If you would like a chicken yard that is not 100% pitted raw dirt, here are some plants I have found my hens leave alone. I don't think any are toxic, but if anyone knows otherwise, please chime in. I routinely toss my weedings over the fence and let them scratch around, eat bugs, and snack if they want. Some reseed themselves inside the yard.

Vervain, both regular and blue - they don't touch these and they are quickly colonizing the yard. It's a pretty look when in bloom but I am fearful they will continue to spread into my adjoining field and lawn, so I wack them back before they go to seed.

Ladies' mantle - I only have a couple of these volunteering in the chicken yard.

Blue fescue - I was surprised at this one, I figured any type of grass would be fair game. Instead, I'm slowly getting little balls of fescue here and there which softens up the yard considerably.

Thistles and burdock: Of course they leave these eyesores alone. I dig them out now and again.

Fennel: I only have one volunteer, a bronze fennel, and so far they are leaving it alone. I'll keep an eye on this as it can become invasive.

Feverfew: I wouldn't mind this spreading here and there, jolly little daisy.

Fens' Ruby Euphorbia: This has become a persistent invasive for me, and my guess is the latex is likely not good for the birds. So far they haven't touched it and it has only a small footprint. I'll likely keep after that to eradicate it from their yard.

Lamb's Ears: Another softening look, these are growing along the fence.

In general, I find the chickens just won't touch things that are toxic to them, especially if given enough other greens to choose from. We also feed food scraps, scratch, and chicken feed. Happy hens.

Another good feature to having plants at the edge of the chicken yard is it helps to keep the birds from digging their way to freedom. Here are some I have purposely planted just outside the yard fence, which the chickens do peck at but can't quite kill them off. They are helping stabilize the perimeter, and most are good for the birds:

Comfrey - yum, they love this, but it is a robust plant and can take a good amount of pecking if the rootball is out of reach.

Daylillies - they will peck at the leaves that wander inside but so far have not outright killed any. I suppose they may also eat the blooms as I've not noticed any blooming inside the yard.

Sweet cicily - another one they likely peck at, but this is planted far enough out to stay healthy.

Mints - I have not yet planted any, but this would seem like a good fit for an outer perimeter plant. My guess is the chickens will keep it under control on their side and the lawnmower will keep it under control on my side.

Mint flavored eggs?

Ha - too bad I have no chickens that lay greenish eggs...

I just visited my cousin who keeps 3 hens. She has mint growing right alongside her coop, which helps keep the odor down. I suspect the birds may nibble on it, as it has not taken over her whole yard.

Oregano- mine do not touch it (even though, didn't I see it touted as good for their health?) It is perennial here and could make at least a small mat of roots to resist digging.
Parsley, mine won't eat.
Garlic chives they don't eat and those do make a thick tough root mass.
Mine roam a portion of the yard. They focus on scratching for bugs and picking in the 'lawn' and have not grazed most of the ornamentals .. yet. they do worse by scratching out seedlings and exposing roots.
Last year they left my cannas alone and I might plant those to give them a late day shade cover in summer.

I often toss my weed clippings over the fence to the chickens and then I find out what will self-sow that the hens don't eat. One is a tufty clump grass, maybe Elijah Blue? Valerian is taking over the yard at a steady clip. I've been weedwacking it and randomly digging it out.

Nettles the hens leave alone, as well as thistle. I also dig those out and leave the rootballs laying down for the hens to pick at.

i had a volunteer brassica weed this winter, I tasted it and found it just nasty, and the chickens never touched it either. It had cream colored flowers with a little purple in the petals, not one of the yellow flowered ones.
They have wiped out everything in their yard except wild garlic.

They do not eat mock strawberry- sure wish they did
Mock Strawberry (Potentilla indica)

How about horse herb?

porkpal, referring to this?
https://www.illinoiswildflower...

I don't think I have had it, so I can't say.
Chickens have not bothered my daylilies at all.. really haven't eaten much foliage of anything except vegetables that they can reach through the fence. Again, it's the scratching that kills, here.
I protected tomatoes and peppers and okra when small, and they are not eating them even now.

I was thinking of horse herb, Calyptocarpus vialis , also called straggler daisy. However, I think the hens would eat it, but maybe not fast enough to kill it...

I wish the horses would eat it, I could feed them all winter.

Horses will eat it, especially if it's all there is that is green.

I have a big patch of giant Rudbeckia (laciniata?) which the hens ignore when the are ranging. . I planted it into their yard and they decimated it
I DID use the cannas to dress up the chicken yard, they liked the shade and cover. They did not eat any leaves.
I have some comfrey that they are nibbling a little, and more they are not. The blooming ones are not getting eaten, maybe they taste better young, only..
Somehow I got a lot of red Amaranth come up and they are not eating it at all.

This is off subject and I had to share my duck/bug experience. One summer we had a horrible infestation of grasshoppers. Couldn't walk outside without hundreds crossing in front of us. Our ducks kept them off our garden plants and the veggie garden.
I don't have chickens but the ducks are tenacious bug hunters and great at keeping the veggie garden free of any type of insects. They don't eat the plants and they do lay eggs. The egg whites make the best meringue for lemon meringue pies. That's my story and I'm sticking to it.

The only thing the chickens don't eat when closed in the small area around the coops is stinging nettle - not valuable information, I know.

Porkpal, that's good to note, kind of the conclusion I wanted to point out- when confined they might eat things they might have not eaten otherwise.
I have not seen them yet eat the 'Eupatorium chocolate' or whatever this related thing is that I have coming up everywhere, snakeroot, or something, I stuck that in their yard two weeks ago with the Rudbeckia.

Duck eggs make the best cakes too.

Chickens have not eaten zinnia that self sowed. Nor pokeweed. Nor celosia, even though they ate from mature ones that I threw in their yard last year. They're funny- part of it may be just how desperate are they at the moment.
They LIKE the berries from Duchesnea indica, darn weed I have eveyrwhere.