The dreaded garlic mustard (Alliaria petiolata). This plant is a biennial: not quite an annual, it spends its first year of life as low rosette of kidney-shaped leaves. The next spring, the plant bolts upwards with a new crop of triangular leaves and a raceme of cross-shaped flowers (characteristic of the family Brassicaceae). After flowering, the plant dies --- but not after aggressively seeding the surrounding area with a new cohort.
Garlic mustard is a poster child for invasive species. Introduced from Europe, it was probably brought over intentionally by colonizers as a sharp-flavored, aromatic potherb (many folks still find it delicious and it contains more vitamin C than orange juice). Unfortunately, the phenology of this shade-tolerant plant is such that it emerges quickly in the spring in North America; invading woodlands, outcompeting and replacing native plants. To make matters worse, garlic mustard secretes very effective natural toxins from its tissues which suppress the growth of nearby plants (a process known as allelopathy) and is known to kill the emerging larva of insect eggs laid on its leaves (such as the endangered West Virginia white butterfly).
The traditional use of garlic mustard as food raises some common debates around invasive species management: does garlic mustard's obvious resilience and usefulness to humans outweigh the threat it poses to local biodiversity? There is some evidence that any given population of garlic mustard loses its allelopathy over time, and some claim that plant invasions as a whole are another symptom of increased human disturbance, rather than a crisis in itself. Either way our native plants need all the help they can get, so pulling garlic mustard up by the roots seems like an easy decision --- especially if you plan to eat it (while being careful about potential pesticides and local pollutants)!