Anagallis arvensis L.
Scarlet pimpernel is a widely distributed annual or rarely perennial weed that is not recorded above 1,100 ft in Britain. It prefers soils in the pH range 5.5 to 8.0 and is said to be an indicator of loam. At early growth stages it grows better in light shade than in dense shade or in full sun.
Scarlet pimpernel is common on arable land in cereals, sugar beet and other crops. It is a common garden weed.
Numerous forms and varieties exist. Sub-species arvensis with red flowers is found on cultivated land, by roadsides and on dunes throughout the UK. Sub-species foemina with blue flowers is found in arable fields in S & W England but is rare.
Scarlet pimpernel is poisonous if ingested by man, dogs and horses. Under normal conditions animals are unlikely to consume sufficient of the plant to suffer injury. Scarlet pimpernel may cause dermatitis if handled.
Scarlet pimpernel flowers from April to August, sometimes into October. The flowers are self or more rarely insect pollinated. There are 35-40 seeds per seed capsule. The average seed number per plant is 900 but a large plant may yield 12,000 seeds. Seed shedding can result in over 1,000 seeds per m². Four percent of seeds may be viable just 15 days after flowering. The seeds are fully mature after 6-8 weeks. Plants can be found in fruit for 4 months of the year.
Seed from different sources may vary in the level of dormancy. Seeds contain a water soluble germination inhibitor. Dormancy is broken by chilling and light is then required for germination. Scarlet pimpernel seeds germinate better at moderately low temperatures. Germination will occur at 2-5°C but the optimum is 7-20°C.
Seedlings can emerge throughout the year but emergence is mainly in March-May and August-September. Dry conditions in summer probably limit germination at that time. In a sandy loam soil, seedlings emerged from the top 40 mm of soil with most coming from the upper 25 mm. Plants from seedlings that emerge in the autumn can overwinter and will continue to grow at moderately low temperatures.
Seed is able to remain viable in soil for at least 10 years. Seed recovered from excavations and dated at more than 30 years old is reported to have germinated. Seeds in dry storage remain viable for 8 years.
Scarlet pimpernel seed was a common impurity in clover and in vegetable seeds. Viable seeds have been found in wormcast soil. Seeds have been found in the droppings of gulls, and seedlings have been raised from the excreta of various birds.
Normal tillage operations should keep the weed in check. If scarlet pimpernel is plentiful, surface cultivations and the inclusion of one or more root crops will reduce it. Scarlet pimpernel seedlings can grow at relatively low temperatures and light levels so can be more of a problem early in the year.
In set-aside, scarlet pimpernel populations were greater under natural regeneration than under a sown ryegrass or ryegrass/clover cover. Seed numbers were higher when the cover was cut once a year rather than twice. In pasture, cattle avoid scarlet pimpernel allowing it to increase.
Fallowing for 1 year reduced seed numbers in soil by 75%. Seed numbers were reduced but to a lesser extent by cropping with winter wheat for the same period.
Updated November 2007.