There are 3 different kinds of celery plants; normal (sometimes called blanched or bleached) celery, leaf celery (sometimes called zwolsche krul) and celeriac (sometimes called celery root). These are all three related, and will cross pollinate if grown next to each other, so you can only save seeds from one at a time. Of these, normal and leaf celery are much more difficult to grow than celery root.
The two main ‘tricks’ for growing celery root are choosing the right variety and not planting it in the garden too early.
My favorite variety is Giant Prague, but I know several others do well too. The best thing is to ask fellow gardeners and find out what kind does best in your climate. If you are experimenting with different varieties, don’t be discouraged with some failures. There are normal and ‘smooth’ types. I don’t care for the smooth type, and in any case if you do grow it be aware that it is very different from the normal kind. Smooth refers to it’s texture when eaten.
Celeriac is a biennial. This means there are two seasons that make up the growing cycle of this plant. The first season it will grow and form the roots, and the second put it’s energy into reproduction, grow a flowers stalk and go to seed. The problem is celeriac gets easily confused, and if you transplant plants out too early into cold ground, it can think it has passed through it’s first growing season and into it’s second. What usually happens then is either the roots remain very underdeveloped or sometimes they will bolt (send up a flower stalk). In either case it is usually inedible. For some reason this is often not well documented, and it can be hard to understand what is going wrong with your plants if someone doesn’t explain it to you!
Because celeriac has a long growing season, the plants are best started indoors about 7-12 weeks before setting them out into the garden. I usually set them out between 15 May and 1 June, but this will vary according to your climate.
I prefer to start the seeds in a flat (seed tray) and after they have developed their first set of true leaves (after the cotyledons) transplant them into their own containers. The reason for doing it this way is the seed is very small, and it is easier to broadcast over a tray than plant 1 or 2 seeds at a time in pots. This way also allows you to more easily select the strongest seedlings and, if you have old or unreliable seed, you can easily compensate by sowing a little more densely. When transplanting them into their own containers, the soil can be loosened with a table knife or similar instrument, and the seedlings plucked out of the dirt with your fingers. While in the tray, a bottom heat source is not necessary, but can help the seeds germinate faster. Garden centers sell special appliances for this. Make sure you keep the seeds and seedlings moist but not soggy at all times. Make sure you provide the seedlings with enough light, either fluorescent tubes or a growlight.
Once in the garden celeriac appreciates good soil, rich in organic material. It is also very important to give it sufficient water. It can be dug up in the late fall, after the roots have grown to a good size. Celeriac is not very frost hardy, but it may overwinter in mild climates.