In soil and soilless
Two ways to grow the saffron crocus — the centuries-old field and stacked indoor shelves — and what each one gains and costs.
In soil: the open field
Saffron has grown in fields for centuries, on little water and a great deal of labour. The crocus copes with hard conditions, but a few things it insists on.
Climate and soil
- A hot, dry summer. The flower buds form in summer heat; summer rain and wet soil rot the corm.
- A cool autumn. Cooling air starts the flowering.
- Free-draining soil. Sandy loam that doesn't hold water, chalky or neutral. Heavy, waterlogged soils are saffron's worst enemy.
- Sun. Leaves that get winter sun build up next year's corms.
From planting to harvest
Corms are planted 10–15 cm deep at the end of summer, while the soil is still warm. Rain is usually enough; in a dry autumn, watering after planting and before flowering will do. The flowers are picked by hand every morning through October and November.
Planted corms stay in the same field for three or four years. They multiply each year and in time crowd one another. Then they're lifted, sorted by size and planted in another field. Replanting the same ground straight away feeds soil-borne disease.
What makes the field hard
- Corm rot. Soil fungi rot the corm and travel from field to field with the corms.
- Rodents. Mice and voles eat the corms.
- Weather. Rain or frost in the week of flowering can spoil a year's harvest.
- Labour. The harvest is squeezed into a few weeks and done entirely by hand: bending, every morning, flower by flower.
Soilless: indoors
Growing indoors takes advantage of one feature of saffron: it needs no soil to flower. The corm carries everything the flower needs in its own store. Given the right temperature and humidity, it flowers in a tray, a crate or on a shelf, without touching soil.
So indoor saffron splits into two jobs:
- Flowering. After a dark, warm incubation, the corms are cooled and the air is made more humid. Flowers open on the trays within two to three weeks. No water, nutrients or soil are needed.
- Growing corms. After flowering, leaves grow and new corms form. This stage needs roots, water, nutrients and light. In soilless systems, the roots are fed by mist (aeroponics) or by periodic flooding (ebb and flow).
Some growers do only the flowering indoors and plant the corms back in a field to grow them on. Tarming House runs both stages indoors.
The saffron programme at Tarming House
At Tarming House, every cell lives the saffron year by the programme below. The values are based on published research into soilless indoor saffron.
| Stage | Months | Temperature | Relative humidity | Light |
|---|---|---|---|---|
| Incubation | June – August | Steady 25 °C | 60–70% | Complete darkness |
| Flowering | September – October | Day 10–13 °C, night 7–10 °C | 80–85% | 12 hours a day |
| Leaves and corms | November – April | Day 12–15 °C, night 8–12 °C | 65–75% | 10 hours a day |
Flowering starts with the cooling that follows incubation and arrives within two to three weeks. The high humidity of this stage protects the stigmas during harvest.
The two methods side by side
| In soil | Soilless, indoors | |
|---|---|---|
| Space | Needs a wide field | Shelves stack upward; several harvests from one floor area |
| Climate risk | Rain, frost and drought hit the harvest directly | Temperature, humidity and light are set per stage |
| Calendar | Set by the seasons | Set by the programme |
| Harvest | In the field, bending, every morning | From the shelves, standing, in a clean room |
| Corm health | Exposed to soil-borne disease | No soil; each cell's water is separate |
| Cost | Low investment, high labour | High investment and energy cost |
| Experience | Centuries of knowledge | New; knowledge still building |
The two methods aren't rivals. The field is saffron's home of thousands of years and will stay that way. Indoor growing makes saffron possible where the climate isn't suitable. It also lets us learn what saffron really likes, in a place where every degree and every percent of humidity is measured.