Soil Preparation

• Select a site that receives full direct sunlight. 
• Celery needs compost-enriched soil. Loosen the soil to a depth of 12 to 15 inches with a garden fork or tiller. Mix 2 to 4 inches of aged manure and/or compost into the soil. Or, work in some 5-10-10 fertlizer. The soil should retain moisture, bordering on wet but still draining.
• Celery prefers a soil with a pH between 5.8 and 6.8. Get a soil test if you’re not sure of your soil pH.
• It’s important for celery to grow in moisture retentive soil that doesn’t drain too quickly. Wild celery grows in boggy ground, so you’ll need to ensure consistent moisture for this thirsty vegetable, while a sunny spot should ensure good, even growth.

Sowing Seeds Indoors

• Due to a long growing season, it’s best to start celery seed indoors. For a spring crop, start seeds 10 to 12 weeks before the last spring frost date. (For a fall crop, start seeds in time to transplant seedlings 10 to 12 weeks before the first autumn frost date.)
• Note: The seeds are tiny, so you’ll need to sow with care and a keen eye.
• Before planting, soak seeds in warm water overnight. This will speed germination.
• Fill seed flats or pots with good-quality seed starting mix then gently firm it level.
• Press soaked seeds into seed-starting soil; to get good germination do not cover with soil. The easiest way to sow the seeds is to carefully tap the packet above the surface of the potting mix and watch carefully as the seeds fall. Ideally you want them to fall about an inch apart. Once you’re done, firm the seeds into place.
• Cover starter trays/pots with plastic wrap to retain moisture. Germination should occur in about a week but it can take up to three weeks; be patient.
• Soon after seedlings appear, place a fluorescent grow light 3 inches above them for 16 hours a day (plants need dark, too).
• Maintain an ambient temperature of 70° to 75°F during the day and 60° to 65°F at night. 
• Mist regularly.
• When seedlings are 2 inches tall, transplant them to individual peat pots or to deeper flats with new potting soil. In flats, set the plants at least 2 inches apart.
• Harden off seedlings before transplanting by reducing water slightly and putting them outdoors for a couple of hours each day.

Transplants in the Ground

• Plant celery outdoors when the soil temperature reaches at least 50°F and nighttime temperatures don’t dip down below 40°F. (Cold weather after planting can cause bolting.)
• Begin acclimatizing celery to the outdoors two weeks before planting: Leave your plants outside for progressively longer each day, taking care to bring them back under cover if frost threatens. 
• Work organic compost into the soil prior to planting. (Learn more about soil amendments and preparing soil for planting.) Or mix in fertilizer (about one pound of 5-10-10 per 30 square feet). 
• Water thoroughly.