Gadenin日本語Get the appPrune once growth restarts, not in the cold
The right time is after the worst of the cold has passed and before the new growth flush extends, when the tree is about to start moving. Cuts made then callus quickly and the tree covers the wound with the first flush. Cuts made in cold weather sit open, and citrus wood is prone to dieback from an open cut in the wet and cold.
There is a second, much lighter opportunity straight after picking, but limit it to removing broken wood and obvious crossing branches. Anything structural done late pushes soft growth that will not harden in time.
Never take out a large limb and a heavy thinning in the same year. Kabosu responds to hard cutting with a mass of vertical water shoots covered in the longest thorns the tree makes, and you spend the next two seasons removing them.
Work in this order
Every cut on a citrus should either let light in or bring the crop within reach. Working through in a fixed order stops you standing back and taking out too much.
- Suckers from below the graft Anything growing from the rootstock, below the graft union, comes off flush at its base first, every time. Rootstock growth is more vigorous than the top and will take the tree over if you leave it a season. Rub it off with a thumb while it is soft rather than cutting it later.
- Dead, damaged and crossing wood Take dead wood back to live green wood, and where two branches rub, remove the weaker one. Rubbed bark is the commonest entry point for the canker-type problems that spoil citrus fruit skin.
- Vertical water shoots Straight vertical shoots inside the canopy do not fruit and shade everything below them. Remove them at their point of origin. Leaving a stub simply produces three more shoots from the same place next year.
- Open the center Thin the middle until you can see sky through the canopy in patches and could put a hand through in several places. Fruit only sets and colors where light lands; a dense kabosu carries its whole crop as a shell on the outside.
- Reduce the height last Take the leaders back to a lower outward-facing side branch rather than cutting straight across. Aim to keep the top of the tree at a height you can pick from a three-step ladder, and take the reduction over two or three years rather than all at once.
Height is the decision that matters
This is the one place kabosu differs sharply from a small sour citrus. Left alone it goes up, and a fruit at fourteen feet (4.2 m) is a fruit you will not pick. Unpicked fruit stays on, turns yellow, and loses both its acidity and its aroma, so height that you cannot reach is not just inconvenient, it costs you the crop.
Decide your working height early, in the third or fourth year, and hold the tree there. Ten feet (3 m) is a sensible ceiling for most gardens. Holding a young tree at a height by shortening its leaders each year is easy; bringing a mature sixteen-foot tree down to ten is a two- or three-year project that costs you fruit while it happens.
| Tree age | Main aim | Roughly how much to remove |
|---|---|---|
| Year 1 | Nothing but damaged wood | Under 5 percent |
| Years 2-3 | Choose 3-4 well spaced main branches | 10-15 percent |
| Years 4-6 | Set the working height, open the center | 15-20 percent |
| Mature | Hold height, thin, remove water shoots | Up to 20 percent a year |
| Neglected and overgrown | Reduce in stages | No more than 25 percent in any one year |
Handling the thorns
The thorns on kabosu are woody, up to two inches (5 cm) long, and stiff enough to go through a glove and a sleeve. They are worst on young vigorous wood, which is exactly the wood you are cutting. Puncture wounds from citrus thorns often carry bark bacteria into the wound and go septic more readily than a clean cut, so this is not a matter of toughing it out.
Wear a long sleeve of thick material and gauntlet gloves that cover the forearm, and use loppers or a pruning saw on a handle to keep your arms out of the canopy. Cut a branch out in short sections from the tip inward rather than pulling a whole thorned branch through the tree at you.
Rake up every piece you drop before you move around the tree. Thorns in the grass find bare feet and bicycle tires for years, and they do not rot away quickly.
Photograph the tree from the same spot each year just after pruning. Season to season the shape drifts without you noticing, and a run of photographs shows you whether you are actually holding the height you decided on or quietly losing a foot a year.