Celery is a rewarding crop for the home gardener but requires a long growing season and a precise indoor start to reach maturity. Celery (Apium graveolens) is a slow-growing plant, often cultivated as an annual, demanding patience and attention to timing. Because of its extended time from seed to harvest, starting seeds indoors is necessary for most climates to ensure a successful yield before the season ends. Determining the date to begin the indoor process is the most influential factor in growing celery.
Determining the Optimal Indoor Start Time
The calculation for starting celery seeds indoors revolves around the last expected spring frost date in your specific location. Celery seedlings require a long lead time indoors before they are robust enough to be transplanted. Most sources recommend starting seeds 10 to 12 weeks before the last frost date, though some growers extend this period to 13 or 14 weeks for the strongest starts.
This extended indoor period is necessary because celery seeds are slow to germinate, often taking two to three weeks to sprout. The initial growth rate of the tiny seedlings is also quite slow, necessitating several weeks under controlled conditions to develop into sturdy transplants. Calculating the start date backward from the last frost date ensures the plants have sufficient time to establish themselves before the outdoor growing season begins.
Starting celery seeds directly outdoors is generally not recommended unless you live in a very warm climate with an exceptionally long growing season. A delayed start will likely result in a smaller or failed harvest, as the plant cannot tolerate the high temperatures of mid-summer.
Creating the Right Conditions for Seed Success
Maximizing germination depends on providing specific environmental controls. Celery seeds have particular requirements that make initial sprouting challenging. The tiny seeds need light to break dormancy, so they should be surface-sown or covered with only a slight dusting of fine material, such as vermiculite.
Warmth is important immediately after sowing, as seeds germinate best when the soil temperature remains consistently between 70°F and 75°F (21°C and 24°C). Using a heat mat is recommended to maintain this temperature range from below, ensuring a stable microclimate. The seed starting medium should be sterile and fine-textured to prevent disease and allow small roots to penetrate easily.
Maintaining uniform moisture is crucial for successful sprouting and early growth. The soil must be kept evenly moist but never saturated or allowed to dry out, which can be accomplished by covering the trays with a humidity dome or plastic wrap. Once the seeds germinate, the seedlings should be moved to a slightly cooler location, around 60°F to 70°F (15°C to 21°C), to continue development. Celery seedlings also require supplemental artificial light for approximately 16 hours per day, as a sunny windowsill is usually insufficient for robust growth.
Preparing Seedlings for the Outdoor Garden
The transition from the sheltered indoor environment to the garden involves careful timing and a gradual acclimation process. Celery seedlings should be ready to transplant when all danger of frost has passed and the soil temperature reaches at least 50°F (10°C). Planting seedlings out too early, especially if exposed to prolonged cold temperatures below 50°F (10°C) for a week or more, can trigger bolting.
Bolting causes the plant to prematurely send up a flower stalk and produce seeds, resulting in bitter, woody, and undesirable celery stalks. To prevent this, the seedlings must undergo “hardening off,” which slowly introduces them to harsher outdoor conditions. This transition involves placing the seedlings outside in a sheltered, shaded spot for a few hours each day, gradually increasing their exposure to direct sunlight and wind over a seven- to fourteen-day period.
Reducing the amount of water during hardening off is advisable, as it helps toughen the plant’s tissues and slow its growth slightly. This gradual exposure prepares the plant to cope with the increased stress of the garden environment, reducing the likelihood of transplant shock or premature bolting. Once fully hardened, the young celery plants can be planted in their final garden location, where they require consistent moisture.