Gadenin日本語Get the appThe job for the first several years is size, not shoots
A koshiabura is picked by removing the terminal bud from a stem, which is a real cost to the tree. It can only carry that cost once it has enough leaf area and enough stored reserves to rebuild. Until then, every year of care is aimed at one thing: getting a stem tall enough and thick enough that losing its growing point in spring is a setback rather than a wound it cannot recover from.
The working threshold most growers use is a stem at least an inch (2.5 cm) thick at the base and at least six feet (1.8 m) tall, on a tree that made strong extension growth the previous summer. Below that, leave it. A young tree picked once will often survive; a young tree picked in consecutive years usually will not.
Never pick from a tree in its first three or four years in the ground, however tempting the bud looks. The plants that die in gardens are almost always young ones that were picked too early, not old ones that were picked too hard.
Water while it establishes, then mostly leave it
The critical period is the first two or three summers. Koshiabura roots are shallow and the tree comes from ground that stays cool and moist under leaf litter, so a young plant in an exposed garden dries out much faster than its natural habitat would suggest. Once established the tree is far more self-sufficient, and constant watering after that does more harm than good on heavy soils.
| Stage | Watering | Mulch and feeding |
|---|---|---|
| First growing season | A thorough soak weekly in dry spells, more in heat above 85F (29C) | Renew leaf mold each spring, keep the stem clear |
| Second and third seasons | Soak during dry spells of more than ten days | Top up mulch in autumn as it breaks down |
| Established, not yet picked | Only in prolonged drought or scorching heat | Mulch in autumn, no fertilizer needed on decent soil |
| Established, in production | Water through a dry late spring so the tree recovers after picking | Leaf mold in autumn, light feed only if growth is weak |
Shade, and how it changes as the tree grows
Young koshiabura are far more sensitive to sun and drying wind than older ones. A plant that is happy under fairly deep shade for its first few seasons will often need a little more light later, when it is tall enough to compete and its roots reach cooler soil. If a young plant is scorching in a site you thought was shaded, look at where the sun actually falls between midday and mid-afternoon in high summer, not at how the spot looks in spring before the canopy closes.
If the surrounding trees are growing too, the reverse problem creeps up on you: a site that was dappled when you planted becomes deep shade a decade later, and the koshiabura stops extending, holds thin pale leaves and produces small buds. Thinning a branch or two out of the canopy above it, in winter, is usually all the correction it needs.
Feeding, and why less is right
On soil with reasonable organic matter, koshiabura needs no fertilizer at all. An annual autumn mulch of leaf mold feeds it in the way it is adapted to be fed: slowly, from the surface, as litter breaks down. That is the default and it works.
Where growth is genuinely weak, with less than four inches (10 cm) of extension on the leader in a season and small pale leaves, a light dressing of a balanced general fertilizer at bud break is reasonable, applied over the root zone and watered in. Do not push it with high-nitrogen feed. Soft, sappy growth on this tree is more prone to dieback at the cut after picking and more attractive to aphids, and it does not shorten the wait for a first harvest.
Light shaping, and what never to cut
Koshiabura needs very little pruning. Take out dead, broken, crossing or rubbing wood in winter while the tree is bare, and remove anything growing into a path or across your access. That is the whole routine in most years.
The one rule that overrides everything is to stop all cutting as soon as the buds begin to swell in late winter. Every stem tip you shorten from then on is a shoot you will not eat, and it takes years for a tree to replace the picking positions you removed. If you want to encourage more pickable stems, do it by keeping the tree in good light and good heart rather than by heading back stems, which on this species tends to produce weak, spindly regrowth.
The records that tell you when to start
Once a year at leaf fall, measure and write down three things for each tree: the total height, the extension growth the leader made that season, and the number of stems that meet your picking threshold of an inch (2.5 cm) at the base and six feet (1.8 m) tall. It takes two minutes and it turns a vague sense that the tree is getting on into a decision you can actually make.
Keep a note of drought weeks and any scorch you saw as well. Read across several years, that record tells you whether a slow tree is slow because the species is slow or because the site is too dry, too dark or too bright, which is the difference between waiting and moving on to a better spot while the plant is still small enough to move.