Although couch grass (pronounced coo-ch, Latin name Elymus repens sometimes known as Agropyron repens) is said to have uses in herbal medicine, most gardeners and allotment holders will know it as an invasive weed that is difficult to control.
Our own little plot is riddled with it.
It might seem as though you’re fighting a losing battle when it comes to couch grass, but with perseverance and a little bit of understanding about how it grows and survives, it is possible to keep it at bay. (Although that is likely to take us many years of vigilant work!)
Why is it a problem?
Couch grass grows rapidly through the top layer of soil, creating a thick mat of roots, removing water and nutrients from the soil and making it difficult to plant other things.
Like all plants, it exudes chemicals to help it survive and some of these are poisonous to other plants. These toxic chemicals are known as phytotoxins and are designed to prevent other plants thriving nearby.
How does it spread?
Couch grass may look like just another grass, but it is what happens under the soil surface that makes it such a problem.
The plant grows on a modified underground stem, called a rhizome. At various places along the rhizome, it sends out roots and shoots (the above ground stem, known as the culm, and its leaves).
The points on the rhizome where the plant grows roots and shoots are known as nodes. The areas between the nodes are known as internodes.
The internode sections grow fast, extending the distance between nodes and spreading the plant further. Not only that, nodes can divide, creating new rhizomes – a process known as tillering.
As if that wasn’t bad enough, even the smallest section of a rhizome with a node can develop and become a new plant. You might pull out a clump of the grass, but leave behind any nodes, and new plants will follow.
So, it is easy to see how a single plant can quickly invade a garden, or move from a neglected allotment plot or overgrown path into well cultivated beds.
And, of course, couch grass produces seed which can also lead to new plants. However, the seeds are a secondary method of propagation because each plant needs another to cross-pollinate with, and the seeds, produced between July and August, are only viable for around 3 years.
The plot next to ours is riddled with couch grass which means that we will be fighting the spread of couch grass coming from there, also.
Controlling couch grass
Weed-killing chemicals or herbicides
Herbicides for couch grass work in two main ways – contact action or translocated action.
Contact action herbicides act by killing the parts of the plant that they come into contact with – usually above ground. These types of herbicides can be a good way to control annual weeds but they do affect any plants they come into contact with such as your prized flowers or vegetable plants.
Contact action herbicides may be chemical in origin or made from natural fatty acids. However, although natural fatty acids are sometimes regarded as natural or organic, bear in mind that they are still chemicals.
Translocated action herbicides are applied to the above ground part of the plants where they are absorbed and translocated to the root system. They then act on and kill root cells, therefore killing the plant. These may also be known as systemic herbicides.
Cultural methods are any means not using chemical or biological controls.
Weeding – loosen the soil and remove the rhizomes. It is good practice is to use a fork as you can easily chop through the rhizomes with a spade, inadvertently creating more plants. You might want to choose a fork with more prongs, especially with a sandy soil or you could try using a soil sieve. I think we’ll be doing a lot of weeding!
Dealing with the rhizomes – allow them to dry out and die by laying them out in the sun on a dry path or suspended on metal grid or fine wire mesh. When totally dried, they can be composted. This not always as easy as it sounds.
Incinerate them – stick them on a bonfire or in a garden incinerator.
Bag them and send to garden waste or recycling centre.
Some gardeners suggest that burying couch grass about half a metre below the soils is an effective way of killing the plant and removing the problem. The theory is that because couch grass rhizomes live in the top 150-200mm of the soil, burying them at or below 500mm (perhaps in a bean trench) will prevent them from reappearing. However, there is a danger that this could affect the plants you are trying to grow in that area because of the phytotoxins given off by a high concentration of decomposing couch grass roots and rhizomes.
The idea with mulching is to cover the soil, depriving weeds of light so that they become weakened or die altogether.
Natural materials – loose natural materials, such as bark chippings, can sometimes be successful. It should be applied at least 100mm deep to exclude the light.
Manufactured materials can be quite successful excluding the light and controlling weeds. These might include cardboard or plastic sheets. But you need to select your material carefully. Many allotment gardeners use tarpaulins or other plastic sheets that don’t just exclude light, they also exclude water and prevent air movement, having a detrimental effect on the soil as a living biome by killing soil organisms.
You can get specialist weed suppressing membranes, which exclude light while still allowing movement of air and water. This is likely to be the route we will take.
If you have a problem with the idea of covering your allotment with plastic, you might want to consider cardboard, which will eventually rot down providing more nutrient for the soil. But you may need to check on any glue used or whether it is printed with inks containing plastics.
Some people cover their allotments in old carpet. You could try this but only if it is made from natural materials – a wool carpet with a hessian backing, the older the better because newer carpets are likely to have been treated with preservatives. Modern acrylic carpets with plastic backings may mean you’re digging bits of plastic out of your soil for years to come. They may also contaminate the soil with chemicals as they degrade.
In a garden situation, you could try turning the affected areas into lawn and keeping it close mown until the weed is gone. But that may take some time, is not always a successful method for couch grass, and isn’t practical for an allotment situation.
The RHS website has some excellent information about couch grass including a fact sheet to download on what weedkillers are available to the gardener.