Our Organisation Search
Quick Links
Toggle: Topics

Grass Weed Identification and Biology

The following section will introduce you to some of the more problematic grass weeds found in Ireland.

Before deciding on how to control a weed, first it must be accurately identified and its growth pattern and biology understood, this will enable you to develop the right strategy that will give you sustainable and cost effective control.


Grasses have many common characteristics that can be used to make an accurate identification. When identifying grass weeds look out for these main characteristics: 

  • the presence of rhizomes and stolon’s
  • whether the leaves are rolled or folded within the stem
  • the presence and appearance of the auricles
  • the presence of hairs on the leaves and leaf sheaths the shape of the ligule
  • the structure and appearance of the seed head 

Using a guide such as the Bayer Weed Guide is a useful tool to help identify many of these weeds as it provides a structure to the identification process.

There are many other characteristics which are less common and maybe more distinct to individual species such as a reddish/purple colour at the base of the stem or on the leaf sheath, characteristics of the leaves, such as colour and twists in the leaves. These are very useful to know but as there are many it is maybe not practical to be familiar with them all

Below is more information and a series of videos that will assist in the identification of the following grass weeds:

This first video demonstrates how to get started with the identification and recognising particular characteristics


Unfortunately, blackgrass is becoming increasingly common in Irish tillage fields and has seen a large increase in numbers in the last 3 years. An incredibly competitive and fecund grass weed the presence of large populations within a field will quickly make the production of tillage crops unviable. When identifying it the key characteristics are:

  • Leaves rolled within the stem,
  • No auricles,
  • No stolon’s or rhizomes,
  • Ligule is medium in length (2.5mm), blunt and serrated
  • And the seed head is a compact spike up to 13cm in length

Image shows Blackgrass heads above the canopy of a crop of winter barley

Blackgrass lifecycle


  • Approximately 80% of blackgrass germination occurs from August to October 
  • This will vary from year to year as weather conditions during flowering and seed formation will impact on seed dormancy.
  • Blackgrass will emerge in the spring time, typical from April onwards as air and soil temperature increase.
  • Light helps to induce germination.
  • Plants can produce up to 1000 seeds (100 seeds per head) but this can be much higher (up to 6000 seeds per plant) in the absence of competition.
  • There is a 70% seed decline in the seedbank per year
  • Deeper burial (> 5 cm) will reduce seed emergence of freshly-shed seeds.
  • 8 to 12 plants per m² can cause yield losses of 2 to 5 %.
  • Resistance to ACCase and ALS herbicides has been confirmed in Ireland 

Image is an example of a well tillered Blackgrass plant

Sterile Brome (Anisantha Bromus)

A very common grassweed, it can be found predominantly at bases of ditches and hedgerows and in field margins. It is becoming increasingly common in fields as it encroaches in from the margins and can be found from time to time throughout a field as its seeds are spread via machinery and cultivations.

Large populations of sterile brome in cereal crops will cause significant yield losses 

When identifying it the key characteristics are:

  • Leaves rolled within the stem,
  • No auricles,
  • No stolon’s or rhizomes,
  • Fine hairs will be present on the surface of the leaves and on the leaf sheaths
  • Ligule is medium in length (2-4mm), pointed and deeply serrated
  • And the seed head is a heavy drooping panicle. Which turns from a green to deep purple as it matures

Sterile brome lifecycle

  • Sterile brome is a widespread weed, found in a range of fertile soils throughout Ireland.
  • About 90 % of sterile brome germinates from August to November, it flowers from June onwards begins to shed seed in Jul
  • it is not unusual to find sterile brome emerging in the spring time, particularly where winter crops are thin or have failed
  • Plants can produce anywhere from 200 to 2000 seeds
  • Seeds have poor persistent in the soil with a seed decline of almost 90% per year
  • Exposure to light will induce dormancy in seeds so shallow cultivations after harvest will help germination
  • Seed emergence is reduced with increasing seed burial depth (> 10 cm).
  • 5 plants per m2 can cause yield reduction of 5 %

Spring Wild Oats (Avena fatua)

One of the most widespread grassweeds, it can be found on almost every tillage farm in the country. It can become a severe problem where continuous barley is produced and the discovery of a number of populations in the South and South east which are resistant to the ACCAse chemistry makes it even more challenging to control.


Image is of resistant wild oats in a crop of spring barley in Co. Wexford

When identifying it the key characteristics are:

  • Leaves rolled within the stem,
  • No auricles,
  • No stolon’s or rhizomes,
  • Fine hairs will be present the margins of the leaf and sometimes on the leaf sheaths
  • Ligule is long (6-8mm) and rounded
  • The seed head is a large loose drooping panicle

Spring Wild Oats lifecycle 

  • This is the most common type of wild oat in Ireland
  • Predominately spring germinating, but the odd seedling may  germinate from September to May
  • It flowers from June onwards and it will begin to shed seed from July
  • Seeds have the ability to survive in soil for several years and are thus unaffected by seed burial depth.
  • Most seeds emerge from the top 10 cm soil, but some emerge from greater depths, up to 15 to 25 cm.
  • Light promotes germination of seeds.
  • A single well-tillered plant has the ability to produce up to 2,000 seeds.
  • A population of 1 plant per m2 has the potential to cause a yield loss of 1 %.
  • Resistance to ACCase herbicicdes has been confirmed in a number of populations across the south of the country

Winter Wild Oats (Avena sterilis)

Winter wild oats is less common than spring wild oat (Avena sterilis) it can also be found in Ireland. The best time to differentiate between the two species is when seeds are about to be shed. There are two features which will allow you to identify between the species.  

1.  The presence or absence of an awn on the 3rd seed in a spikelet 

2. Whether seeds remain attached to each other or separate at shedding

Image above: Spring Wild Oats - seeds separate when mature and shed singly. 

Image above: Winter Wild Oats - the two or three seeds within the spikelet remain attached and shed as a unit
Rothamsted Research technical publication – Identification of Wild Oats:   Click Here 

Lesser Canary Grass (Phalaris minor)

Canary grass seed heads appearing above a crop of spring barley

The least common of the four weeds that the ECT project is focusing on. It is becoming increasingly common in parts of the South and South East especially where continuous spring barley is the predominant crop. The ECT project is looking to fill the knowledge gap around the growth habits and crop losses that it can cause in our climate.

Pinkish red exudates on the tips of the root of a Canary grass seedling

When identifying it the key characteristics are:

  • Leaves rolled within the stem,
  • No auricles,
  • No stolon’s or rhizomes, the roots produce a bright red/pinkish exudate which is very distinctive
  • It does not produce tillers but instead produces lateral or side shoots
  • Ligule is medium to long (3-8mm) and pointed

Detailed information about the growth habit and biology of Canary grass is sparse in an Irish context.

  • It is becoming increasingly common, particularly in East Cork, Wexford and parts of Kildare.
  • Once thought to be a mainly spring germinating weed, farmer and advisor experience would suggest it has a protracted emergence pattern.
  • It is becoming an issue in both winter and spring crops.
  • It has a prolific seed return
  • Severe infections can lodge crops and make combining very difficult