Coriander is a herb that can be a great addition to your polytunnel. The plant is known for its pungent leaves, which are not to everyone's taste, and for its round seeds, which are also used in a range of curries and other recipes. You should choose the correct variety – one that is best for leaves or best for seeds. Either type can fit well in a mixed polyculture section of your polytunnel and is well-suited to growing with other herbs.
Once you have chosen your coriander, you can either sow in containers inside from April or direct in the soil between June and September. Sow coriander shallowly at a spacing of around 30cm. It is important to keep the soil around plants moist but you should also be careful not to overwater. If the roots dry out or are swamped, this can cause the plants to bolt or flower prematurely, which will reduce the quantity of leaves that can be harvested.
Coriander does not like too much competition and so you should weed regularly around your coriander in order to prevent any check to growth of the plants. Sowing coriander at the right time can also help to reduce the chances of a plant grown for its leaf going to seed prematurely.
Coriander plants can attract beneficial predatory insects to your polytunnel and it is suggested that planting coriander close to cabbages and other brassicas may reduce the incidence of pests in those plants. By attracting hoverflies, coriander may be beneficial as a companion plant to a whole range of crops.
If you are harvesting the leaves then these are best harvested when young. They can be used fresh or frozen for later use. The harvesting can begin as early as July and with the right varieties you can be picking fresh coriander leaves right through until the autumn.
If you are harvesting the seeds then these can be ready for harvest, usually, from late summer onwards. After flowering, the coriander plants will go to seed, creating seed pods filled with round, ball-like seeds that are around the size of pepper corns. Cut the stalks as soon as the pods are ripened and just before they fall to the ground. These can then be dried on paper sheets or in a paper bag. Leave the seeds to fully ripen before separating them from the rest of the stalk and storing them in airtight containers. The seeds can be used whole in recipes or ground into a fine powder using a mortar and pestle.