List of beneficial weeds

This is a list of undomesticated or feral plants, generally considered weeds, yet having some positive effects or uses, often being ideal as companion plants in gardens.
Beneficial weeds can accomplish a number of roles in the garden or yard, including fertilizing the soil, increasing moisture, acting as shelter or living mulch, repelling pests, attracting beneficial insects, or serving as food or other resources for human beings.

Categories of beneficial weeds

Blackberry—blackberries are rich in nutrients, and their thorny stems can form a barrier against some larger pest mammals.
Burdock—roots are edible.
Chickweed —used in salads and also as ground cover.
Cornflower—various colors; can be served as edible garnish to decorate salads.
Flatweed—leaves are edible raw, while roots are edible after being roasted.
Horsetail—primeval plant that is high in silica; tops are very similar to and may be eaten like asparagus.
Lamb's quarters—leaves and shoots, raw, also prevents erosion, also distracts leaf miners from nearby crops.
Nettle—young leaves collected before flowering used as a tea or spinach substitute. Plants have use as compost material or for fibre.
Purslane—prepared raw for salads or sautéed.
Shepherd's purse—leaves are edible and often sautéed or blanched.
Watercress—can be eaten raw or cooked; is considered a weed in some cultures.

Trap crops draw potential pests away from the actual crop intended for cultivation.

Cowpea—attracts ladybird beetle, so planting around cotton fields protects them from sucking insects. It serves as source of food and niche.