If you haven't met this plant while weeding a garden or helping out at a local farm, you'll want to take some time to get to know this weed. Red-rooted Amaranth (Amaranthus retroflexus) is a delicious member of the same plant family as White Goosefoot and the cultivated spinach.
It has red pigment on its root and often grows in large groups. It is an annual that doesn't show up in quantity until the soil warms sufficiently. I started to gather Red-rooted Amaranth in mid-June this year.
I look for fast growing plants that have yet to produce flowers. In a good row at a local organic farm, I can gather a pound of this plant in just a few minutes – enough to last me several days.
Before consuming any wild food, I take care to positively identify the species by consulting trusted wild food resources – a crucial step that should not be dismissed. While some garden weeds are edible, others are most certainly not, and even edible plants have particularities (i.e., specific parts to be collected at specific times and prepared in specific ways) that need to be learned. If independent research doesn't give you total confidence, I recommend spending time with an experienced forager. Some things are best learned in the field.