The oft repeated definition of a weed as a plant growing in the wrong place may be something of a cliche, but is perfectly true. But the reasons why these particular plants, or should I say weeds, have arrived there can be many and varied. Another old saying that “One man’s meat is another man’s poison” applies equally to weeds which may be prized flowers in one region, yet an invasive pest in another. These ideas are explored further in an article by Martin from his Gardening Step by Step website.
You will often hear gardeners say that weeds are plants growing in the wrong place and, although something of a cliche, this is probably the best working definition there is.
Weeds are plants like any other. They have the same requirements for light, water and nutrients as other plants and they come in the form of annuals and perennials or even trees or shrubs.
The real defining feature of weeds is that they are growing where they are not wanted.
They may be plants that we cultivate elsewhere in the garden but which have spread from their allotted place, by stealth or self-seeding. Or they may be plants that were in the garden before we were and which tenaciously hang on to their place.
They may even be plants that we unwittingly introduce – growing from seeds carried in the potting mix of plants we buy in, or from seeds which survive the home composting process and gleefully spring up when we spread the compost on our borders, bursting with the energy that the compost provides.
This explains the tomatoes, beans and other vegetable seedlings that you may occasionally find popping up in your ornamental beds.
Weeds are especially problematic, in the garden and in the wider environment, when they are introduced into conditions for which they are supremely well adapted. Many is the plant that grows unremarkably in one environment but rampantly in another.
These are the real problem weeds that can irreparably harm native environments and cause significant financial loss to farmers and landholders.
See more at Gardening Step by Step