Start in flats eight to ten weeks before the frost-free date. The seed is very small, so it should just be pressed into the surface of the potting soil, watered, and the flat covered to preserve moisture until the seed sprouts, which is likely to take a week or two. If you don’t need a full flat, put your parsley and other slow-growing herbs in the same tray.
To sow thinly enough, you can use a pen or pencil to pick up the seed: just touch the tip of the pencil to your tongue, then to the seed; the moisture will cause a few seeds to stick to the tip; then you can push them off into the waiting flat with your finger.
During their time in the tray, keep the plants above 50F/10C to avoid triggering their biennial bolting reflex. As soon as the plants have their first true leaves thin to the strongest plant per cell of the tray, or to an inch apart if you are using an open tray.
Set the plants out twelve inches apart once the weather has become reliably mild. An occasional light frost won’t hurt them, but persistent daytime temperatures below 50F/10C will cause them to bolt, or at the minimum, slow down their growth enough to hurt the quality of the final harvest. If planting in rows, allow two feet for a path. We give celeriac seedlings a shot of fish fertilizer at transplant time to get them off to a good start.
We mulch the beds to help keep down weeds and to help regulate moisture. To get the smoothest celery roots, pull away the mulch and soil once the knobs have reached an inch or so in diameter, rub off the small feeder roots above ground level with your finger, and then hill them to blanch that part of the root. Celeriac roots are suitable for harvest from the first fall frosts until temperatures get below 20F, and will keep up to a couple of months in a root cellar, or under moist refrigeration.