Recognising blackgrass

The ability to identify blackgrass early is key to preventing it from building in your fields.

Andy Doyle

14 June 2017

An infestation of a crop of winter wheat with blackgrass in Ireland.

Blackgrass heads in flower

Flowering heads of blackgrass in a crop of winter wheat.

Blackgrass has a huge ability to tiller and compete. This plant grew in a good crop of Avatar winter wheat and it produced 23 fertile tillers.

Blackgrass has a long and indented ligule.

There is an increasing awareness among tillage farmers of the necessity to be able to identify blackgrass. It was also pointed out to me that we have not published good pictures of the headed-out stage to aid recognition (see picture). Can I point out that there was a very useful guide to the recognition of a number of grass weeds written by Ronan Byrne of Teagasc in our Crop Protection 2017 magazine published last April. This provides a simple and good guide to a number of different grass weeds.

Can I point out that there was a very useful guide to the recognition of a number of grass weeds written by Ronan Byrne of Teagasc in our Crop Protection 2017 magazine published last April. This provides a simple and good guide to a number of different grass weeds.

Blackgrass, Alopecurus myosuroides, is also known as slender fox-tail, which describes its general shape. The heads will often take on a dark purplish to black colour and this is what gives it its most-used name.

The plant mainly germinates in autumn, but individuals can germinate in spring also. By the time you see the heads, you have missed any potential selective control opportunity by six to eight months, but you should seriously consider removing infested areas to help prevent seed return.

Spot or patch spraying with glyphosate now might even be too late, as some seeds have been fertilised for about two weeks at this point and viable seeds would be possible by the time the plant dies.

Plants will grow in fields put down to grass, so it is essential that these be managed to prevent seed return. And it seems that plants can produce multiple heads in a year.

Blackgrass is notorious for developing resistance to herbicide families. If you have infestation, the first thing to do is get your sample checked for herbicide effectiveness.

Many recent infections have originated from importation of highly resistant English seed, so most or all herbicides may be ineffective. But some infestations have been around for much longer and many such samples are still sensitive to at least some herbicides.

The important thing is to prevent seed return if you have it and then to recognise and treat it next autumn. But the key is to delay planting in such fields, as early gives the advantage to the grass. Plants tiller profusely in autumn, especially when early sown. And 20 to 40 fertile tillers per plant is common.

Young plants have their leaves rolled, like a cereal plant, and they have a long ligule. This is the white tissue (picture left) that grows at the join of the leaf and the leaf sheath.

This can be an identifying feature and it has a very irregularly indented top edge. The leaf blade tends to have a pronounced groove in its centre. Also, the leaf sheath is generally not fully closed where it joins the leaf. And it has no auricles, which are the little fingers which protrude from the base of the leaf to wrap around the stem.