Siberian Iris info - Knowledgebase Question

kalamazoo, MI
Question by dfgquilter
June 5, 2008
I have several Siberian Irises planted. They have been there 3 years and they don't get buds and bloom.
Do they have to be at a certain depth? What kind of soil, compost, fertilizer do they need? Please help.

Answer from NGA
June 5, 2008


There are several reasons that iris may not bloom. I didn't get much specific information so let's cover several scenarios.

Age. Some iris varieties need several seasons to get established.

This means that the rhizomes may not be old enough or have enough stored food for flower production. These may need a year or two before they begin flowering. They should be planted far enough apart to allow them space to grow.

Planting depth. Iris rhizomes need to be planted shallow, not too deep, or they will not bloom but produce plenty of leaves. Iris rhizomes should be planted so the tops of the rhizomes are slightly below the soil surface.

If you plant them 2 to 4 inches deep they will not produce flowers until the rhizome enlarges enough to be closer to the soil surface or the soil erodes away. You can cover with a bit of soil or mulch to protect them from the sun, but not too deep.

Adequate light. Bearded iris need about five to six hours of direct sun a day to bloom well. Morning and early afternoon sun is best. Late afternoon sun or reflected light off of west facing exposures are worse.

Fertilizer. An appropriate iris fertilizer will be high in phosphorus, the middle number, to promote blooming. Straight nitrogen fertilizers, or excess fertilizer, will promote plenty of leaf growth but not flower development.

Water. Iris are quite tolerant of drought, but they might not bloom well if they experience long periods with limited water.

During new growth and the flowering period they need to be watered so they avoid any stress. After blooming they can handle much more water stress. It still isn't good for the plant, but they can handle it. Mulching helps conserve water and avoids stress but if kept wet it could promote diseases, such as rots, in the rhizomes.

Poor bloomer. There are some iris varieties that are just poor performers.

The next scenario is for established iris with a track record of flowering, but have stopped blooming.

Good bloomers will reduce or stop flowering if overcrowded. They will not resume good flowering again until they are divided and produce new growth. Iris are best divided, separated and replanted in the fall -- not too late. They need time when temperatures are still favorable for growth to recover from the shock they received when they are divided.

With iris, you throw away the old fans and save the young ones formed this year. Get rid of everything in the middle of the clump and keep the plants on the outside. Make sure each iris rhizome has at least one fan of leaves, a good root system and is about 3 to 4 inches long.

After replanting in the fall, cut the fans back by half to help avoid transplant shock.

Hope you can determine why your iris is not blooming through the information provided. Best wishes with your irises!

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