One way to have celery year round from the garden is to grow celeriac root. Weird looking but quite flavorful, celeriac root is edible raw or cooked, and tastes similar to the stalks of common celery.
It grows well in the North, keeps well in cold storage all winter, and apparently can have a shelf life of approximately six to eight months if stored properly. You can serve it roasted, stewed, blanched, or mashed, or added to your favorite stews or casseroles. Peel it and chop it and use it in place of fresh celery in cooking. Excellent combined with potatoes when cooking mashed potatoes!