Common velvetgrass, meadow soft grass, tufted soft grass
Holcus lanatus L.
Perennial Grass Weeds
Yorkshire fog is a tufted, perennial grass, native on rough grassland, lawns, arable land, waste ground and in open woods. It is generally distributed in the UK and often abundant. It occurs over a wide range of soil types being found in fen-meadow communities, poorly-drained and water-logged soils, low fertility and nutrient rich soils. It is well adapted to growing in wet conditions but can survive a moderate drought, although growth is restricted. It is tolerant of soil pH but grows best between pH 5.0 and 7.5. Yorkshire fog exhibits climatic tolerance over a wide altitude range. Severe frost has been found to kill Yorkshire fog under certain conditions. It does not survive trampling and puddling.
Yorkshire fog is a widespread weed of herbage seed crops and can reduce the seed yield. In a survey of weeds in cereals in central southern England in 1982 Yorkshire fog was found in 2% of fields. In set-aside in Scotland, it was one of the most frequent grasses present and in a seedbank survey Yorkshire fog seed was found in 5% of arable soils in Scotland.
Yorkshire fog forms a dense stand that excludes other plants and reduces species diversity. It was considered a weed in lowland ryegrass swards because of the low palatability to gazing animals when it begins to flower but there is some disagreement about this. On drier pastures the plant is hairy and stock avoid it. Yorkshire fog has been used for land stabilisation and for sheep grazing on soils of low nutrient status. Young shoots are readily eaten and digestibility is good, mineral status is relatively high but dry matter content is low.
Yorkshire fog exhibits considerable variation in morphology and growth habit. Some plants are stoloniferous, others are short-lived. Ecotypic variants with tolerance, to salt, nutrient and pH status of soil and the presence of pollutants have been reported. There are also a number of agricultural cultivars. Natural hybrids are formed with creeping soft-grass (H. mollis). The hybrids resemble creeping soft-grass in morphology.
In Britain, plants of Yorkshire fog require vernalization in order to flower, with a minimum exposure of 25 days at a temperature of 5°C. Yorkshire fog flowers from June to September. Flowers are wind pollinated and out-crossing predominates. The seeds start to become viable 5 to 9 days after flowering and are completely viable after 20 days. Seeds are shed from June to early autumn. Seed numbers per panicle range from 100 to 380. The average seed number per plant ranges from 177,000 to 240,000 depending on time of emergence. Plants from later emerging seedlings have progressively less seeds because fewer tillers develop in time to become vernalised over winter and hence flower. Nevertheless, the panicles on these plants tend to produce more seeds to compensate for the smaller tiller number, so the seed production of the late-summer and early autumn seedlings is still relatively abundant.
Seeds germinate over a wide range of soil temperatures immediately they become mature. Seeds germinate better in the light than in the dark, and at fluctuating rather than constant temperatures. Seedlings emerge mainly from April to October.
There is little winter growth but Yorkshire fog is able to begin growth at relatively low temperatures in early spring. It produces profuse tillers, and regenerates vegetatively by developing new shoots and roots at the nodes. In established swards it forms large spreading clumps. Plants are relatively deep rooted, an advantage in soils of low nutrient status. There is evidence that Yorkshire fog may have an allelopathic effect on other plants in the sward.
Persistence and Spread
Seed production is very high and just a low level of dormancy can allow a substantial seedbank to build up. Although seeds show little dormancy, 14% remained viable after 10 years burial at 12.5 cm deep in soil. Around 37% of the seeds in the surface seedbank produce seedlings but less than 8% of these reached maturity. Invertebrate activity and greater openness at the soil surface increases the chance of seed burial. Seeds in dry-storage had 5% viability after 12 years. In granary conditions, seeds had 82% viability after 1 year and 6% after 4 years.
Most reproduction is by seeds that may be dispersed by various means including the wind, birds and crop seed contamination. Seeds that are eaten by rooks remain viable but those eaten sparrows do not. Moles bring Yorkshire fog seeds to the soil surface in their hillocks. Viable seeds are also found in worm cast soil. Yorkshire fog was a common contaminant of grass seed samples. In a survey of weed seed contamination in cereal seed in 1970, it was found in 1% of samples. All of this was home saved seed.
Yorkshire fog spreads vegetatively by developing new shoots and roots at its nodes. Plants form a blanket of runners on the soil surface. Semi-prostrate rosettes of shoots called mops may form at the end of the runners. These mops root readily in contact with moist soil.
It is essential to avoid sowing the seed as a contaminant when laying land down to grass. Yorkshire fog is favoured by undergrazing. Intensive mowing or grazing suppresses the establishment and spread of Yorkshire fog. Regular grazing also keeps it in a vegetative and palatable condition. Yorkshire fog benefits from undergrazing. In fields cut for hay Yorkshire fog has time to ripen and shed seeds.
Burning, ploughing and limited soil moisture reduce the relative abundance of Yorkshire fog in pastures. It is grazed by rabbits and is also susceptible to treading. Yorkshire fog is attacked by a variety of insects and fungi.
Updated October 2007.