Lamium purpureum L.
A native winter to summer annual weed common on cultivated land and waste places throughout the UK. It occurs on all soils but prefers loose, nutrient-rich, loamy or sandy soils. In an early survey of Bedfordshire and Herefordshire it was characteristic of light sandy soils.
Red dead-nettle is common in cereals where it has benefited from the control of more competitive weeds. It is a weed of gardens as well as arable land.
Red dead-nettle can carry cucumber mosaic virus both in the growing plant and in its seeds. It is also a host of the potato leaf-roll virus.
The closely related cut-leaved dead-nettle (L. hybridum) is sometimes found growing alongside the red dead-nettle but is much less common.
Red dead-nettle flowers from March to October, sometimes into the winter. The flowers are self- and insect pollinated. Seed is set from May to July and into November. Viable seed number per plant when grown in isolation is estimated at 27,634 but in competition with winter wheat the number of seeds per plant may range from 1,075 to 4,594 depending on crop density. Seed numbers are well correlated with plant dry weight. Red dead-nettle can be found in fruit for 8 months of year.
In the UK, germination occurs throughout the year but seedlings emerge mainly from March to May and July to October. In the USA, seedling emergence is generally restricted to autumn. Seed shedding from these plants occurs in spring and fresh seeds are dormant. The dormancy cycle of the seeds plus the hot dry summers in the USA ensures germination occurs only in autumn. Seeds that do not germinate become dormant again. Plants in the USA may have a different genotype from those in the UK or there may be winter and summer races in the UK or it may simply be the difference in climate. Light is required for germination.
Seedlings and plants are frost tolerant. Over the winter the prostrate shoots may root at the nodes.
Red dead-nettle seeds broadcast onto the soil surface, ploughed in and followed over a 6-year period of cropping with winter or spring wheat had a mean annual decline rate of 20%. The estimated time to 95% decline was 13-14 years. Seedbank decline was also studied in a succession of autumn-sown crops in fields ploughed annually for 3-4 years with no seed return. The annual rate of loss was 55% and the time to 99% decline was calculated at 5.4 years. Annual seedling emergence represented 3.5% of seedbank. Seeds recovered from excavations and dated at 30 years old are reported to have germinated.
Some local seed dispersal by ants has been observed.
Control is by surface cultivations in spring and autumn. However, in moist conditions, detached shoots can re-root after spring cultivations. In winter wheat, crop density is an important factor in limiting seed production by red dead-nettle through the effect on weed biomass.
Updated November 2007.