A weed is a plant growing where it is not wanted. Often a plant becomes a weed when it is brought from one place and introduced in another, becoming an exotic nuisance. This process began early in North American history and continues to the present. European colonists brought essential herbs with them. By the late 1600s, two of these, dandelion and wild carrot, were naturalized. Much later, Japanese honeysuckle was considered a lovely vine for the home garden, but it wouldn't stay behind the fence. Kudzu was first imported as a forage crop for cattle in the southern states and was once advertised as having more potential than cotton. Multiflora rose, sold as a living fence, is now one of our greatest pests.

In 1890 and 1892, C. F. Millspaugh, a botanist at West Virginia University, published three Agricultural Experiment Station bulletins on West Virginia's weeds. The second contained a list of the 25 worst weeds plus 25 others for a secondary list. It also contained botanical descriptions and ecological observations.

The "worst" list started with ox-eye daisy and followed in order with broomsedge, pasture thistle, burdock, bitterdock, wild carrot, elderberry, ironweed, yarrow, buck plantain, cockle-bur, blue thistle, ragweed, Spanish needles, whitetop, sand-briar, sorrel, wild garlic, white devil, blue devil, Canada thistle, morning glory, wild sweet potato, dog fennel, and cinquefoil. Twelve of the worst 25 were imports from Europe, one was from tropical America, and 12 were natives that had adopted the open soil of cultivated fields.

In 1975, a botanist from the state Department of Agriculture resurveyed the state's weeds. This resulted in roughly the same 50 except that wingstem, blackeyed Susan, and crabgrass had moved from the lower 25 into the top 25 and nine newly introduced weeds were found to be doing as much damage to the state's ecosystems as all 50 of the earlier ones. The nine were kudzu, multiflora rose, autumn olive, tree of heaven, Japanese honeysuckle, Japanese knotweed, coltsfoot, mile-a-minute, and purple loosestrife.