Growing & Caring For Iris

iris beardedAt last count, there were more than 200 different species of Iris plus thousands of colorful varieties. For most of us, when we say Iris, we are thinking of the Bearded or German Iris with which we grew up in grandma’s garden. These are the tall flowering plants, often with blue or violet flowers and fuzzy growth on the three petals (actually sepals) which hang down in front. The fuzzy growth gave them the name ‘Bearded’ Iris. All the rest of the Iris are referred to as ‘Beardless.’

Bearded Iris now come in many colors and even combinations of colors; all except scarlet red and bright orange. They bloom for about three to four weeks usually in May or June. Some Iris bloom a bit earlier or later than others and so by planting these different varieties, the bloom season can be extended.

Iris need at least a half day of sun but they grow best in full sun. They must have well-drained soil and so it is essential that you prepare our heavy clay soil with organic matter and gypsum (see our Soils-Its Care and Improvement Care Guide). Master Nursery Camellia, Azalea and Rhododendron Fertilizer is recommended because it contains Sulfur to lower soil and water pH which the Iris need. This fertilizer should be applied around Valentine’s Day and the 4th of July.

iris depthIris ‘bulbs’ are really rhizomes; swollen plant stems which grow along the soil surface or slightly below the soil surface. The leaves and flower stems grow from the forward tip of the rhizome. As the plant matures, the rhizome continues to grow forward in the direction of the tip. The old leaves die and new leaves form at the tip. After one or two years, the rhizome develops side branches which grow and behave as the original rhizome did. Within four to five years, there are rhizomes and rhizome branches growing in all directions and it becomes necessary to divide them. In the Bay Area, division is best done in September or October. Prior to this time, when the foliage has yellowed, it is cut back to five or six inches long.

After dividing the Iris rhizomes, they are planted in the well prepared soil. The whole rhizome must not be covered with soil. The top of the rhizome remains above the soil and the trimmed roots, 3 to 4 inches long, are spread out flat, perpendicular to the rhizome and coved with soil. The Iris are generally planted 12 to 24 inches apart and watered thoroughly. During the first year keep the soil moist so the roots can become established. Later, watering every two or three weeks should be sufficient. Bulb rot is one of the greatest dangers for Iris and so we do not recommend mulching which covers the rhizomes and holds moisture which will contribute to rot.