In part 2 of this weed control series, I try to answer the simple, but not entirely straightforward, question – what exactly are weeds?
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.This is well illustrated by my own experience in moving from the UK to Australia.
I was shocked to see that many plants I was used to growing in the UK, such as Verbena bonariensis (Verbena or, in Australia, Purple Top), Ricinus communis (the Castor Oil plant), Arundo donax (Spanish Reed Grass), are considered to be noxious weeds.
Some are specifically banned from sale on account of the threat they pose to native vegetation and ecosystems. Likewise, Ligustrum spp (Privet) and Lonicera japonica (Japanese Honeysuckle) are respectable garden plants in the UK but on many lists of invasive weeds in the USA.
One gardener’s weed is another gardener’s prize bloom because the degree to which a particular weed species is troublesome in a particular area is determined to a large extent by the growing conditions in that area.
Differences in soil type, or the availability of light and/or water that that may be present in adjoining properties or even in different parts of the same garden, can account for the presence or absence of a particular weed in each place.
Broader climatic considerations, such as minimum temperatures and the amount of sunny days, can account for the different growing characteristics in the UK, compared to elsewhere, of a plant such as Privet.
Therefore, the identification of individual weed species is for the most part a local matter.
There are some weeds that are recognised as such, and are widespread, in many parts of the world. However the main purpose of these weed control posts is to arm you with the knowledge and techniques that you can apply to whichever plant or group of plants are growing in the wrong place in your garden.
It is for that reason that you will not find exhaustive lists of weed species here. Instead, I’ll try to point you to some useful resources that can help you with identifying the weeds that may be troubling you.