Not many gardeners would agree with James Russell Lowell's description of a weed as “no more than a flower in disguise.” Weeds aren't a total curse: some attract beneficial insects, some add organic matter to the soil and those with deep roots break up compacted soil, while other weeds, such as dandelions, are even harvested for salad greens. The seemingly never-ending job of weeding makes you wonder where the heck they all come from. No sooner do you pull one out when up pops another.
So, what is a weed?
Broadly speaking, a weed is any plant growing where it's not wanted. For most gardeners, that usually means plants that are invasive, or ones that harbour pests or diseases harmful to desirable plants. But in certain circumstances, it can also mean trees (Norway maple), shrubs (European buckthorn), vines (oriental bittersweet), bulbs (wild garlic) or herbaceous plants (borage).
Trying to distinguish weeds from desirable plants can be perplexing, especially at the seedling stage. Books such as Weeds of Canada and the Northern United States, by France Royer and Richard Dickinson (Lone Pine Publishing and the University of Alberta Press), are helpful guides to identification, but there are on-site clues as well. Soil is one: desirable plants tend to self-sow readily in gravel or light, sandy soils; weeds prefer poor soil. Speed of germination is another: if it came up lightning fast—almost overnight—it's probably a weed. How are the plants distributed? Randomly and in a mixed pattern, resemblng a native, planted meadow? It's likely a weed. And one more clue: weeds are hard to remove; plants that practically fall over with the slightest tug are probably the real McCoy. Still can't tell? Try smelling the plant: weeds, particularly their broken stems, have a green, decidedly weedy smell—rank to some, but to my nose, similar to that of a mowed field.
As part of their survival strategy, weeds produce abundant seed—often with a tough coat that increases viability. If conditions are not good for germination one year, seeds lying in the soil may germinate the following year or even the year after. To prove seed longevity, botanist William Beal began an experiment in 1879 that continues today. He buried 20 pint jars—each containing sand and 1,000 common weed seeds—at the University of Michigan. Initially, some jars were opened every five years and their seeds tested for viability: the interval has since been lengthened to every 20 years. One jar was unearthed after being buried for 120 years and its seeds planted, resulting in a vigorous group of 26 seedlings of moth mullein.
Weeds also adapt well to adverse conditions, so it's important to understand their life cycles to identify and eradicate them effectively. Annuals, such as crabgrass, lamb's quarters and ragweed, complete their life cycle in one season, growing from seed in the spring, then flowering and producing copious amounts of seed before dying. One ragweed plant can produce 117,000 seeds in a single year. The common groundsel ensures reproduction by a different means: the ability to bloom even when temperatures are below freezing.
Plants such as Queen Anne's lace, mullein and burdock are biennial weeds. They die at the end of the second year, after they have flowered and set seed.