Celery can be tough to plant. It is very temperature sensitive, needing a cooler spring and summer to grow fully. For warmer areas, celery will need afternoon shade or to remain potted to make it through summer heat. If you experience extremely high summer temps, save some of your seeds for a late fall crop (planting in late summer). Ideal growing temperatures are between 60-70ºF (which can easily be achieved indoors).
Celery does best when planted in acidic soil. If you are in a place with alkaline soil or hard water, keep celery potted and try to use distilled or rain water for maintaining soil acidity. Celery also needs a lot of water as is grows - this will adjust depending on whether you’re growing indoors or outdoors, and whether the space is in full sun or partial shade.
If you have springtime temperatures above 50ºF, plant the celery immediately. Celery requires 3-5 months of growing time, and if it gets too hot before it is fully grown, your celery can be ruined. If you still have some cold times ahead, start the celery indoors in soil on a heat mat. Do not cover the seeds - they are small and need light to germinate. If you must, use sandy soil to lightly cover them. Keep them in a sunny window so that the seeds can go through day and night light changes. Since celery seeds are very small and can take some time to sprout, spread them as thinly as you can. The more seeds that are clumped together, the harder it will be to separate them when it comes time to transplant. To decrease germination time, soak seeds overnight in warm water.
Thin celery to 12-18” apart. Taller celery varieties can be up to 30” tall, so don’t plant them next to something that they will shade. The tighter the plant spacing, the longer/taller the stalks should grow. Celery has a shallow root system, so consider a drip irrigation system or consistent, regular watering. Celery doesn’t compete well with weeds, but cultivating can disturb the shallow roots. Consider trimming weeds with scissors or tackling them regularly when they are small!
Celery is completely usable, and needs very fertile soil to grow. Make sure that you are fertilizing throughout the growing period, in 4-6 week intervals, as it is a heavy feeder, and can strip the nutrients from your soil. Also mulch your celery bed continuously, either with fresh grass clippings, leaves, or straw. This will help to keep the soil cooler and retain moisture. Once the stalks thicken up to a usable size, cover the celery plant completely with straw or a shade cloth to “blanch” your plants. 2 weeks after this, you can harvest the whole plant. You can also side harvest (like lettuce) as the plant grows, but the stalks will be thin and stringy. If planting a fall crop of celery, be sure to pull the plants before any freezing weather.
As a side note, if you chop the bottom of the celery plant off, you can re-root it in water and grow a whole new plant! It will never be as large as the previous one, but you can use it for soups and salads.