look around on the ground where the iris were, they aren't suppose to be planted very deep, you should see some rhizomes from them, if you leave them alone the fans should start to come back, it may take a couple of years before they bloom though. also if the rhizomes look packed in togather you probuably need to divide them and move them, if there are alot of rhizomes in my experiance they will still grow fans but they won't bloom, also what I have always been told is that iris need moved every three years to keep their color, or they will fade to the original parent color.
on the peony issue if you can dig around and find their tubers they may come back as well, I had a really friendly neighbor that sprayed mine to be 'helpful' it upset me greatly, but they came back the next year.

It's likely that at least some of the irises are still there, especially the hardiest cultivars in your mother's collection. Mowing alone won't kill the irises, but if they have been disallowed to grow enough foliage to feed themselves, and if they have been poisoned with herbicide on top of that, they may have weakened substantially. Also, they've apparently been walked over repeatedly over the last six years, which means that their soil is likely badly compacted and, I'm guessing, overrun with grassy weeds. With that sort of neglect, some of the irises may have rotted and disappeared permanently, while others may have become badly overcrowded and undernourished. The strongest cultivars may have overtaken and choked out the weaker ones.

I recommend you leave the irises where they are for now. Fertilize them in the spring about six weeks before their anticipated bloom period. That'll give you an idea of how many, if any, have survived the abuse of the past few years and might even promote a little bloom for you. After iris bloom season is finished next year, dig the area and rescue any iris rhizomes you find. Dig a new bed for them, properly amended, in a sunny location, and replant them. It may take a few years for them to recover and start blooming again, but the surviving irises will bloom eventually.

Oh, and irises do NOT "fade to the original parent color" under any circumstances. It is possible, however, for iris plantings that become overcrowded to lose certain cultivars as the stronger ones choke out the weaker ones over time. That's why it's important to give each cultivar enough space when planting to avoid the clumps growing into each other, and to divide the clumps when they do start to grow too close.

Happy irising,

Laurie

just found this, been growing iris 25 years. mowing usually doesnt hurt them. however weeds do! it wont hurt to pull them before blooming, and i advise you do. remember they are probably starving for room and nutrition, they most likely wont bloom and if they do they will be small, deformed blossoms.
also there is the likely chance you could have borers. they are devistating to iris. small pink grub-like worms. waiting only gives them more time to eat and multipy.
when you dig watch for the smallest even rizome it will grow! also i have found from experiance they actually do better pulling to prevent blooming helps since you stopped that cycle. they wil grow and nourish all summer and the following spring you will have show of beauty.
but then i break alot of rules that most iris gowers would never do. just for the record there are tousands of them in my bed!!!
good luck,
laurie

& if/when you dig them up, you can soak them in a bleach solution (is it 5%, knowledgeable people?) to kill borers.

Have fun!