It is in the nature of chickens to scratch about in the soil searching for small insects. They are omnivorous and eat pretty much anything. Unfortunately this also means chickens will peck away at any leaves or plants that they find in the garden, and which they think will make a quick snack for them.
Chickens eat garden plants, given half a chance
The best way to keep your chickens from temptation is to restrict them to a Walk In Chicken Run or some other area or enclosure away from any of your prized plants. However, you need to make sure that the chickens have enough room in their enclosure. Too little space can make a chicken unhappy.
Even if you don’t have any plants in your garden, just a lawn, chickens will still scratch up the grass to make dirt baths for themselves and search for food. Given time they will reduce a small lawn to bare earth. Keeping healthy free range chickens sometimes means having to sacrifice part of your garden.