What is the plural form of 'Iris'?

Is it "Irises" or "Irisi" or is it just "Iris" (like fish)? The reason I ask is that my lovely big sis split her Bearded Iris (Iris germanica) and Siberian Iris (Iris sibirica) and gifted me with quite a few extra (yay)! I had to sneak out yesterday morning to get them into the ground (and find a place for them).

I am super excited because...I was complaining to my Mom that though my garden looks great now, it looked really terrible in early June after the tulips and crocus (crocusi?) had bloomed; really green, and scraggly...she mentioned that I needed Iris, and now I will have them. I hope.

In other news, I fought another battle against the Japanese Knotweed. The war began last year when I decided to tackle the mess growing along my fence in the backyard, and discovered that when I cut down the nasty stuff, it grew back in like...10 more places. Today I'm hoping the strategy of going over to my neighbor's side of the fence (with his permission, of course) and digging up some largish clumps will turn the tide.

-SG

P.S. my Sweet Pea is blooming!