature fall toward 17 °C, which tells the corm to come up.

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• divideJUL – AUG · EVERY 4–5 YR

  Lift and divide

  Every few summers, dig up the dormant corms and sort them by size. Replant the big ones, which will flower, and grow on the small ones, which won't yet. Saffron farms differ a lot here: Iran and Spain leave corms in the ground for 4–8 years, while parts of Italy replant every year or two. UVM suggests every 4–5 years. Store lifted corms dry and at room temperature, with their husks on.

  see it on the strip ↑

Botanical drawing

Köhler's Medizinal-Pflanzen, pl. 164 (1890) — Crocus sativus L.: the corm, the plant, and a flower split to show its three stigmas