Celery plants are warmth-loving summer annual vegetables known for their high water content. Celery stalks are often eaten raw or cooked, and the leaves are also used in salads or for juicing. These vegetables demand a long growing season of 125 frost-free but cool days. This can present a problem for cold-winter climates, because the frost-free fall and spring months aren't long enough for celery to reach maturity. In most climates, you'll need to start the celery seeds indoors during early spring for a summer harvest.
Start the celery seeds indoors about 10 to 12 weeks before the last expected spring frost. Soak the seeds in lukewarm water overnight before planting them.
Plant the celery seeds in a seed tray or into 3- to 4-inch-diameter planter pots filled with loose, well-draining potting soil. Plant the seeds 2 to 3 inches apart and about 1/5-inch deep.
Water the celery seeds and seedlings once every three days to keep the potting soil evenly moistened and not dry or soggy. Keep the celery seedlings in bright, direct sunlight.
Transfer the celery seedlings into individual 3- to 4-inch-diameter planter pots when they're about 2-inches tall.
Prepare a planting bed for your celery seedlings that's in full, direct sunlight and has fast-draining soil. Loosen the top 12 inches of soil using a pitchfork or rototiller. Spread a 2- to 4-inch-thick layer of organic compost on the planting bed and mix it into the soil.
Transplant the celery seedlings outdoors in spring after all chance of frost has passed and the soil has warmed up to at least 60 degrees F. Plant the celery seedlings 8 to 10 inches apart in rows that are 10 inches apart.
Spread a 3- to 4-inch-thick layer of organic mulch around the celery plants. Water the celery plants deeply and thoroughly to soak the soil to a depth of at least 6 inches once every week.
Feed the celery plants two or three times during the growing season by spreading a "side-dressing" of organic compost or compost tea along the rows.
Harvest the whole celery plants when they're about 12-inches tall. You can begin harvesting the outer stalks of the celery plants when they're just 6- to 8-inches tall.