Fiorin, marsh bent grass, running twitch, white bent
Agrostis gigantea Roth. (A. nigra, A. alba)
Perennial Grass Weeds
A native, perennial, rhizomatous grass, black bent is a serious weed of arable land. It is distributed throughout the British Isles but is commonest in the south and east. It prefers moist but well drained soils of low to high pH and light to medium texture. It grows equally well in marshy or dry places and varies greatly in appearance. Black bent is said to prefer lighter soils than common couch (Elytrigia repens) and to be less tolerant of tillage.
In a survey of weeds in conventional cereals in central southern England in 1982, black bent was found in 10, 9 and 6% of winter wheat, winter barley and spring barley respectively. In a study of seedbanks in some arable soils in the English midlands sampled in 1972-3, black bent seed was recorded in 41% of the fields sampled in Oxfordshire and 22% of those in Warwickshire but in moderate numbers only.
Black bent is a carrier of take-all disease of cereals, the disease is present in the rhizome internodes. It is said to reduce barley growth allelopathically.
In the vegetative state black bent is often confused with common couch. In the literature too there is often confusion over the identification of the weedy rhizomatous grasses. The same common name has been used for several species and the Latin names of the different species have changed periodically too.
Flowers are formed from June to August and seeds develop rapidly after flowering. One third of the seeds can be viable just 1 week after flowering. In the north of England, black bent sheds its seed several weeks earlier than common couch. A single panicle contains about 1,000 viable seeds.
Mature seeds are non-dormant and germinate readily on moist soils. The fresh seed requires light and alternating temperatures for germination but older seeds will germinate at constant temperatures in the dark.
Seed sown on the soil surface germinates better than seed lightly covered with soil and few seedlings emerge from seed buried deeper than 25 mm down in the soil. The optimum depth of emergence is 0 to 5 mm. The buried seeds germinate when brought to the soil surface during soil cultivation. The main period of seedling emergence is from March to October.
Black bent seedlings grow rapidly and initiate rhizomes at the 6-leaf stage when some plants already have 10 tillers. Seedlings are much more susceptible to competition from a growing crop than plants derived from rhizome pieces. However, the later growth of the weed is much faster than that of the cereal mainly due to its greater leaf area.
Black bent rhizomes only branch occasionally and have distinct scale leaves. The rhizomes occur mainly in the top 5 cm of soil but are found down to 15 cm deep. The fibrous roots may penetrate further. The aerial shoots can also root at the nodes.
Persistence and Spread
Rhizome multiplication is considered to be the main form of reproduction but seed production is also important. Most seeds germinate during the first autumn but a few may persist and remain viable in cultivated soil for at least 3 years. Seeds persist longer when buried and left undisturbed.
Seed that had passed through the digestive system of sheep germinated better than fresh seeds. However, after three months in a manure heap the germination potential was just 3.3%.
The prevention of seeding and removal of the creeping rhizomes is important in the control of black bent. Forking out may be practised on a small scale. In the field, roots and stems gathered up by the harrow should be collected and burnt. Machinery has been developed with two banks of rigid soil-loosening tines fitted with 30 cm wide wing- or duck-foot shares that tear up the stubble ahead of a pto-driven horizontal rotating shaft fitted with long curved tines. These flick the rhizomes out onto the soil surface where they can be left to desiccate or can be collected up for burning.
In barley stubble, control can include one or two passes with a rotovator, the second when regrowth has 1-2 leaves. The land is then ploughed, cultivated and drilled with a spring crop. There is foliar growth at the time of ploughing but not appreciable rhizome growth. In the spring, shoot numbers are reduced by 80 to 90%, the higher figure from 2 passes with the rotovator. The treatment does not lead to complete eradication and needs repeating in subsequent years. Black bent will increase under intensive cereals and minimal cultivation.
In pasture, the grass should be cut before black bent sets seed. Cutting at intervals of longer than 4 weeks does not reduce rhizome biomass. Cutting intervals of 2-4 weeks are needed to reduce black bent populations but the composition of the sward can also have an effect. Perennial ryegrass, Lolium perenne and cock.