When is a weed not a weed?

Published on 25/04/2012

People often ask us 'what is the definition of a weed?' In horticulture, the accepted definition of a weed is simply 'a plant that is growing in the wrong place'. But what happens if we turn that definition on its head? When most people think of weeds, they think of plants like grounsel, chickweed or dandelion. These plants are easily described as weeds because they aren't particularly attractive or showy and are considered a nuisance for invading the vegetable patch or growing in between paving slabs. But if we stop and think about it, some common and popular garden plants can also behave like weeds, and they do! When we think of weeds as nuisance plants, it is because they fall into one of four categories - they are either invasive, in the wrong place, detrimental in some way, or spread themselves around easily.

Invasive plants

Two main definitions of a plant becoming invasive come to mind. The first is for those species that have been introduced from other countries and once introduced adversely affect native habitats. A common example of such a plant that often causes problems in domestic gardens is Japanese Knotweed. Introduced to Britain in the 19th century as an ornamental plant, over time it has become widespread in a range of habitats including roadsides and riverbanks, as well as domestic gardens.It can grow up to a metre in a month and has has even been known to grow in concrete and tarmac! It spreads by producing rhizomes and can remain dormant in the ground for up to twenty years and still produce plants. It can be very difficult to get rid of and the plant is considered such a threat that you need to contact your local authority if you want to dispose of it.

Another less serious but still cumbersome plant in the garden is Convolvulus or Field Bindweed. This is actually a very attractive plant, so much so that a number of less invasive cultivars have been produced for the domestic garden with flowers ranging from pinks to blue/purple. The common variety is perhaps the one most people experience trouble removing from their garden. Convolvulus is a perennial climber or creeper that can grow up to 2m high, with attractive funnel-shaped white flowers. In a garden it climbs up shrubs and other plants, choking them as it intertwines up stems and competes with them for sunlight and moisture. It too can be difficult to eradicate as it has a deep extensive root system which if not completely removed can sprout repeatedly from just tiny segments. Its seeds can also remain viable in the soil for up to twenty years.

Plants in the wrong place

We all admire the buttercup as one of our most pretty wildflowers and when a few plants sprout up in our lawns we initially don't mind, finding its bright yellow flowers an attractive addition to our grassy areas. However, if not controlled this pretty little plant will start to pop up all over a lawn, competing with the grass. Known as creeping buttercup, it has vigorous creeping stems that run along the rhizome. Hand weeding is the preferred method of removing it, but if it spreads too much then a chemical weed killer is most effective.

Another attractive perennial I often find myself having to remove is the foxglove. I do love foxgloves and indeed have a number of varieties in my herbaceous borders, but I do find that the common variety tends to pop up just about anywhere. It especially seems to like the vegetable garden. These plants are easy to remove as they have shallow root systems. If like me, you feel a little guilty at their removal, then try and find a space for them elsewhere in the garden.

Plants that can be harmful to other plants or humans

Some plants we might like to grow in our gardens can become detrimental to other plants if allowed to. One example of this is ivy. Ivy is a self-clinging climber that can grow quickly into the canopy of a tree. In most cases ivy growing on the trunk of a tree is not damaging in itself, but when it starts to infest the tree canopy it is time to control it. To remove it, cut the stems back down to the ground. I prefer to wait a week or so until the ivy begins to die off. This weakens its hold on the tree trunk and makes it a little easier to remove. Ivy is often grown in a border to infill space. But its often dense growth can swamp other plants and so again this is a time when it will need to be controlled.

Some plants can be harmful to humans and as responsible gardeners it is our duty to make sure harmful plants are not present or properly managed if there are children using the garden or if for example the garden is open to the public. Plants can be harmful through their sap or chemicals in the plant that can cause allergies or more serious problems. Some examples include Aconitum or Monkshood which is poisonous and an irritant to the skin, Laburnum which is also poisonous, Passiflora or hardy passionflower and even tulips produce sap that can be a skin irritant.

Those plants we just can't get rid of

There are a few plants in my garden which I purposefully added because of their attractive or other qualities but which have now become rather a headache. Firstly, lemon balm. This useful and attractively scented herb has many uses in the garden. However, it is one of those plants that is easily propagated by wind and I now find it popping up all over the garden, even in between the slabs on my patio. It can also be difficult to remove as it has strong fibrous roots. Another such plant is the common violet. I initially planted this lovely little plant in one of my large containers as a companion to lavender, but am now finding clumps of it all over the garden. Another attractive and popular garden favourite which can become a problem is the blue bell. I find that it is best to dig up a number of clumps of the bulb each year to stop it spreading all over the flower borders. It is not so much the flower itself, but the extensive foliage which can swamp emerging plants in spring.

So are there any beneficial weeds?

Believe it or not, there are a number of plants commonly described as weeds that are actually beneficial to grow in your garden. These plants can have a number of roles such as increasing moisture, acting as shelter, fertilising the soil, repelling pests and attracting beneficial insects. Dandelion is actually a good companion plant for tomato plants, it attracts bees and is edible. Nettle is a good companion for broccoli and tomato. It also attracts bees and is used for many herbal remedies. Purslane is good for breaking up hard soil and it brings water and nutrients up from deep in the ground. Wild Vetch provides ground cover for beetles and fixes nitrogen into the soil.

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