How to Plant Siberian and Japanese Iris

Plant Siberian and Japanese iris with the rhizome buried, not on the surface like bearded iris: 1 inch deep for Siberian and blue flag, 2 inches for Japanese. Keep them well watered until they are growing strongly.

How to Plant Siberian and Japanese Iris
When to plant
Spring once the soil can be worked, or August through October at least 4 weeks before your first hard freeze
Depth
Siberian and blue flag iris: rhizome 1 inch deep, a little deeper in sandy soil. Japanese iris: about 2 inches deep
Spacing
Siberian: 18–24 inches apart. Japanese: 12–18 inches, or 18–24 inches to delay crowding
Sun
At least 6 hours of direct sun; midday shade in hot southern areas
Water
At least 1 inch a week after planting; Japanese iris want 1–2 inches a week and damp soil year-round
Soil
Rich in organic matter. Siberian: pH 6.0–7.0. Japanese: acidic, pH 5.0–6.5, no lime

Step by step

  1. Check your label. Siberian and Japanese iris are planted differently from bearded iris; their rhizomes go under the soil, not on top of it.
  2. Choose a sunny spot with at least six hours of sun, with rich soil that stays evenly moist.
  3. For Japanese iris, the soil must be acidic. Work in compost or peat moss, and never add lime or bone meal.
  4. Plant Siberian and blue flag iris with the rhizome 1 inch below the surface, a little deeper in sandy soil. Plant Japanese iris about 2 inches deep, because they grow new roots on top of the old ones.
  5. Firm the soil around the roots and water thoroughly.
  6. Keep new plantings well watered at all times, at least an inch a week, until they show strong new growth. These should never dry out, even though bearded iris can.
  7. Blue flag and 'Gerald Darby' iris can grow at a pond edge in up to 4 inches of standing water. Japanese iris like wet feet in summer but should not sit in water that freezes over in winter.

After bloom

Remove spent flower stalks to stop reseeding. Feed with a balanced fertilizer such as 10-10-10 in early spring and again after bloom. Leave Siberian foliage until it browns and withers in late fall, then cut it back. Cut blue flag foliage to just above the crown after the first frost. Divide Japanese iris every 3–4 years, and Siberian iris only when clumps get crowded or the center dies out.