Yes, it is a thing. I just found out about it recently and had to try it out. Photo of experiment below (or wherever this post decides to place it).
I first prepared some muslin for dyeing by soaking in alum, then overnight in alum/borax and then hanging it outside to dry. You can buy prepared-for-dyeing fabric, but I didn't wish to pay $30/yard for that. You can bypass this step altogether if you don't think you'll be needing to "set" the dye, if you're not planning to wash the finished item.
Next gather some flowers, tape them face down on your fabric and pound away. I used my little ball-peen hammer. I used asters, brown-eyed Susans, coleus leaves, and forget-me-nots.Attachment 4825
I will probably embellish these with some stitchery, make little table linens out of them (for under plants or candles, whatever) or something.
