Lamb's-quarters
Chenopodium album
Annual; spreads by seeds; height to 3 feet; found across U.S.; flowers June through October.
Hoe or pull plants as you see them. Mulch seedlings heavily. Leaves are edible.
Photo: (cc) Franken Stoen/Flickr
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Common Ragweed
Ambrosia artemisiifolia
Annual; spreads by seeds; height to 3 feet; found across U.S. (not northern ME, WI, or MN) and central and eastern Canada; flowers July through October.
Hoe seedlings and mow larger plants. Mulch or plant cover crops on fallow areas. Plants can be composted if haven't gone to seed. The pollen is a common cause of hay fever and allergies.
Photo: (cc) Frank Mayfield/Flickr