Start seedlings 10-12 weeks before planting date Sow seeds at a depth of 3 mm (1/8“) Celery prefers cool moist conditions, so be sure to maintain even moisture throughout the summer especially in dry hot weather. To harden off plants, reduce water for one week. Plants may be planted outdoors 2-3 weeks before last frost. Space plants 15-25 cm (6-10”) apart leaving 90cm (36”) between rows.