Gadenin日本語Get the appWhat the plant is doing once the flowers are over
A Japanese water iris flower lasts two or three days, and a stem carries two or three of them in succession, so a clump is in flower for perhaps two weeks. What follows is the most productive part of its year. Roots are running hard, new fans are expanding at the edge of the clump, and inside those fans the buds that will flower next season are being formed. Almost everything you can do to influence next year's display happens now.
That is why the standard mistake, cutting the whole clump down once the flowers finish, is so expensive. The foliage looks scruffy and the temptation to tidy is strong, but a clump cut back to the ground in mid season has to rebuild its leaves out of stored reserves rather than putting those reserves into buds. It will flower, but noticeably less, and the effect compounds if you do it every year.
The correct treatment is short: take off what has flowered, leave what is still green, feed once, and keep the water up until the leaves yellow of their own accord.
Deadheading, and where to cut the stem
Remove each flower as it goes over, snapping the spent bloom off cleanly where it joins the stem. Faded Japanese iris flowers turn into a wet, translucent mass that clings to the stem and sits against the next bud in the sheath, and that damp tissue is where botrytis starts. Taking them off is worth doing every couple of days while the clump is in flower, not just once at the end.
Once every bud on a stem has opened and finished, cut that whole flower stem down to the base, following it into the clump with your fingers and cutting where it leaves the crown. Leave no stub. Stubs die back slowly, stay damp at ground level and give soft rot an entry point straight into the crown.
Unless you are deliberately raising seedlings, do not let seed pods develop. The pods are fat, three-sided capsules that swell quickly after flowering, and ripening a full crop of them draws heavily on the same reserves that would otherwise go into next year's buds. If you do want seed, leave two or three pods on the best plant and cut the rest away.
| What you are looking at | Do this | Why |
|---|---|---|
| A flower that has just gone over | Snap it off where it meets the stem | Wet dead petals against the next bud start botrytis |
| A stem with all its buds finished | Cut it to the base with no stub | Stubs rot at ground level and let soft rot into the crown |
| A swelling seed pod | Cut it off unless you want seed | Ripening seed costs you flower buds for next year |
| Green leaves looking untidy | Leave every one of them | They are building next season's buds right now |
| A single leaf gone fully brown | Pull or cut that leaf out | Dead tissue in the fan holds damp and hides pests |
| A clump that flowered poorly this year | Mark it for division rather than more feed | Declining flowers are a congestion problem, not a hunger one |
The one feed that matters most
Feed the clump within a couple of weeks of the last flower fading, using a fertilizer for acid-loving plants. This is the feed that pays. The plant is actively building buds, the roots are working, and the nutrients go straight into next season's display rather than into a flush of soft growth.
Spread it over the whole root area, not just against the crown, because the feeding roots run well out beyond the fans. Water it in thoroughly. If the clump is in a container, a liquid acid feed at every second watering for a month does the same job and suits the smaller root volume better.
Keep the water up through this period as well. It is easy to relax about watering once the show is over, but a clump that goes dry in the weeks after flowering makes fewer and weaker buds, and the damage does not show until the following year when you have long forgotten the dry spell that caused it.
Letting the foliage die back on its own
Through the second half of the season the leaves stay green and do their work, then yellow from the tips as the plant shuts down. That yellowing is the plant withdrawing everything useful from the leaves back into the roots, and interrupting it wastes what it was reclaiming. Let it run its course.
Reduce watering as the leaves go over. The plant does not need bog conditions once it has stopped growing, and easing off now is the beginning of the drier dormancy it must have. A pot standing in a water tray comes out of the tray at this point.
Once the foliage is properly brown and collapsed, cut the whole clump down to about 2 inches (5 cm) and rake every fragment of leaf out of the bed. Burn it or bin it rather than composting it. Iris borer eggs overwinter in dead iris foliage and in debris around the crown, and a thorough clean-up now is worth more than any spray next season.
- Deadhead as the flowers go over Every two or three days through the flowering period, snap off finished blooms so wet petals are not sitting against unopened buds.
- Cut spent stems to the base Once a whole stem has finished, follow it down into the clump and cut it flush with the crown, leaving no stub behind.
- Feed with an acid fertilizer Within two weeks of the last flower, spread a feed formulated for acid-loving plants over the whole root area and water it in well.
- Keep watering until the leaves turn Deep soaks continue right through the rest of the growing season. Ease off only as the foliage begins to yellow on its own.
- Mark any clump that flowered badly Put a stake or a note against clumps with few flowers per fan. Those are the ones to lift and divide, and you will not recognize them once the foliage is gone.
- Clear all dead foliage at the end of the season Cut the browned leaves to 2 inches (5 cm) and remove every scrap from the bed. This is the main defense against borers, and it only works if it is thorough.
What to record while you still remember it
Write down the date the first flower opened, the date the last one faded, and how many flower stems the clump carried. Divide that stem count by the number of fans in the clump and you have the one number that predicts trouble: flowers per fan falls off a season or two before a congested clump looks congested from above.
Note the date you fed after flowering, and whether the clump went through a dry spell at any point while it was in bud. Add the position of each clump in the bed, because the foliage is about to vanish and dormant crowns are invisible. Next season those notes let you tell the difference between a clump that needs dividing and one that simply got thirsty at the wrong moment.