Why didn’t my irises bloom?

(Question)

A gorgeous section in my south facing garden/front lawn in mid-town Toronto has about 20 tall purple irises. For about 7 years they always bloomed. Last year, only a few flowers bloomed.This year, despite healthy looking tall green shoots that came up there were NO flowers. Could the other ground cover be impacting them? I didn’t fertilize them, was I supposed to? DO I pull them all out and start over again? I don’t know soil type, I have sprinklers, so they would have been watered well, and there is sun.

(Answer)

The usual reasons that irises might not bloom are not enough sun, wrong planting depth or overcrowding.  As the plants did well for a number of years, the first two items likely don’t apply – unless there are other plants nearby that have grown tall enough to hog the sunlight (irises need at least 6 hours of sunlight a day).

Here’s the most likely culprit:  As the iris patch grows and the plants multiply, they are prone to overcrowding.  Irises should be divided every 3-5 years, after blooming.  They grow from rhizomes (modified stems that look a bit like ginger root), which need to be planted only slightly under the surface of the soil – no deeper – or no blooms!  After transplanting, it may take a couple of years for them to re-bloom.

You say that you have not fertilized the plants, but if you do, you should be aware that fertilizing with a high-nitrogen fertilizer can decrease or totally suppress flowering – you will see lush, green leaves, but no flowers.  Fertilize with bonemeal or 6-10-10 fertilizers (the first number is the nitrogen).

From your description, it does not sound like the irises are being bothered by pests or disease – both of which could prevent blooming.  However, it is worth mentioning that iris borers  (the larvae of moths) are nasty pests to keep an eye out for.  Generally, though, the leaves would look a bit chewed – you describe healthy, lush leaves.  In any event, watch for evidence of these critters – they leave notches on the plants or leaves and you may see moist material that looks like sawdust.  In any event, it’s important to clean up all the dead foliage in the iris bed in early spring, to minimize the chances of an infestation.

Canadian Gardening’s Irresistible irises: Secrets to eight weeks of blooms  includes tips on how to grow irises as well as how to deal with problems that might arise.  Another terrific resource is the Canadian Iris Society website, which has as a “cultural info” section that provides care tips and information sheets for various iris types – including how to divide and replant them. The website also has information about the iris borer.

Additional references

Ask the master gardeners — why didn’t my iris bloom this year? https://archive.lohud.com/article/20110806/LIFESTYLE01/108060306/Ask-master-gardeners-8212-why-didn-t-my-iris-bloom-year-

The Morning Call. Why won’t my irises bloom? https://articles.mcall.com/2012-05-18/entertainment/mc-iris-bloom-problems-20120518_1_irises-bloom-angel-trumpets