pale-flowered persicaria, pale smartweed
Persicaria lapathifolia (L.) Gray, (Polygonum lapathifolium, P. tomentosum L.)
Pale persicaria is a native annual found throughout the UK in waste places and cultivated ground especially on damp soils. It also occurs in ditches, manure heaps, on river gravels and by ponds. Pale persicaria is sometimes troublesome on moist arable soils in good condition. It is found on a range of soils from light sands to heavy clay. It is not recorded above 1,250 ft in Britain.
It occurs as a weed in cereals and other arable and horticultural crops.
Pale persicaria flowers from June to October. The flowers are both self- and cross-pollinated by insects. Emergence to flowering may take around 6 weeks. The seed number per plant ranges from 10 to 19,300. Numbers are lower when pale persicaria is growing as a weed in arable crops. The seed demonstrates considerable variation in weight both between and within populations and even between inflorescences on the same plant. Soil fertility can influence plant size, seed number and seed weight. The 1,000 seed weight ranges from 0.8 to 3.6 g.
Pale persicaria seeds require a period of after-ripening at low temperature before germination will occur. Scarification and stratification at low temperatures will promote the germination of after-ripened seeds. The germination of buried seeds is enhanced by exposure to light. Dormancy is overcome by low temperatures over the winter but dormancy is re-imposed by increasing temperatures in late spring. The main period of seedling emergence is April to May with a peak in April.
In sand, clay and peat soils most seedlings emerged from the surface 40 mm of soil with the odd seedling emerging from down to 80 mm. Emergence is better from shallowly buried seeds than from seeds on the soil surface.
Adventitious roots may develop on the lower nodes of stems in contact with the soil.
Pale persicaria seeds broadcast onto the soil surface, ploughed in and followed over a 6-year period of cropping with winter cereals had a mean annual decline rate of 22%. The estimated time to 95% decline was 10-17 years. Dry storage reduced seed viability by around 50% after 4 years and by 100% after 7 years. Seed submerged in water for 5 years retained 59% viability.
Pale persicaria seeds have been found as impurities in cereal and clover seed, particularly home saved seed. Seeds have been found in cattle droppings. The viability of seeds in dung was reduced to 1% after 2 months storage. Birds eat the seeds and seedlings have been raised from the excreta of various species. The seeds are also eaten and spread by rabbits Seeds have been recovered from irrigation water and may float for up to 6 months if the outer covering remains intact.
It is important to ensure only pure crop seed is sown. Control is by surface cultivations in spring and early summer and the inclusion of hoed root crops in the rotation. Seed shedding must be prevented.
Exposure to an arbuscular-mycorrhizal fungal inoculum has caused a 60% reduction in the biomass of pale persicaria.
Updated October 2007.