Early wild greens for homemade dog food?

I've been given a bunch of freezer burned ground beef. It's pretty rich so I'm cooking up "dog stew" to stretch it out and buffer the fats. The stew includes last year's potatoes, carrots, beets, and oatmeal.

(Note this will be 50% of their diet -- they still get good quality commercial pet food.)

Gardening season is a ways off. I'd like to add some "wild" early spring greens to the mix.

Grass: My hounds naturally graze on quackgrass (no chem) on the fencelines. If I cut it when it's young (maybe 4") and chop it up fine, that could be a source of vitamins/minerals and fibre.

Weeds: I'm not sure what would be safe. Don't need a vet bill. Chickweed? Lambs Quarters? Poplar tree leaves?

Thoughts?

Chickweed, in moderation, should be fine, lambs quarters will be great. Not sure about the poplar. Go easy with grass, it's something dogs usually chew on when they feel nauseated, and want to puke - kinda like epicac.