This morning, one of my hens was dead inside their run. Lately I have been letting them into my garden as the season wanes. Are there any known veggies that are bad for chickens?
There's really not enough information, and a lot of information may be symptoms of a lot of different things. It's hard to know what they ate.
I doubt that one of your hens would have died from eating vegetables from your garden, unless you had put insecticide, herbicide, or something like that on the plants. And then you would probably have seen other chickens at least get sick. How old was the hen?
About 16 month old Australorp. No, I never put pesticides in my garden, It's mainly tomatoes and pepper plants. They all looked fine when they went into the coop the night before.