This grower is sure of this plant identification.
Lactuca genus (Lettuce). This genus has about 100 species. They do produce a number of common weeds but is best known in the form of 'Lactuca sativa', the common lettuce. Some species may be annuals, biennials or perennial and range in size. They can have large sprays of small blooms, which are often mauve or yellow. The species which are non-edible may contain very bitter compounds that have sedative properties and some have been used medicinally.
They will grow in any moist but well-drained soil with full sun or partial-shade. In a cool climates sow the seeds from early spring to late summer, in a warmer climate sow from autumn to mid-spring. You can propagate from seed. For tender, succulent leaves it is important that there is no check in growth, so ensure the soil remains moist. Shade the young plants in hot weather and feed at intervals with weak liquid manure.