The Sorbonne Iris (Iris germanica ‘Sorbonne’) is a striking and popular addition to many gardens, known for its distinctive beauty and relatively straightforward care. Gardeners often select this variety for its unique color patterns and dramatic presence in floral displays. It is a favored choice for those seeking a visually impactful perennial.
Identifying Sorbonne Iris
This Tall Bearded Iris reaches heights of up to 38 inches (97 cm) with clumps spreading 12 to 18 inches wide. Its blooms are large and intensely ruffled, earning it an Award of Merit from the American Iris Society. The complex coloration features deep, almost black-red standards, sometimes with yellow speckles. The intricate falls display beaming yellow and white dappled blazes with deep brown-to-black margins and reddish-brown specks. The beard, a fuzzy line on the lower petals, is brown with subtle white tips. It is not typically fragrant. Sorbonne Iris generally blooms mid-to-late spring, with some varieties reblooming late summer to early fall.
Planting and Site Selection
Sorbonne Iris thrives in full sun, requiring at least six to eight hours of direct sunlight daily for abundant blooming. Insufficient light can lead to poor flowering.
Well-draining soil is crucial, as soggy conditions can cause rhizome rot. They prefer slightly acidic to neutral soil (pH 6.1-7.5) but tolerate various soil types, provided drainage is adequate. To prepare the soil, loosen it to a depth of 12 to 15 inches and mix in compost or aged manure to improve fertility and drainage.
When planting rhizomes, create a small soil mound in the hole’s center, place the rhizome on top, and spread its roots down the sides. The top half of the rhizome must remain exposed to the sun, as this exposure is essential for flower cell development. Plant individual rhizomes or groups 18 to 24 inches apart to ensure proper air circulation and prevent overcrowding. The best time for planting is late summer to early fall, typically August to October, allowing roots to establish before winter.
Ongoing Care for Sorbonne Iris
Established irises are drought-tolerant, but moderate moisture is important, especially during their first year and dry periods. Water thoroughly every 7 to 10 days during dry spells, avoiding overwatering to prevent rot.
Fertilization supports robust growth and flowering. Avoid excessive nitrogen, which promotes leafy growth at the expense of blooms and increases rot susceptibility. A low-nitrogen, balanced fertilizer (e.g., 6-10-10 or 5-10-10) is recommended. Apply fertilizer once per year, either about a month before spring bloom or one month after blooms finish. Scatter it around plants and water it in, taking care not to place it directly on the rhizomes.
Deadheading spent blooms encourages rhizome development rather than seed production, potentially promoting new growth or reblooming. Cut flower stems to their base after blooming, but leave the sword-shaped foliage intact. This foliage continues to photosynthesize and nourish the rhizome for the following season’s blooms. Dividing overcrowded clumps every three to four years maintains vigor and prevents reduced blooming. This process, ideally performed in late summer after flowering, involves lifting the entire clump, cutting away healthy outer rhizomes with leaves, and discarding older, depleted sections.
Troubleshooting Common Issues
Bacterial soft rot, characterized by mushy, foul-smelling rhizomes and yellowing leaves, is often linked to excessive moisture, poor drainage, or too much nitrogen. If soft rot occurs, carefully remove all affected, soft tissue from the rhizome with a clean tool. Then, expose the cut area to air and sun to dry, or dust with powdered sulfur. Disinfecting tools between cuts prevents further spread.
Iris borers are destructive pests that can lead to bacterial soft rot by tunneling into rhizomes. Signs include dark streaks or ragged edges on young leaves. Eggs overwinter on dead leaves, so removing and discarding plant debris in the fall helps control their population. Larvae can be crushed by hand within the leaves in spring, and severely infested rhizomes should be discarded or carefully cleaned and replanted.
Leaf spot, a fungal disease, appears as small brown spots that enlarge and can cause leaves to die back, especially in wet, humid conditions. To manage leaf spot, remove infected foliage promptly and ensure good air circulation by proper plant spacing. Avoid overhead watering, as splashing water can spread spores. Removing all dead debris in the fall is a preventative measure since the fungus overwinters on plant material.
Lack of blooming can stem from insufficient sunlight, overcrowding, or planting the rhizomes too deeply. Ensure plants receive adequate sun exposure and have sufficient space to prevent shading. Confirm that the rhizome’s top half is exposed to the sun after planting. Yellowing leaves, if not associated with rot, can be a sign of transplant shock, especially after initial planting, and often resolve as the plant establishes.