One man's weed...

Found growing in my yard.  We hesitated to cut it down although we knew it was a weed and it was prickly and almost in the way.  We rarely cut down plants until we know what they are and mostly wait until they bloom.  I went searching and found it's a Milk Thistle.  Not only is it used in bird food but it's been used medicinally for about 2000 years. Originally it was a Mediterranean plant, used for liver problems and based on current studies might actually help our poor abused livers. They are also now studying it as a cancer remedy for helping control liver damage form chemo especially in children. It's also good at protecting you from certain kinds of mushroom poisoning.   It's supposed to be edible but honestly it's so prickly I really didn't want to try it.   Ours isn't as tall as most of them - mainly because we walked over it, didn't water it and it was growing in fairly horrible soil.   I'm going to try to collect the seeds and share them with out resident birds and try to encourage the plant to grow in a regular bed.  
Volunteer plants are truly fun.  I've had a number of them in the back yard - not quite as deadly as the milk thistle.  I also, em, help volunteers by collecting seeds in the flower beds at work or near bus stops.  I always follow the gathering rule which is take a few and leave many.  I never strip a plant.  I like to think I encourage plants to reclaim parts of my back yard and of course the birds help.