Compass plant, lobed prickly lettuce, wild lettuce
Lactuca serriola L. (L. scariola L.)
Annual Broad-leaved Weeds
Prickly lettuce is an annual, rarely biennial weed probably native in waste places, rough ground, disturbed areas and on walls. It also occurs in cultivated fields and along roadsides. It is frequent in England especially in East Anglia and the south east.
In the USA, prickly lettuce populations have been found with resistance to sulfonylurea herbicides following five years use of the chemicals for weed control in continuous wheat crops.
Prickly lettuce flowers from July to September and is automatically self-pollinated. Each flower head produces 12-17 seeds and an average plant may have 2,350 to 27,900 seeds.
In laboratory germination studies the seeds appear relatively indifferent to light but when seeds are put to germinate under a leaf canopy in the field there is much lower germination than in diffuse white light.
Seedlings emerge from February to September but the main emergence peak is in April with a smaller peak in September. Emergence tends to follow soil cultivation.
The plants form a strong taproot.
Persistence and Spread
Seeds buried in soil may remain viable for 3 to 6 years. In a cultivated soil seeds do not persist for longer than 4 years. Seeds in dry storage gave 54% germination after 2 years but seeds submerged in water gave only 1% germination after 3 months.
The seeds have a pappus of hairs that aids wind dispersal and seeds have been blown at least 100 m by the wind. Seed has also been recovered from irrigation water.
Phytoparasitic bacteria have been considered as potential biological weed control candidates for prickly lettuce.
Updated September 2007