Position: The roots of celery plants are limited, usually stretching just six to eight inches away from the plant and only two to three inches deep, so the top part of the soil not only has to have enough moisture, it must also contain all the nutrients the plants need. Celery likes soil that is capable of retaining moisture and so soils that have had organic compost or manure are well suited, otherwise dig in organic compost or manure fertiliser into the soil a few weeks before planting out the celery seedlings. If your soil is well drained ensure that the celery receives adequate regular watering in warm periods.
Sowing: Sow indoors September to October. Sow seeds indoors 10 to 12 weeks before the last frost. Outdoor sown seed rarely germinates satisfactorily. Sow on the surface of large cells, tray or pots containing fine seed compost. Celery seed is very small, so mix it with sand or vermiculite to make it more manageable. It is one of the few plants that germinate in the light, so scatter seed as thinly as possible on top of the moist compost. Celery germination rates aren't high so if you are sowing seeds in stations, sow about 5 seeds per station, a few inches apart.
Gently water in, you may wish to sit the tray / pots in a pan of water and allow the water to rise through the compost using capillary action. Once watered, cover the pots/ trays with a transparent cover - sheet of glass, clingfilm, or propagator lid and transfer to a warm bright position, a north facing window sill is ideal, but make sure that it is out of direct sunlight. Once the seeds have germinated you can remove the cover, and water as necessary.
Germination takes from around 14 to 21 days and is successful when temperatures are around 21°C (70°F). When the plants are 5cm (2in) tall, about 2 weeks after germination, transplant them to individual pots or to another, deeper, flat containing new potting soil to grow on indoors. If you use flats, transplant the plants at least 5cm (2in) apart.
Transplanting Outdoors: Being a biennial, celery should flower in the second year, but if it thinks winter is coming and the temperature drops below 10°C (50°F), it will probably panic and flower in the first year and all your efforts will be lost. For this reason, it is vital to delude it that it is summer right from the start. However, because of the need for a long growing season, it's often worth the gamble to set a few of the plants out early. Transplant some seedlings to the garden as early as a week or two before the last frost date. If the weather does turns cold after you set your celery out (night temperatures consistently under 10°C/50°F for about two weeks), the plants may go to seed prematurely. Plant them with the base at soil level. Cover them with fleece or cloches for a few weeks and protect against slugs.
Plant out the rest of the seedlings in November or December, once temperatures have risen above 13°C (55°F). If it is still cold when they are ready to go out, trim them down and keep them in the warm a little longer. Plants should be at least 10 to 15cm (4 to 6in) tall when you set them out. Be sure to harden plants off first for a week to 10 days to get them used to spring weather.
Water the plants well an hour before transplanting. Remove some of the outside leaves from each plant before setting them in. As with head lettuce, this trimming helps the roots recover from the transplant shock and resume normal growth more quickly. Either space your seedlings about 20cm (8in) apart in rows that are about 30cm (12in) apart or plant out in a square pattern 20cm (8in) apart. If they are planted in tight-knit blocks rather than rows they blanch each other as they grow, resulting in even paler stems.
Sidedressings of 5-10-10 or a similar balanced fertiliser or manure tea in the second and third month of growth will help keep celery growing steadily. Use one tablespoon per plant and sprinkle it in a shallow furrow 7 to 10cm (3 to 4in) from the plant and cover it with soil. Continue to apply manure tea weekly as you water the plants.
Mulch the plants after they're about 15cm (6in) tall to help keep the soil moist and roots cool. It will also help to keep down weeds, which is important because celery grows slowly and doesn't appreciate any competition from weeds. If you don't mulch, be careful not to weed too deeply near plants. Celery has a shallow root system that can be harmed by deep cultivation. Careful watering is vital to good celery yields. A hot spell without adequate watering will result in the stems becoming tough and stringy. They can also develop hollow or pithy stalks in dry spells.
You can blanch your celery by covering up the stems to prevent light reaching them. The colour of the stalks will lighten, and their flavour will become milder.
To blanch celery, wrap and tie the plants in brown paper, or use cartons or cardboard tubes and use them as ‘sleeves’. Cover the stems of the plants a week, 10 days or even longer before you want to harvest. There's no need to blanch the top leaves. Some people place boards close along each side of the row, others simply bring soil or mulch up around the plant to block out the sun about a month before harvesting, although this method may let dirt fall into the interior of the stalks, making them hard to clean. Plants should be dry if blanched with soil or else they may rot.
Harvesting: February until the first frosts. You can remove a few stalks at a time rather than harvesting the whole plant. Remove the outer stalks first and let the less developed inner stalks continue to grow. Take care not to damage the rest of the plant if removing individual stalks. Harvest the celery plant when it has reached the desired size. Pull up the whole plant and trim off the roots so that all stalks are still as one unit. Wash the stalk bulb and dry.
Storage: Celery will keep in the fridge for a couple of weeks. Celery stores really well - you can keep it for many weeks with no trouble. Dig up the plants carefully, disturbing the roots as little as possible. Replant them in boxes of sand in your root cellar or set them close together in a trench in your cold frame where you can keep them from freezing. As long as the roots stay moist and the stalks dry, they'll really keep. Temperatures in the range of 2 to 5ºC are best for good storage. Celery will blanch naturally when in storage.
Companion Planting: Growing celery with companion plants and vice versa can be successful. Dwarf beans with their nitrogen fixing roots benefit celery, but tall beans would overshadow them. For cabbages, the white butterfly is repelled by celery. Tomatoes and dill are good neighbours of celery and leeks can be grown in the same trench and earthed up for blanching along with the celery. Flowers for celery are cosmos, daisies and snapdragons.
Wild celery is a Mediterranean marsh plant that thrives in damp ditches, often near the coast in many parts of Europe. Ancient literature documents that celery, or a similar plant form, was cultivated for medicinal purposes before 850 B.C. It was widely used as a medicinal plant by the Greeks and Romans. Although celery is believed to originate from the Mediterranean, indigenous ‘wild’ relatives of celery are found in southern Sweden, the British Isles, Egypt, Algeria, India, China, New Zealand, California and southernmost portions of South America.
Known as smallage in England, it was used to flavour soups. The modern equivalent is cutting celery. The celery as we know it today and celeriac (which is grown for its bulbous lover part of the stem) were bred in the 17th and 18th centuries respectively.
The Italians domesticated celery as a vegetable in the 17th century resulting in selections with solid stems. Early stalk celery had a tendency to produce hollow stalks. After years of domestication, selection eliminated this characteristic as well as bitterness and strong flavours. Early growers found that the naturally strong flavours could be diminished if grown in cooler conditions and also if blanched.
There are two types of stalk celery varieties, self-blanching or yellow, and green or Pascal celery. In North America green stalk celery is preferred and mainly eaten raw although it is also eaten cooked. In Europe and the rest of the world self-blanching varieties are preferred.
Nomenclature: The genus name Apium was the classical name for celery-like plants. The word was used by Pliny but was called apion by Dioscorides. (Some relate it to the Celtic ‘apon’, water, as its preferred habitat.) The species name graveolens means 'strong smelling', it derives from the Latin gravis meaning 'grave or 'heavy', and olens from the verb 'oler' meaning 'smelling'. Similarly, the word oleracea means 'garden herbs or vegetables used in cooking' The Greek writer Homer referred to celery as 'selinon'. The Latin name was 'selinun' and the French name 'celeri' is similar to the name we use today.