Crops Evening primrose (Oenothera spp.)

Seedling

Both annual and biennial forms of evening primrose are grown, but plant structure is very similar There is a strong taproot, often with lateral roots at some depth. Leaves are of a narrow oval shape, pointed and with a prominent midrib; they are usually dark green, but are sometimes tinged with red or purple, particularly when stressed. Initially the leaves form a rosette, in which form biennial crops overwinter. During late spring, a strong central stem is produced, round which leaves are spirally arranged: the stem may grow to 2 m in height and may also produce branches.

Plants in flower

Flower

Oblong-shaped buds, formed on the upper parts of main stems and branches, open in sequence up the stem to produce the 4- petalled yellow flowers; these open in the evening, each lasting only 1 or 2 days.

Ripe seedpods

Seeds

Pods also ripen successively up the stem, often splitting open when ripe, thus allowing seed to shed; frequently the earliest-formed pods may ripen before buds have opened near the tip of the stem. For sowing, the soil should be worked down to a fine tilth, into which the tiny seeds may be shallowly drilled: irrigation may be necessary to ensure germination at the right time. The biennial varieties (O. biennis) are usually sown in July or early August, the high soil temperatures aiding germination; annual types (often O. lamarkiana) are drilled in spring. Seed rates of 2-4 kg per ha, at row widths of about 25 cm, may be suggested as suitable, to establish a crop density of some 30 plants per square metre which is probably best for reducing branching and encouraging evenness of maturity Herbicides may be used to control weeds during the rosette stage: nitrogen fertiliser at a rate of 50 kg per ha is best applied in late spring. Harvesting is usually performed during late September or October, timed to maximise the amount of ripe seed obtained; combine harvesting is usually preceded by chemical desiccation or swathing. Yields are variable, usually between 0.3 and 0.8 t per ha. Evening primrose is now considered to be an established minor crop. Breeding work has resulted in the release of varieties in which the ripe pods do not split, thus reducing seed losses and increasing potential yields. Current research into the properties of GLA is concerned with the correction of metabolic imbalances of the body, extending into the possible use of some derivatives of this substance as an aid to treating certain forms of cancer.