Controlling Weeds

Dealing with weeds is an important part of keeping the garden in good health. Weeds attract many pests into the garden and are often hosts for fungal and other diseases, which then spread to other plants.

What is the difference between an annual and a perennial weed?

Weeds can quickly colonize areas of
the garden if not controlled.

Many weeds are annuals – they complete their growth cycle within a year. These are relatively easy to control, but it is important to do so before they set seed. If they have already shed their seed, this will germinate and grow to produce a fresh crop of weeds. Perennial weeds are more difficult to eradicate. Some, such as ground elder, disappear under ground in winter but form dense mats of roots. Woody weeds, such as brambles and elders, can also prove troublesome. Tap-rooted weeds can be difficult to eradicate, as any scrap of root left in the soil will regenerate.

Do I need to get rid of all weeds?

Weeds are a problem in all gardens. Not only are the majority of small ornamental merit, but they will compete with your plants for moisture and nutrients. They are a food source for many common insect pests, which will then move to other plants. Some are also prone to mildew and other fungal diseases, so it is best to get rid of as many as you can.

How do I deal with annual weeds without weedkillers?

Hand weeding is time-consuming but a highly effective way of dealing with weeds. At the seedling stage in spring, simply hoe over them to sever the topgrowth from the roots. You can leave them on the soil surface to break down – the roots will break down under ground. Larger weeds will need to be removed individually.

White vinegar can be used to kill young weeds. Heat the vinegar first for the best results. Repeated applications may be necessary.

I have heard that weeds can be killed by excluding light. How do I do this?

If you have a large area to treat, 
you can cover it with thick black plastic or pieces of old carpet. 
To be effective, all light must be excluded. It can take up to year, or even two or three, for weeds to be killed by this method.

What are weed-suppressing membranes?

Weed-suppressing membranes are sheets of material that are spread over the soil and prevent weeds from coming through. They are permeated by tiny holes that allow free passage 
of moisture and air. For maximum effectiveness, the ground should already be weed-free before a membrane is laid – the membrane can really only prevent weed seeds from coming into contact with the soil and germinating. They are usually most effective on flat ground.

How do I use a membrane?

Clear the site of weeds, then rake 
over the soil to level it. Lay over the membrane and cut it to fit, if necessary. Secure it at the edges either by weighing it down with stones or bricks or dig a trench with a spade into which you can tuck each edge. 
To plant, cut crosses in the membrane for each plant, fold back the edges and excavate a suitable planting hole. Once the plant is in position, fold back the edges. Top the membrane with grit, pebbles or chipped bark to hold it in place.

I am trying to be more environmentally friendly. Is there any justification for using garden chemicals?

Chemicals still have a place in gardening, even though there are far fewer available today than previously. Used wisely, they play an important part in keeping the garden healthy, provided their use is localized.

How do weedkillers work?

Any weedkiller that is applied to a plant is carried around it in the same way that all plants take up water from the ground.

A contact weedkiller is painted on to
the leaves of a weed but does not
affect any surrounding plant material.

What are contact weedkillers?

Weedkillers work in a variety of ways. Contact weedkillers – sometimes called spot weedkillers – have to be sprayed or painted on to the leaves by hand. They kill only the part of the plant with which they are in contact. They are useful where isolated weeds have to be removed from among 
other plants and hand-weeding is impractical, but can also be used 
over a wider area. Annual weeds are usually killed outright, but perennials may need two or more applications, and often regrow strongly, as the roots are not affected.

What are systemic weedkillers?

Systemic, or translocating, weedkillers enter the leaves of the plant and are then carried through the plant’s entire system. Depending on the vigour of the weed, more than one application may be necessary.


Will these weedkillers stay in the soil?

Nowadays, nearly all weedkillers, both contact and systemic, break down 
on contact with the soil. They do not leach out of plant roots into soil. Once the site is cleared, the ground can therefore be replanted straightaway.

What are soil-acting weedkillers?

Highly effective, soil-acting weedkillers are seldom used today because of concerns over the environment. They are applied to 
soil and enter weeds through the roots, killing all types. However, 
they persist in the soil for months, 
in some cases up to a year. You will therefore not be able to replant for a considerable time. If you buy a new-build property with areas of bare soil in the garden, check with the contractor whether the soil has been treated with such a weedkiller.

When is the best time to apply weedkillers?

It is best to apply chemical weedkillers during periods of good growth when the sap is moving around at a higher rate and the chemical will be taken with it. Weedkillers are most effective at the seedling stage for all types of weeds and since most weeds spread by seed, applying the weedkiller before flowering and the plant sets seed is essential. Biennial weeds are more easily treated in their first season, 
as seedlings or small rosettes. 
For perennial weeds that are difficult to control, an application of a systemic weedkiller in the autumn can be very effective. Plants under stress such as those in areas of waterlogging, drought or low temperature are less susceptible to the weedkiller as they are growing poorly.

What are the best conditions for applying weedkillers?

Choose a dry, still day. Avoid use during windy periods – the wind will blow the product on to other plants, which may themselves be killed.