Scarlet Flax Seeds (Linum rubrum). This introduced specie native to North Africa and southern Europe enjoys sun to partial shade and is very tolerant of dry soils. Scarlet Flax is an excellent annual for quick, intense summer colour and contrasting rich green foliage. Plants grow to between 30-60 cm (12-24″) in height. While each flower typically only lasts a day, due to the large number produced by each plant, flowering is pretty well continuous from late June through to August.