Broussonetia kazinoki Seeds | Kozo
LATIN NAME: Broussonetia kazinoki
COMMON NAME: Kozo
Plant Profile
The plant is a deciduous shrub, usually two to four metres tall, with a spreading habit and a tendency to sucker freely from the roots. Leaves are broad and rough-textured, coarsely toothed, and strongly variable in shape: leaves on vigorous young shoots are often lobed while those on flowering wood are entire. Rough, scabrid hairs on the leaf surface are a family habit shared with the mulberries.
Broussonetia papyrifera is the taller, more aggressive species, with larger leaves and a wider naturalised range across the tropics; the kazinoki is the smaller, more manageable plant, and in the paper villages of Japan and Korea the two are grown side by side, with hybrids between them also planted for fibre. Molecular work has pointed to the paper mulberry being of hybrid origin involving a kazinoki-like parent, which is why the two are so hard to separate on leaf characters alone.
The harvest is a winter job, done when the stems are leafless and the sap is down. Nothing in the sequence is complicated, but each step decides how white and how strong the finished sheet will be.
The scraping is the step that takes practice: too little and the dark outer tissue shows in the sheet as brown flecks, too much and the long fibres are cut and the paper loses strength. Fine, white, translucent sheets depend on stripping long ribbons in one pull and scraping them on a flat board with a sharp blade held almost flat.
Science & Biotechnology
The root bark of this species yields a group of prenylated flavonoids that carry the plant’s own name: the kazinols, compounds built on a flavanone or chalcone core with an extra isoprenyl side chain. Kazinol A and the related kazinols B to F were characterised from the root bark, and the family has since grown to include many more members.
Alongside the kazinols sit the broussonins, another set of prenylated phenolics named after the genus; together they form the chemical signature of the root bark used in East Asian medicine. Prenylation changes how a flavonoid sits in a membrane and how it interacts with enzymes.
Uses & Benefits
The flowers are tiny and crowded into separate unisexual heads, the male heads long and catkin-like, the female heads rounded. These ripen into a compound fruit — a syncarp — made of many small fleshy segments fused around a central axis, turning orange-red and then dark, sweet and edible, and spread by birds that take the fruit and drop the seed.
The useful fibre is a bast fibre, the same class as flax and hemp: long, thin-walled cells rich in cellulose and bound by pectin, lignin and hemicellulose. Cooking the stripped bark in an alkaline liquor, traditionally wood ash lye, dissolves the pectin and much of the hemicellulose and frees the fibres, so that beating separates them into a pulp rather than chopping them short.
Growing from Seed
Seed is best gathered when the fruit colours and softens, then mashed and washed to separate it from the pulp. Sow shallowly in a fine, moist seedbed and keep it shaded; seedlings establish quickly, and the plant can also be raised from softwood cuttings or rooted suckers.
Fibre quality depends on long, unbranched stems, so the stools are cut to the ground every winter and allowed to throw up fresh canes through the following season. Plant on 60 to 90 cm centres where a harvesting block is wanted, and feed with compost each spring since annual coppicing removes a great deal of material.
The shrub spreads by suckers and can become a nuisance in a small garden. Scale insects and mealybugs gather on young stems in sheltered positions, and the roots are shallow enough to be damaged by deep cultivation around the stools.
Our Take
If you want to compare the two fibre plants, the taller paper mulberry grows faster and yields more bark per stem, while paper mulberry seed lots from cooler districts are usually the more manageable kazinoki type. For fruit rather than fibre, the sweet mulberry of the same family is the better garden plant.
Related Reading
Gardeners who grow Kozo often pair it with these related subjects: