Celery seed is a spice used in salads, sauces, dressings, brines, and dry rubs. It's available in whole seed and ground. In its whole form, it's a small seed, about the size of a poppy seed, with a dark brown color. When ground it is a fine, dark greenish-brown powder.Celery seeds are a versatile spice with many uses. They're sold as whole seeds, crushed, or as a ground spice. Ground celery seed is often referred to as celery powder. Unsurprisingly, the flavor of celery seed resembles that of a celery stalk.