Iris tectorum aka Shenan

Iris tectorum is a perennial in the iris family. It comes from China and grows from spreading rhizomes. Gardeners call it roof iris or Japanese roof iris. These names recall its past use on thatched roofs. People grew it this way in China and Japan. Wild plants grow on rocky slopes and woodland edges. They also occur in meadows, scrub, and damp sites.

Plants form upright clumps of leaves in broad fans. Most reach 1–1.5 feet [0.30–0.46 m] tall. RHS gives a higher range, reaching 3.3 feet [1 m]. Clumps spread 1–3 feet [0.30–0.91 m] across, depending on conditions. The narrow green leaves have clear ribs and smooth edges. They reach 1–1.5 feet [0.30–0.46 m] long. Young leaves stand upright, then may bend or droop. Sources describe the foliage as deciduous or partly evergreen. This herbaceous plant has no woody trunk or bark.

Flowers open in late spring, from May into June. Each bloom spans about 0.5 feet [0.15 m]. Blue, lilac, or purple shades mark the flowers. Dark streaks or blotches add contrast to the petals. Three short, upright standards rise above three broad falls. The falls carry white crests rather than beards. Ruffled edges give the blooms a soft outline. Small capsules follow the flowers but add little garden interest.

Grow roof iris in rich soil that drains well. It accepts sand, loam, and clay, with acid to neutral pH. Full sun and partial shade both suit it. In full sun, keep the soil moist. Plants in shade can cope with drier soil. A sheltered spot near a wall suits this iris. Plant rhizomes shallowly and mulch them through winter. Sources rate it for USDA zones 4–9 or 6–10. RHS rates its cold hardiness as H6. Use it along borders, near ponds, or in rock gardens. Its spreading clumps can form ground cover or broad drifts. The flowers also suit cutting, and deer tend to avoid it.

Slugs, snails, and thrips may damage the plants. Viruses can stain leaves; soft rot and grey mould also occur. Divide clumps to make more plants. Remove old flower stems in summer and dying leaves in autumn. The plant harms people and pets when eaten. Roots, seeds, and sap can also irritate skin. Reported root starch use does not make raw roots safe. Evidence lists no known medicinal use.