Wall speedwell
Other names
Corn speedwell
Latin names
Veronica arvensis L.
Weed Type
Annual Broad-leaved Weeds
Occurrence
Wall speedwell is a native winter or summer annual found on walls, banks, open acid or calcareous ground and cultivated soils. It is common on grassland and heaths, usually on dry soils throughout the UK. In surveys of Bedfordshire, Hertfordshire and Norfolk in the early 1900s it occurred chiefly on lighter soils but was rare on chalk. Wall speedwell prefers nutrient rich, moderately acid sandy loams in the pH range 6.0 to 8.0.
This taprooted weed can be plentiful in autumn-sown wheat. In a survey of weeds in conventional cereals in central southern England in 1982, wall speedwell was found in 2% of winter wheat and winter barley but not at all in spring barley. In a study of seedbanks in some arable soils in the English midlands in1972-3, wall speedwell seed was recorded in 94% of fields sampled in Oxfordshire and 31% of those in Warwickshire.
Tricotyledonous seedlings occur at the rate of 1 in 20,000.
Biology
Wall speedwell flowers from March to October. Plants require vernalization and long days to induce flowering. Seed is set from June onwards. Wall speedwell has around 18 seeds per seed capsule. There can be 17,000 seeds on a large plant. The 1,000 seed weight is 0.090 g.
Fresh seed is generally dormant. Over the summer, the seeds after-ripen and the proportion and rate of germination increase. The temperature range over which germination will take place gradually widens but light is required. Seedlings emerge in two main flushes, April to May and August to November but odd seedlings emerge at other times too. High temperatures and low soil moisture limit summer germination.
Dry stored seed kept at ambient temperature had an optimum germination temperature of 10°C after 5 weeks and required light for germination. The optimum temperature increased with seed age to 20°C after 15 weeks and by then, light was not required for germination. The rate of germination increased with seed age.
Seed that remains on the soil surface germinate mainly in the autumn or over winter. Seedlings from buried seed generally emerge over a longer period. The optimum depth of emergence is 0 to 5 mm and the maximum is generally 10 mm. Seedlings are frost tolerant.
Persistence and Spread
Wall speedwell forms a relatively persistent seedbank. The annual percent decline of seeds in cultivated soil is around 58%. Most seeds germinate in the first 3 years after shedding and few viable seeds remain after 5 years in cultivated soil. Seed recovered from excavations and dated at 30 years old is reported to have germinated.
Seeds are small enough to be blown around by the wind. Viable seeds have been found in cattle droppings.
Management
Sow only pure crop seed. Harrow cereals well early in the season and keep root crops clean to control wall speedwell. Stem fragments can root readily but this is not seen in the field. Unlike other broad-leaved weeds, wall speedwell does not decline following a change to minimum cultivation techniques. In winter wheat, crop density is an important factor in limiting the growth and seed production of wall speedwell.
Seed numbers in soil were reduced by 50% after a 1 year fallow and by 80% if the fallow was continued for a second year. Under winter wheat grown for the same period, seed numbers increased in year 1 but returned to the original level in year 2. Seed return is obviously dependent on seedling survival under the different conditions each year. Fallowing at 5-year intervals over a 15-year period reduced seed numbers in soil but not consistently. Seed numbers were 260, 137 and 33% of original levels after the different fallow years but seed numbers often increased in the intervening cropped years.
The seeds of wall speedwell are consumed by several species of ground beetle.