"A surprising number of so-called weeds have edible greens, including dandelion, chicory, pigweed, lamb's quarters, chickweed, sheep sorrel, and cleavers. In Ashland, Oregon, permaculturist Tom Ward has cultivated an intimate relationship with his weeds. He has encouraged edible weeds in his lawn and prepares diverse and highly nutritious salads from his front yard...'There's probably more nutrition along the edges of most gardens--in the weeds--than in the crops. So mothers should be telling their children not, "Eat your greens," but, "Eat your weeds."
Weeds are supremely multifunctional plants. They are the pioneers, covering, protecting, and fertilizing bare soil, preparing it for others. Many weeds are superb nutrient accumulators--in fact, that is often their primary role, pulling widely scattered nutrients from deep in the earth and concentrating them in the surface soil. That also explains why they are so nutritious: They accumulate health-giving minerals in their tissues." (GAIA'S GARDEN 140)
"Certain species draw specific nutrients from deep in the soil and concentrate them in their leaves. The long taproots of these plants dredge up important nutrients such as potassium, magnesium, calcium, sulfur, and others. As these plants lose their foliage in fall, the nutrients build up in the topsoil. Such plants are obvious candidates for the ecological garden because they keep nutrients cycling within the yard and reduce the need for purchased fertilizers.
Nutrient accumulators include yarrow, chamomile, fennel, lamb's quarters, chicory, dandelion, and plantain." (GAIA'S GARDEN 131)