I'm thinking of putting the planter in the chicken run for the girls to take care of but I want to make sure it isn't a plant that will harm them first. I really need to get a field guide. There are so many unknown plant popping up in my yard.
Looks kinda like a lady's-thumb to me (a weedy Polygonum), with the little dark smoodgey chevron on each leaf. If this is what it is, it will get not-unattractive 1-2" spikes of pinky white flowers that are in the shape of tiny hard balls, like jimmies you would put on a cupcake. I believe that I have read that young leaves are edible (er, if this is indeed what your weed is ) but have not tried it myself and do not make any guarantees.
(e.t.a. - smartweed! that's its other name, Google reminds me. Doh. A.k.a. Polygonum persicaria = P. vulgaris.)
Got that stuff everywhere. it even braves growing between rows of corn and stems can get to be like dang near 4 feet long before popping out pink and white flowers. Luckily, they are easy to pull out and the chickens and turkeys like to nibble on it. Unfortunately they prefer my beans!!!!