Oxalis, wood sorrel

(Oxalis corniculata, Oxalis debilis and Oxalis latifolia)   Several species of oxalis are attractive garden plants. But there are some species that can become difficult weeds to deal with, if they are left to grow out of control.

There are several species of oxalis that are attractive garden and indoor plants. But there are also some species that can become difficult weeds to control, due to their many tiny bulbs (bulbils) and seeds. The three main weedy species are Oxalis corniculata (creeping woodsorrel), Oxalis debilis (pink-sorrel or pink wood sorrel) and Oxalis latifolia (broadleaf wood sorrel).

Oxalis corniculata can be annual or perennial, its stems rooting where they touch the soil. It produces numerous seedpods and seeds, which are scattered over a long distance as they are forcibly ejected from the seedpods.

Oxalis debilis rarely produce seeds, but the numerous tiny bulbils spread easily when the soil is cultivated and can remain dormant for several years.

Oxalis latifolia spreads both by its forcibly ejected seed and by bulbils.

The leaves are eaten or added to drinks for their acidic taste.

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

Oxalis species have clover-like leaves, but this has an acidic, sharp flavour when tasted, due to their oxalic acid content. They flower for a long time over summer.

Oxalis corniculata produces small, yellow flowers that are followed by upright seed capsules, and creeping stems that readily root as they grow. A purple-leaved variety is very common.

Oxalis debilis produces deep pink flowers.

Oxalis latifolia produces deep pink flowers. As its common name (broadleaf woodsorrel) suggests, this has broader leaves.

As with most perennial weeds, never allow oxalis to become established where you don’t want them to grow. This will make it more difficult to fully control them. Early eradication is important to stop them taking over the garden.

You can start by digging out the plants. However, this is fairly laborious and you have to make sure to remove all the bulbils.

Hoeing the plants will weaken them over time, but this will probably take several years to completely kill them.

Mulching the soil with organic matter, such as a bark mulch, will help reduce or eliminate growth. For mulches to work properly, they need to be a minimum of 5cm (2in) thick, but 7.5cm (3in) deep works better.

Covering bare soil with weed-control membrane (landscape fabric) or even thick black polythene will exclude light and may starve the bulbils, so they die. This can take a couple of years until the bulbils are completely exhausted, especially as they can lay dormant in the soil.

In lawns, regular mowing throughout the year may weaken and eventually kill them.

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

Contact weedkillers will burn and kill the foliage, but will have no effect on the bulbils. Constant spraying whenever leaves are present may weaken and kill them in time.

For best results, spray with a systemic weedkiller. A systemic weedkiller, which is absorbed by the leaves, then moves down to the bulbils to kill them.

To ensure the weedkiller works effectively:

• Spray the leaves when the plants are growing actively from spring to autumn.
• The larger the leaf area present, the greater the amount of weedkiller that can be absorbed.
• Use a fine spray to thoroughly coat the leaves in small droplets.
• Don’t spray when the sun is fully out. Ideally spray in the evening to prevent the spray evaporating and to give maximum time for the spray to be absorbed.
• During the summer, spray in the evening to prevent the spray evaporating and to give maximum time for the spray to be absorbed. In spring or if overnight dew is forecast, spray earlier in the day to allow the spray to dry before dew falls.
• One application of weedkiller probably won’t completely kill oxalis. You may need to make repeated applications over several years.

Most contact weedkillers are total weedkillers – that is they will damage or kill any plants whose leaves they are sprayed on. 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 is pretty easy in your garden, it’s more difficult to stop the seeds coming in from a neighbour’s garden, any surrounding fields and waste ground and even further afield.

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

• Weedkillers