Sow celery seeds from March to April on the surface of a free draining, moist seed compost and cover with a fine sprinkling of vermiculite or compost. Place in a propagator or seal the container inside a polythene bag at constant temperature of 15-18C (59-64F) until after germination which takes up to 21 days. When the seedlings are large enough to handle, carefully transplant them into 7.5cm (3″) pots and grow on at a slightly reduced temperature.
When all risk of frost is passed gradually acclimatise celery plants to outdoor conditions over a period of 7 to 10 days prior to planting outdoors. Grow celery plants in moist, rich fertile, well drained soil in full sun. Prepare the soil in early spring by adding plenty of well rotted garden compost to the soil to improve its structure and fertility. A week before planting, apply a general fertiliser to the soil surface and rake it in.
Plant celery at a distance of 25cm (10″) apart, making sure that the crown of the plant sits at ground level. Plant self blanching celery in a grid formation to promote self blanching. Water the plants in well after planting.
Celery requires a moist soil at all times to avoid the stems becoming tough and stringy. Water celery plants frequently, particularly during hot dry periods. Provide a thick mulch of organic matter around the base of the plants to help conserve moisture and reduce weed growth. Celery can be harvested from August onwards, or when the stems reach around 30cm (12”), by lifting the plants with a garden fork. Lift all plants before the first frosts.