A lot of people don't know this, but an iris sets it's flower buds for next year, after blooming. So, it would only make sense that the best time to fertilize is late summer and early fall, after they've bloomed.
Fertilizer in the spring is not going to get them to bloom that year. It may help grow bigger and healthier plants. But what you do after bloom show up in the next years flowers.
So keep those beardless fertilized and watered!
A lot of people think it's going to hurt a beardless iris to fertilize it before winter. Not so. The top foliage will die down anyway, so what difference does it make?
They are not like shrubs, where new top soft growth can be killed and hurt the shrub.
Because of the bearded iris rhizome being so close to the top of the ground, I would also not fertilize them late in the season.
But your sibs, and Japanese...... go for it! See next year how it works.
I think my one Japanese Iris fried and then died in the July heatwave when it was 103 here this year. It came up this spring just fine but its not there now.
So my beardless are the Siberians. At least they are doing well. I did much better with the ones I bought this year than last year as they didn't mostly die on me like last year. So I know not to fertilize new ones. But I will go out tomorrow with the organic granular fertilizer and do all of last years Sibs. They are looking good and growing great so maybe they will have more bloom next year.
Name: Dee Stewart Willamette Valley OR Snowpeak Iris
I will my regular Polly, what I do is just before the garden goes down for the year, trim them all almost to the ground, and take the cheapest steer manure and dump it to cover the entire plant in a little hill and I do this to rhubarb and then next year oh so lovely.
I leave the leaves on to protect them over the winter here, and then clean the leaves off first thing in spring, but I do what you told me, and put composted cow manure over them. Sure works great. The beardless are so tough. Right now I'm putting a 10-10-10 on them, and lots of water up to late fall when I do the manure thing.
What should I do here about trimming the foliage? Our cold weather really doesn't set in until December. The ground may or may not freeze. We don't usually get snow cover because what little snow we get normally melts in a day or two. I say normally, but who knows what the new normal is.
Most of the foliage right now looks pretty ratty, between what the grasshoppers have chewed and the drought and high temps.
I will do the basic 10-10-10 fertilizer this week and maybe some fish emulsion also. Then I'll add the bags of manure in October.
I have clumps that have not been divided in 4 years. Should I do that this year, even after all the stress they've gone through?
I would suggest dividing, too, but in early spring. Believe me when they get to be large, it's a job.
If your foliage looks ratty, I say trim it. You might even get new if you fertilize it now. If you do get new foliage, even though it might look good, cut it back in early spring.
Everyone~~~always cut your old foliage off the beardless in early spring. Even though it might not look too bad, and you might not be having any problems with the beardless, the old foliage can harbor insects. Right Lucy? Versicolor, especially is known to do that.