Gadenin日本語Get the appWinter wet is what actually kills these plants
The great irony of a bog iris is that drowning is the commonest cause of death. Through the growing season the plant can stand water over its crown indefinitely, because it is actively pumping water up through its leaves. Once the foliage dies back that stops, and a dormant crown sitting in cold standing water simply rots. The gardener sees nothing until spring, when the clump that was thriving in summer never reappears.
The symptom, if you go looking in late winter, is a crown that has gone soft and brown and pulls apart with no resistance, often with a sour smell. There is no treatment at that stage. Cut back to clean white tissue if any remains and replant it somewhere that drains, and accept the loss if none does.
Prevention is entirely a siting decision. In a pond or bog garden, plant on a shelf or a raised pocket so the winter water line sits below the crown. In a border, make sure the low spot that stays wet all summer actually drains after autumn rain rather than holding a puddle. In a container, take the pot out of its water tray as the leaves yellow and stand it where it drains freely.
Iris borer, and why autumn clean-up is the whole control
Where iris borer is present it is the most destructive pest this plant has. The adult is a night-flying moth that lays eggs in autumn on dead iris leaves and debris around the clump. The eggs sit there all winter. In spring the tiny caterpillars hatch, climb the new leaves and tunnel into them, working down inside the leaf to the crown, where they hollow it out and open the way for bacterial soft rot to finish the job.
The early sign is a dark, water-soaked streak running down a young leaf, sometimes with a ragged notch at the edge where the caterpillar entered. Later the leaf goes limp and yellow and slides out of the fan, and the base of the fan feels soft and smells foul. By the time the smell is there the crown is already badly damaged.
Control is almost entirely sanitation, and it is remarkably effective. Cut all foliage down at the end of the season once it has died back, rake out every fragment of leaf and debris from around the crowns, and destroy it rather than composting it. That removes the overwintering eggs. Through spring, if you see a streaked leaf, squeeze it firmly between finger and thumb from the streak downward, which crushes the caterpillar inside, or pull the leaf out entirely.
| Symptom | Likely cause | What to do |
|---|---|---|
| Yellow leaves with darker green veins, worst on new growth | Iron chlorosis from soil that is not acid enough | Water in chelated iron now, then correct the pH; never lime |
| Dark water-soaked streaks down young leaves | Iris borer caterpillars tunneling inside | Crush the leaf along the streak or pull it out; clear all debris next autumn |
| Soft, foul-smelling base to a fan | Bacterial soft rot, usually after borer damage | Lift, cut away every soft part back to clean tissue, replant in fresh soil |
| Clump vanishes over winter and never returns | Crown rot from standing water in dormancy | Replant survivors above the winter water line |
| Brown spots with a yellow halo, spreading in wet weather | Fungal leaf spot | Remove affected leaves, improve air movement, clear debris in autumn |
| Brown, papery leaf tips on green leaves | The plant went short of water | Water more deeply and more often; this plant has no drought tolerance |
| Silvery streaking and distorted new growth | Thrips feeding in the tight fans | Hose the fans out, remove badly affected leaves, tolerate low numbers |
| Ragged holes and slime trails on new shoots | Slugs and snails on soft spring growth | Trap or pick at night while the shoots are short |
Yellow leaves, and telling the causes apart
Yellowing is the symptom gardeners report most and it has four separate causes that need four different responses. The pattern on the leaf tells you which one you have, so look at it closely before doing anything.
Yellow between the veins with the veins themselves staying green, worst on the youngest leaves, is iron chlorosis caused by alkaline soil. Uniform pale yellow across older leaves first, on a plant growing steadily, is usually nitrogen hunger and responds to an acid feed. Yellowing that starts at the tips and works down, with the tissue going brown and papery, is drought. Yellowing of a whole fan that then goes limp, with a soft base, is rot or borer damage and the fan will not recover.
The one to rule out first is chlorosis, because it is the one people misdiagnose as hunger and then make worse by piling on general fertilizer. If you have limed the bed, used bone meal, planted near fresh concrete or watered with hard water for a long time, assume chlorosis until proven otherwise.
Leaf spot, thrips and the smaller nuisances
Fungal leaf spot appears as small brown spots with yellow margins that enlarge and run together, mostly on older leaves and mostly in warm, wet, still weather. It rarely kills a healthy clump but it does shorten the working life of the foliage, which costs flower buds. Pick off badly spotted leaves, keep the clump open by dividing on schedule, and clear all dead foliage at the end of the season, since the fungus overwinters on it.
Thrips hide deep in the tight fans and rasp at the leaf surface, leaving silvery streaks and puckered new growth. Populations build in hot dry spells, which is another reason to keep this plant properly wet. A forceful jet of water directed down into the fans knocks them back and costs nothing.
Slugs and snails go for the soft new shoots in spring and can shred a fan before it is a hand high, particularly in the damp conditions this plant demands. Deal with them while the growth is short; once the leaves are tall the damage is cosmetic. Deer and rabbits generally leave iris alone, which is one of the few things in this plant's favor.
Do not reach for a fungicide for yellow leaves. In this plant the great majority of yellowing is a soil pH problem, a watering problem or a rot problem, and none of the three responds to spraying. Diagnose the pattern on the leaf first.
What to record when something goes wrong
Note the date you first saw a symptom, which clump it was on, and what the weather had been doing for the fortnight before. Borer streaks, leaf spot and rot all follow predictable conditions, and after two seasons of notes you will know roughly when in your own garden each one turns up, which lets you act early instead of reacting.
Record what you did and whether it worked, and specifically record the date you cleared the dead foliage each autumn. Borer damage the following spring is directly related to how thoroughly and how early that clean-up was done, and it is the only way to prove to yourself that the job is worth doing properly. Add a note of any clump you lost over winter and exactly where it was standing, because that spot will drown the next plant too.