There are weeds for all places in all seasons, including winter annuals, summer annuals, biennials, perennials, grasses, sedges, and broadleaved weeds. If one type is selectively eliminated from a planting, another will take its place. The landscape manager who becomes a master at controlling weeds will always be able to find a job. There are certainly many definitions of the term 'weed'. The one most frequently used is simply, 'a plant out of place'.
Corn is a common weed in soybean plantings because they are planted in rotations.
In landscape and turf plantings everyone recognizes dandelion and quackgrass as two common weed problems. However, there are other cultivated plants that are commonly used in landscapes and turf areas that are weeds in certain situations. Tall fescue in a fine Kentucky bluegrass lawn is a weed. Goldenrod was introduced as an herbaceous perennial flower for colonial gardens. Purple loosestrife is a beautiful flower for today's gardens, but has become a horribly invasive weed in wetlands across the country.