Yarrow

(Achillea millefolium) Cultivated varieties of achillea are attractive, ornamental plants. However, wild yarrow is a problematical weed – especially when it gets into the lawn.

What is yarrow?

Cultivated varieties of achillea are attractive, ornamental herbaceous perennials, which will brighten up borders for many months. However, wild yarrow can become a problematical weed – colonising large areas – especially when it gets into the lawn.

Yarrow grows in any soil, but is particularly problematic in light, chalky or gravelly soils. It is very drought resistant, and often starts to take over in hot weather and during droughts. In such conditions, which don’t favour other plant or grass growth, yarrow quickly establishes and grows prolifically – especially in lawns, which are weakened by dry conditions – producing large patches.

The mats of foliage can smother and kill smaller plants. The roots and rhizomes (underground stems) can grow in between the roots of cultivated plants, making them awkward to remove and control. It produces low mats of foliage in lawns, which can be difficult to control.

Where does it grow?

Beds
Borders
Lawns
Waste or uncultivated ground

Appearance

Yarrow is an evergreen perennial with grey-green, aromatic, soft, ferny foliage. It overwinters as leafy rosettes. Leafy flowering shoots start to appear in spring, which produces white or sometimes pinky-white, flattened, daisy-like flower heads from June to August, sometimes well into October. These produce enormous amounts of seeds throughout the year.

It is a mat-forming plant, which produces rhizomes (underground stems) that grow and spread over quite large distances, making it very invasive.

How to control yarrow

As with most perennial weeds, never allow yarrow to become established. This will make it more difficult to fully control. Early identification and eradication is very important.

Natural control

You can start by digging out as much of the yarrow as possible, including the creeping rhizomes and roots.

Regularly hoeing the new leaves as soon as you see them will weaken the plant over time, but this will probably take a few years to completely kill plants – and needs lots of dedication to not allow them to grow.

Covering the soil with weed-control membrane (landscape fabric) or even thick black polythene will exclude light and may starve the plants, so they die. This can take several years until the plants are completely exhausted and eradicated.

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.

In lawns
Regular mowing throughout the year and the use of lawn fertilisers to strengthen the grass, may weaken yarrow and eventually kill it. Although the mower blade may miss the low-lying plants. In which case, lightly scarify the lawn before mowing to lift plants up towards the blades. Don’t compost the clippings.

Good lawn maintenance, to keep the grass growing strongly, will help it outcompete yarrow. Adding a lawn topdressing every year will help.

Thoroughly watering the lawn once a week during prolonged dry periods or drought will keep the grass growing strongly, and reduce the growth of yarrow.

Weedkillers

There are a number of weed control options available to treat yarrow. 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 roots and rhizomes, which will continue to grow, produce new leafy growth and spread further.

A systemic weedkiller, which is absorbed by the leaves, then moves down to the roots to kill them.

To ensure the weedkiller works effectively:

Spray the leaves when the yarrow is growing actively; this is mainly from March/April to September/October.
The larger the leaf area present, the greater the amount of weedkiller that can be absorbed and move down to the roots.
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 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 may not kill all the yarrow. You may need to spray once, allow the plants to die down, and then spray any regrowth again.

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.

Gel products, which is smeared onto, and sticks to, the weed leaves, may be a better option when trying to treat yarrow growing through or close to wanted plants.

In lawns, use a lawn weedkiller, making repeated applications as recommended in the manufacturer’s instructions.

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

Prevention

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