Iris tectorum
Japanese roof iris
Overview
Iris tectorum is a rhizomatous perennial forming fans of foliage 10-16 inches (25-40 cm) tall and clumps 12-18 inches (30-45 cm) wide, spreading by branching rhizomes that sit at or just above the soil surface. Leaves are sword-shaped and prominently ribbed, 10-16 inches (25-40 cm) long and 1-2 inches (2.5-5 cm) wide, light green and thin in texture compared with the stiff glaucous leaves of bearded irises, and die back to the rhizome in winter. Flowering stems carry 2-3 flowers, each 3-4 inches (8-10 cm) across, lilac-blue with darker veining and a fringed white crest spotted purple-blue on each fall; the falls spread almost horizontally rather than reflexing downward. A white-flowered form occurs in cultivation. Bloom runs April to May over 2-3 weeks. Seed capsules are three-angled, 1-1.5 inches (2.5-4 cm) long. Clumps reach mature width in 2-3 years, then thin at the centre as rhizomes crowd. The thin leaves tear in exposed wind and carry fungal leaf spot as brown blotches in humid summers, and surface rhizomes heave out of the soil during freeze-thaw cycles in zones 4-5.
Native Range
Native to central and southwestern China, recorded from Anhui, Zhejiang, Hubei, Hunan, Sichuan, Yunnan, and Guizhou, on grassy slopes, rock crevices, and in open woodland from 1,000 to 8,500 feet (300-2,600 m). Long cultivated in Japan and naturalised there, where plants were grown on the ridges of thatched roofs during periods when arable land was reserved for food crops.Suggested Uses
Planted at 12-15 inch (30-38 cm) spacing in borders, along retaining wall tops, and in rock gardens where the surface rhizomes are not disturbed. Grows in containers of at least 3 gallons (11 L), with division every 2 years as rhizomes reach the pot edge. Winter dormancy leaves bare ground from November to March, and the thin foliage holds less structure through the season than that of bearded irises.How to Identify
Appearance
Size & Dimensions
Height10" - 1'4"
Width/Spread1' - 1'6"
Reaches mature size in approximately 3 years
Bloom Information
Flowers open April to May in zones 5-8, and in late March in zone 9. Each flower lasts 2-3 days and a clump flowers over 2-3 weeks. Flower numbers drop in the season following division and return the second year. Temperatures above 85F (29C) during bud development shorten individual flower life to about one day.
Detailed Descriptions
Flower Description
lilac-blue with darker veining and a white crest spotted purple-blueFoliage Description
light greenGrowing Conditions
Sun Requirements
Requires 4-8 hours of direct sunlight daily
• Full Sun: 6+ hours of direct sunlight
• Partial Shade: 3-6 hours of direct sunlight
• Full Shade: Less than 3 hours of direct sunlight
Care & Maintenance
Care Guide
Water 1 inch (2.5 cm) per week through the growing season; the thin leaves brown at the tips within a week of the soil drying. Rhizomes are set with the upper surface exposed, since burial below 1 inch (2.5 cm) of soil leads to rot in wet winters. Growth continues in soils from sand to loam at pH 6.0-7.5 with steady summer moisture and free winter drainage. Fungal leaf spot produces brown blotches with yellow margins in humid weather above 75F (24C), and clearing dead foliage in autumn reduces carryover to the next season. Iris borer larvae tunnel into rhizomes in eastern North America, entering through the leaf bases in late spring. Division every 3-4 years after flowering, cutting rhizomes into pieces carrying one fan and healthy roots, holds flower numbers steady.Pruning
Spent flower stems are cut to the base after the last flower fades, which stops seed capsule development where self-sown seedlings are not wanted. Browned leaf tips and spotted leaves can be trimmed at any point in the season. Foliage is cut to 2 inches (5 cm) after frost, or left standing over winter and cleared in early spring. Rhizomes lifted and divided in June or July re-root before autumn.Pruning Schedule
J
F
M
A
M
J
J
A
S
O
N
D
summerfall
Container Growing
✓ Suitable for container growing
Minimum container size: 3 gallons