Charlock
Other names
Field mustard, karlock, kedlock, kilk, shallock, yellows, wild mustard
Latin names
Sinapis arvensis L. ( B. arvensis, B. kaber, Brassica sinapis, B. sinapistrum )
Weed Type
Annual Broad-leaved Weeds
Occurrence
Formerly the most troublesome annual weed of arable land, charlock is found throughout the UK. It can be plentiful in artificial communities subject to disturbance by human activity but is more of a problem in spring-sown than autumn-sown crops. Charlock also occurs on wasteland and roadsides. It is probably native and is frequent on clay and heavy soil, but is also found on chalk and occasionally on sands and light loam. It is especially prevalent on calcareous loams. Charlock was still abundant in cultivated areas in the 1950's.
Biology
Charlock generally flowers from May to July but flowering may begin as early as April in plants that germinated the previous autumn. Successive flushes of germinating seeds flower through the summer as plants mature. Plants are self-incompatible and cross-pollination is performed by a variety of insects. Seed is set from August onwards. The seeds may be dark or light brown and both types can be found on the same plant and even in the same pod. Charlock has 10-18 seeds per seedpod and produces around 2,000-4,000 seeds per plant. Seed number is closely related to plant dry weight. Charlock can be found in fruit for 3 months of the year.
Some seeds will germinate at once while others remain dormant for long periods and germination is generally sporadic. The light brown seeds germinate more readily when seed is fresh. Seed germinates best when lightly covered with soil. Seed dormancy is under genetic and maternal control. Scarification of the seedcoat increases the level of germination as does a period of dry storage. Nitrate can stimulate or inhibit germination depending on the concentration. Light and temperature also have an effect. The optimum germination temperature is 21°C and there is little germination below 11°C or above 30°C. Seedling emergence takes place from November to July with a peak in March-April. However, seedlings that emerge in the autumn are often killed by frost. Rainfall events and soil temperature have a big influence on the timing of seedling emergence. Emerged seedlings represent around 2.5% of the soil seedbank.
In clay and sandy loam soils, field seedlings emerge from the top 70 mm of soil with most seedlings emerging from the surface 40-50 mm.
Persistence and Spread
Seed longevity in dry storage is 12 years and in soil is 35 years. Charlock seeds buried in uncultivated soil will remain viable for 60 years and can germinate when brought to the surface by deep ploughing. Allowing charlock to seed every 11 years is sufficient to maintain the soil seedbank population.
Seeds broadcast onto the soil surface, ploughed to 20 cm and followed over a 6-year period under cropping with winter or spring wheat had a mean annual decline rate of 23% and an estimated time to 95% loss of 11-14 years. In a soil ploughed 3 times a year with additional tillage through the growing season less than 3% of the original seeds remained after 7 seasons, however, this was still equivalent to 2.4 million seeds per ha. In the same period, seed numbers in the seedbank of undisturbed grassland declined by just 50%. Charlock can appear on land that has been under grass leys for decades when it is ploughed for the first time.
The seedpods usually remain intact until crop harvest. Even when dehiscence occurs, seeds are not scattered far from the parent plant. In cereals at harvest, many pods shatter and the seeds fall to the ground but some seeds are gathered with the grain. In the past, charlock seed was a frequent contaminant of cereal seed. Seeds have also been found in clover and grass seed samples. Charlock can also be introduced as a birdseed alien.
Many species of birds feed on charlock seeds and some seeds pass through the digestive system unharmed. Screenings from grain fed to poultry may serve to disperse seeds of charlock and other weeds. Viable seeds have been found in cattle droppings but viability is lost after a few weeks in a manure heap.
Management
Where land is infested with charlock the soil may be cultivated at regular intervals to stimulate and kill successive flushes of charlock seedlings. Although charlock emerges mainly in spring, autumn stubble cleaning is an effective way of dealing with freshly shed charlock seed. The surface soil is cultivated no deeper than 5 cm and the operation repeated at 14-day intervals. Regular, slight disturbance of the soil surface will enhance charlock germination but then control the seedlings of this and other weeds stimulated to emerge. In spring, early harrowing of fields being prepared for root crops will induce charlock germination allowing the mechanical destruction of seedlings during subsequent seedbed preparations. The subsequent harrowing of cereals and hoeing of root crops will help to reduce a bad infestation.
Control is aided by preventing the return of fresh seed to the land at crop harvest and the subsequent re-introduction of charlock seed in crop seed, straw and farmyard manure. Flower heads were once knocked off by means of a switch or special machine to prevent seed production, hand pulling was also a common practice. Preventing charlock from seeding will reduce the soil seedbank by an order of magnitude after 10 years but allowing seeding to occur in just a single year will restore the population again.
Charlock causes yield losses in spring barley (Scragg et al., 1982). Relative time of crop and weed emergence is a major factor in determining the severity of loss. The weed can also be a problem in late spring wheat grown organically. Seed pods usually remain intact until crop harvest. It is important to ensure that seed is all removed at threshing and none is returned to the land in any way, including straw and farmyard manure. The presence of an autumn or spring cereal will reduce seed production by charlock but some cultivars are more effective than others. The relative time of crop and weed emergence is an important factor. In conditions of moisture stress charlock becomes less competitive against spring wheat.
Charlock seed is susceptible to soil solarization. Heating seeds in a loamy soil for 30 minutes at 50°C significantly reduced seedling emergence.
Exposure to an arbuscular-mycorrhizal fungal inoculum has been shown to cause a 60% reduction in charlock biomass.