Sukumo: The Hidden Craft Behind Japanese Indigo Fermentation

Last night, Indigo Week continued with a remarkable presentation from Britt Boles, taking us deep into one of the most fascinating and least understood aspects of traditional indigo dyeing: sukumo.

Many natural dyers have heard the word. Some may have worked with a sukumo vat or read about Japanese indigo traditions. Yet few of us truly appreciate what lies behind that dark, earthy material that eventually becomes a fermentation vat.

As Britt showed us throughout the evening, sukumo is not simply dried indigo leaves. It is the result of months of careful work, observation and transformation.

The journey begins in the field.

Unlike growing indigo for fresh leaf dyeing or pigment extraction, growing indigo for sukumo requires a different mindset. The goal is not simply to produce leaves, but to produce large quantities of healthy plant material that can withstand the long composting process ahead. Throughout the session, Britt shared insights into cultivation methods, harvesting practices and the subtle decisions that influence the quality of the finished sukumo.

From there, the process moves into drying and storage.

For many participants, this was a revelation. While dried indigo leaves can be stored for extended periods, Britt explained how the quality of the leaves, drying methods and storage conditions all influence the success of the next stage. Questions from participants highlighted the practical challenges many growers face, from preserving leaf quality to understanding how different varieties perform under different growing conditions.

But the true heart of the presentation lay in the composting process itself.

Over approximately one hundred days, dried indigo leaves are carefully managed through a controlled composting process. Temperature, moisture, oxygen and microbial activity all play crucial roles. Too wet and problems develop. Too dry and the transformation slows. Too hot or too cold and the process can lose momentum.

What emerged throughout the evening was a growing appreciation for the extraordinary level of skill required to guide this transformation successfully.

Sukumo is often described as composted indigo leaves, but that simple description barely captures the reality. It is a living process involving countless observations and adjustments, with each compost pile responding to its own unique conditions.

One participant commented that the presentation finally explained why good sukumo is so valuable, while others reflected on the hidden labour and knowledge embedded within traditional craft practices.

Perhaps one of the most valuable aspects of the session was Britt's willingness to share both successes and challenges.

Natural dyeing can sometimes make traditional processes appear effortless, yet the reality is often much more complex. Throughout the evening, Britt spoke openly about experimentation, learning through practice and adapting techniques to different climates and growing conditions. This honesty resonated strongly with participants, many of whom are at different stages in their own indigo journeys.

As questions flowed in from Ireland, Europe, North America and beyond, the discussion expanded into topics such as indigo varieties, composting materials, fertilisation, harvesting strategies and environmental conditions. The session became a wonderful example of what makes Indigo Week so special: not simply learning from a presenter, but learning together as a community of growers, dyers and curious practitioners.

What stayed with me after the presentation was a renewed appreciation for the relationship between time and colour.

In a world that often values speed and convenience, sukumo reminds us that some materials cannot be rushed. Before a fermentation vat can come to life, months of work have already taken place in the garden, the drying space and the compost pile. The blue we eventually see on cloth begins long before the vat itself exists.

And this is only the beginning of the story.

Tomorrow evening, Britt returns for the second part, exploring how finished sukumo is transformed into a working fermentation vat. Together, these two sessions provide a rare opportunity to follow the complete journey from indigo plant to living vat.

For anyone interested in indigo, fermentation, traditional craft processes or the remarkable relationship between plants and people, it was an evening filled with insight, generosity and inspiration.

Thank you, Britt, for sharing such a rich and thoughtful glimpse into the hidden craft behind Japanese indigo fermentation.

Indigo Week is part of our Full Membership programme, giving members access to live presentations, discussions and demonstrations from leading indigo practitioners around the world. Every session is recorded, so members can revisit the material at any time and catch up on any presentations they miss. The growing library of recordings has become an invaluable resource for anyone wanting to deepen their understanding of indigo growing, dyeing and fermentation.