Gadenin日本語Get the appWhy this plant has to be divided
Japanese water iris builds itself upward. Each season's new fans form slightly above and outside the previous ones, so the living part of the clump climbs steadily out of the ground while the old center dies away underneath. After three or four seasons you have a ring of fans sitting on top of a dead, woody core, with the feeding roots high and dry near the surface instead of down in moist soil.
The visible symptoms arrive in that order. Flowering thins first, with fewer stems for the same number of fans. Then the middle of the clump goes bare and the fans form a doughnut. Then the whole thing starts drying out between waterings no matter how much you give it, because the roots are no longer in soil that holds water.
None of that is fixable by feeding. It is a structural problem and the fix is structural: lift the clump, cut it up, throw away the old center, and replant young pieces at the proper depth. Doing it on a three or four year cycle keeps the plant permanently in the productive phase. Waiting until it looks bad means a season or two of poor flowers while it recovers.
When to lift, and what condition to lift in
There are two good windows. The first is in the weeks straight after flowering, when the plant is in full growth and roots reestablish quickly; the drawback is the heat, which makes it essential to keep the divisions wet and shaded until they take. The second is early autumn, once the worst heat has passed but with enough of the season left for roots to grow before the ground cools. Both work reliably. Lifting a dormant clump in the depths of winter does not: the cut surfaces sit wet and cold and rot before they can callus.
Do not divide a clump that is under stress. A plant that has just been through a drought, or one showing soft rot at the base, needs to be brought back into good condition first. The exception is a clump that is rotting because it sat in winter water, where lifting is the rescue rather than the risk.
Water the clump heavily the day before you lift it. Wet soil comes away from the roots more easily, and more importantly the plant goes into the operation fully hydrated. These roots have no tolerance for drying and every minute they spend exposed to sun and wind costs you establishment.
| Signal | What it means | Action |
|---|---|---|
| Fewer flower stems per fan than last year | The clump is starting to congest | Plan to divide at the end of this season |
| A bare, dead patch in the middle of the clump | The center has died out under the ring of fans | Divide now, discard the whole center |
| Crown and roots visibly sitting above soil level | The clump has climbed out of the ground | Divide and replant 2 inches (5 cm) deeper |
| Dries out fast however much you water | Roots are above the moisture line | Divide, or at minimum mound acid compost over the crown |
| Three or four seasons since it was last divided | It is due regardless of appearance | Divide on schedule rather than waiting for decline |
| Soft, smelly tissue at the base of the fans | Soft rot, usually after borer damage or winter wet | Lift, cut away all soft tissue, replant only clean pieces |
Dividing a clump, step by step
Have the new planting positions dug and watered before you lift anything. The whole job should take the divisions from soil to soil in minutes, not hours.
- Cut the leaves back first Trim the foliage into fans about 6 to 8 inches (15 to 20 cm) tall before you dig. This cuts water loss while the roots are damaged and makes the clump far easier to handle and see into.
- Lift the whole clump Drive a fork in well outside the fans, all the way round, and lever the clump up whole. Do not try to slice pieces off a clump left in the ground; you cannot see the dead center from above and you will keep the wrong parts.
- Wash the soil off Hose or dunk the root mass until you can see the structure. Until the soil is off you cannot tell live root from dead core, and washing costs nothing while guessing costs you plants.
- Pull it apart into pieces of two or three fans Work the clump apart with your hands where it will come, and use a clean knife where it will not. Each division wants two or three healthy fans with a good handful of white roots attached. Single fans do establish but take an extra year to flower.
- Discard the old center and anything soft Throw away the woody dead core, any piece with no live roots, and any tissue that is brown, soft or smells sour. Cutting back into clean white tissue is the whole point of the exercise.
- Trim the roots to about 6 inches (15 cm) Shorten long trailing roots so they can be spread out in the planting hole rather than folded back on themselves. Never let them dry: keep the divisions in a bucket of water while you work through the clump.
- Replant immediately and deeper than you think Set each division with the crown 2 inches (5 cm) below the surface, spread the roots out over a mound of acid soil, firm in, and water heavily. Space them 18 to 24 inches (45 to 60 cm) apart.
- Keep the new divisions soaking wet For the first four to six weeks the soil must not dry at all, and light shade through the hottest part of the day helps if you divided in warm weather. Expect little or no flowering the following season while the plant rebuilds.
Never let the roots dry out at any point in this process. A division whose roots have gone limp and papery in the sun for twenty minutes may still survive, but it will sit and sulk for a full season. Keep a bucket of water beside you and use it.
Growing from seed, and why the results are unpredictable
Seed is easy to collect and slow to reward. Let two or three pods ripen on the plant until they turn brown and begin to split, then collect the flat brown seeds. Sow them in an acidic seed mix, keep them permanently moist, and give them a cold period over winter outdoors; germination normally comes in spring, sometimes spread over two seasons.
Seedlings take two to three years from germination to their first flower, and they will not resemble the parent. Japanese water iris crosses freely and the offspring are a lottery of color, form and vigor. That is a poor route if you want more of a particular plant, and a genuinely interesting one if you have the space and patience to grow out a batch and keep the best.
Division is the only way to get an exact copy, and it has the added advantage of producing a flowering-sized plant in a single season rather than three. Treat seed as an experiment run alongside the real propagation, not instead of it.
What to record when you divide
Write down the date you divided, how many divisions you got, and where each one went. Then set yourself a note three seasons ahead, because the whole point of a division cycle is that it happens on schedule rather than when the clump has already declined, and three years is long enough to forget.
Record what the center of the clump looked like when you washed it: how much dead core there was, how far the crowns had risen above the original planting depth, and how many fans the clump had reached. Those observations tell you whether your cycle is right for your soil and climate. A clump with a large dead center was left too long; one that came apart easily with almost no waste could have gone another season.