Annual mercury

(Mercurialis annua) Annual mercury is a common weed throughout the garden, but particularly wherever soil is disturbed or cultivated. It sets lots of seeds each year and spreads readily.

Annual mercury is, as its name suggests, an annual weed. It sets a lot of seeds each year and so spreads readily around the garden.

These seeds can remain dormant in the ground for several years. And as “One year’s seeding means seven years weeding”, allowing plants to flower and produce seeds, means several years of trying to control and remove it.

• Beds
• Borders
• Gravel paths
• Between paving slabs
• Containers
• Waste or uncultivated ground

Annual mercury grows up to 70cm (28in) high and has oval, light green leaves. It produces yellowish/light green flowers.

It flowers from July to October and spreads rapidly from seeds, which are dispersed explosively from the plants.

As with most weeds, never allow it to become established, and certainly don’t allow it to flower and produce seeds. This will make it more difficult, and more time consuming, to fully control. Early identification and eradication is important to stop it taking over the garden.

Regularly hoeing seedlings and young plants as soon as you see them is the quickest and easiest method of control. The aim of hoeing is to sever the weed stems at or just below ground level, cutting the top growth from its roots. A sharp hoe blade will make this even quicker and easier, so always sharpen the hoe blade before using it. Hoeing on a warm and/or windy day will mean they quickly dehydrate and die.

Digging out the plants when they are more established is more time consuming and more difficult – especially where they’re growing among wanted plants.

Flame guns and weeders that use an electric current are also effective in some locations.

Covering bare soil with weed-control membrane (landscape fabric) or even thick black polythene will exclude light and prevent seeds germinating. As will mulching the soil with organic matter, such as a bark mulch. For mulches to work properly, they need to be a minimum of 5cm (2in) thick, but 7.5cm (3in) deep works better.

There are a number of weed control options available to treat annual mercury. In addition to traditional weedkillers there are now also a range of more natural alternatives.

Any weedkiller can be used to control and kill annual mercury in beds, borders, waste ground and on paths. Those marketed as “fast acting” are contact weedkillers – killing or damaging the plant tissue they are sprayed onto or make contact with. These tend to be based on “naturally-occurring” active ingredients, such as acetic acid and natural fatty acids.

Systemic weedkillers, that also kill the roots can also be used.

To ensure weedkillers work more effectively:

• Spray the leaves when the plants are actively growing; this is mainly from March/April to September/October. Contact weedkillers will have some effect if used during the colder weather in winter.
• Use a fine spray to thoroughly coat the leaves in small droplets.
• During the summer, spray in the evening to prevent the spray evaporating and to give maximum time for the spray to work. In spring or if overnight dew is forecast, spray earlier in the day to allow the spray to dry before dew falls.

Weedkillers (except lawn weedkillers) are total weedkillers – that is they will damage or kill any plants whose leaves they are sprayed onto. Make sure you keep the spray off wanted plants – including lawns – and, if necessary protect plants by covering with polythene or similar when spraying.

Use weedkillers safely. Always read the label and product information before use.

Never allow plants to flower and set seed. Although this should be pretty easy in your garden, it’s more difficult to stop seeds blowing in from a neighbour’s garden, any surrounding fields and waste ground and even further afield.

• Spade
• Fork
• Hand fork
• Weeding tools
• Hoe
• Weed-control membrane
• Weedkillers