Blue Flag Iris

Fans that yellow and brown from the base and lift away with a gentle tug, and a rhizome that is soft, slimy, and yellowish-brown inside with an unmistakable foul smell. The rot travels along the rhizome from the wounded end.

Cut or scoop the rot back to firm white tissue with a clean knife, disinfecting the blade between cuts, and let the wound dry open in the sun; a dip or swab of 10% bleach on the cut surface helps. Dig and destroy rhizomes that are hollowed or rotted through - do not compost or replant them - and fix the drainage, or the water flow, before replanting the spot.

On bearded iris, plant SHALLOW with the rhizome top at the surface, in sharp drainage and full sun, never mulched over, and feed low-nitrogen (bone meal or 5-10-10, never a lush lawn feed). On water and bog iris, keep the water moving or refreshed rather than stagnant. For all of them: space clumps for airflow, avoid nicking rhizomes with a spade or hoe, divide crowded clumps in mid-to-late summer when plants are semi-dormant, control iris borer as the main wound route, and clear old foliage and debris from the bed each fall.

The disease that actually kills iris. Erwinia (Pectobacterium) carotovorum bacteria enter through any wound or through tissue kept wet, then dissolve the rhizome into a foul-smelling yellow mush that spreads through a clump. Iris-borer tunnels are the classic entry point, but rot arrives on its own too - on bearded iris from deep planting, soggy ground, heavy nitrogen, winter injury, or a careless spade; on water and bog iris from stagnant, oxygen-poor water and overcrowded clumps. No borers required either way.

Ragged, water-soaked notches and pinholes on the leaf fans, sawdust-like frass, slime, and streaking; fat pinkish caterpillars inside; and hollowed, mushy, foul-smelling rhizomes when you dig.

Squash larvae by pinching infested leaves early in the season; dig, cut out, and destroy borer-tunneled and rotted rhizomes (do not replant them). Systemic or spinosad treatments help under heavy pressure - time them to egg hatch in spring.

Cut down and DESTROY all old iris foliage in late fall and early spring to kill overwintering eggs - the single most effective control; keep the bed clean; inspect and squash larvae in the fans early.

The signature iris pest: the caterpillar of a night-flying moth. Eggs overwinter on old iris foliage; larvae hatch in spring, tunnel down through the leaf fans, and bore into the rhizome, hollowing it out. The wounds they leave open the door to devastating bacterial soft rot - borer and rot together are the classic clump-killer.

Blue flag is a water iris and, unlike its bearded relatives, has no drought tolerance at all. Grow it in reliably wet soil or a shallow pond margin, and keep the water level up through summer rather than letting it drop.

Wilting on hot afternoons, browning and crisping along leaf edges and tips, premature leaf drop, collapsed flower stems, and thin, stunted growth - showing up first on young plants and on the driest, most exposed side of the planting.

Water deeply and consistently, then mulch. Mild wilting reverses within a day or two, but a prolonged drought sets a plant back hard and can kill a young one outright. If a plant scorches every summer, the site is wrong - move it to damper ground rather than trying to water it through.

Match the site to the plant: put a moisture-loving species where the ground actually stays damp - a rain garden, pond edge, ditch, seep or low spot - rather than in an ordinary border you intend to keep watered. Mulch to hold moisture and keep the roots cool, water deeply through every dry spell while the plant is establishing, and avoid hot, windy, exposed positions.

Not a pest but a siting problem, and on a moisture-loving plant it is usually the thing that actually kills it. When the soil dries out, leaf edges brown and crisp, foliage wilts and drops early, flowering and fall color are cut short, and growth thins. Hot sun, wind and exposed positions make it worse by pulling water out faster than the roots can replace it. A plant that wants reliably moist or wet ground has no drought tolerance to fall back on, and young or recently planted ones have the least of all.