The surface face of a Japanese tatami mat (*tatami-omote*) is woven from *Igusa* rush (*Juncus effusus var. decipiens*). Harvested primarily in Kumamoto Prefecture, fresh rushes undergo *Tsuchi-zome*—immersion in natural mineral-rich mud slurry that slows drying, preserves chlorophyll green color, and coats stems in protective clay powder before being woven with hemp or cotton warps.
At a Glance: Key Takeaways
- Plant Species: Igusa / Soft Rush (Juncus effusus L. var. decipiens Buchenau)
- Harvest Center: Yatsushiro plain, Kumamoto Prefecture, Kyushu, Japan (>90% of production)
- Mud Dyeing Process: Tsuchi-zome (coating fresh rushes in natural Sendo clay mud slurry)
- Stem Selection: Graded into 4,000–7,000 flawless uniform stems per single tatami mat
- Warp Yarn Options: Single cotton, double cotton, or high-tensile dual hemp (*Asa-ito*)
- Surface Texture: Ribbed tactile grain with cooling summer feel and natural elasticity
Igusa Botany and the Yatsushiro Agronomy
True Japanese tatami rush is not straw or reed, but Igusa (Juncus effusus). Cultivated primarily on the fertile alluvial plains of Yatsushiro in Kumamoto, the plants are planted in flooded paddy fields during late autumn and harvested at peak summer maturity in early July, when stems reach over 1.2 meters in height.
The secret to Igusa's comfort lies inside the stem. Unlike hollow bamboo, Igusa contains a dense, spongy white core called aerenchyma—a three-dimensional microscopic network of star-shaped parenchyma cells filled with air pockets. This cellular cushion provides thermal insulation from cold floors and resilient springiness under bare feet.
The Chemistry and Protective Role of Tsuchi-zome Mud Dyeing
Immediately after harvesting, the fresh green rush undergoes Tsuchi-zome—immersion in a bath of natural colloidal clay mud harvested from ancient mountain deposits (Sendo-tsuchi):
- Chlorophyll Protection: If dried directly in the sun, rush chlorophyll oxidizes rapidly into brittle, bleached grey straw. The mineral mud forms an ultra-thin protective shield over the rush cuticle, slowing evaporation and preserving natural olive-green pigments.
- Moisture Buffer: The microscopic clay coating seals open stomata, preventing the rush stem from collapsing during kiln drying at 60°C.
- Aromatic Fixation: The clay interacts with the natural plant lipids, stabilizing the beloved herbal aroma of tatami. When newly installed, the dry mud is wiped clean with a cloth, revealing the silky, lustrous rush underneath.
Natural Igusa Rush Mud-Dyeing and Loom Weaving in Japanese Tatami Architectural Decision Matrix
Use this reference table to calculate exact physical dimensions, ratios, and specifications before purchasing:
| Warp Thread Typology | Warp Yarn Material | Stems per Mat | Durability / Lifespan | Architectural Application |
|---|---|---|---|---|
| Ichi-men Momen (Single Cotton) | Single cotton thread | 3,500–4,000 | 5–7 Years | Rental apartments & light residential |
| Ni-men Momen (Double Cotton) | Twisted double cotton | 4,500–5,000 | 8–12 Years | Standard residential washitsu |
| Momen-Asa (Cotton & Hemp) | Blended hemp/cotton | 5,500–6,000 | 12–18 Years | Luxury residences and master bedrooms |
| Asa-W-Uchi (Double Heavy Hemp) | Pure dual-ply long-staple hemp | 6,500–7,500 | 20+ Years | Historic temples, tea pavilions, martial arts dojos |
Practical Constraints
Every successful decoration project must account for physical and environmental constraints:
Declining Number of Rush Farms
Strict Room Moisture Limits
From Harvest to Woven Tatami-Omote
Execute this professional sequence to achieve flawless results without backtracking:
Early Morning Harvest
Harvest mature rush stems before sunrise to preserve maximum internal cellular moisture and turgidity.
Sendo Mud Bath
Immerse bundles of freshly cut rush into the Sendo clay bath; agitate continuously to ensure 100% mud coating.
Low-Temp Kiln Drying
Transfer mud-coated rushes to dedicated drying kilns; dry gently at 55°C–60°C for 16 hours to stabilize moisture at 10%.
Mechanical Grading
Pass dried stems through laser thickness sorters; discard discolored, bent, or broken stems to ensure absolute uniformity.
Loom Warp Setup
String 64 high-tensile hemp warp cords across a computerized or traditional shuttle loom at precise tension.
Shuttle Weaving
Insert individual Igusa rushes as weft threads; beat each stem tightly into the weave with the reed at 250 beats per minute.
Quality Inspection
Inspect woven rolls under raking light for skipped threads or thickness steps; roll and wrap in moisture-proof kraft paper.
Common Mistakes & Countermeasures
Avoid these costly errors that compromise aesthetics, ergonomics, and budget:
Using synthetic green dye instead of natural Sendo clay
Weaving with dry, un-humidified rush stems
Wiping newly installed mats with water instead of dry cloth
Project Execution Checklist
Track your physical progress. Checkboxes persist automatically on this device:
Frequently Asked Questions
How can one identify genuine Japanese Igusa from cheaper imported rush?
Authentic Japanese Kumamoto Igusa has a dense, uniform aerenchyma core, deep natural aroma, turns to a clean golden-yellow patina over years, and features high-grade hemp warps. Cheaper imported rushes often have hollow thin walls, use synthetic dyes to mimic greenness, and break down into powdery dust within 3 years.
What is 'Ura-gaeshi' and when should it be done?
Ura-gaeshi ('reverse side turning') is the traditional economy practice of un-stitching the tatami-omote after 3 to 5 years, turning it over to expose the unused, un-faded underside, and re-stitching it with fresh borders, doubling the working life of the rush surface.
