I won't be planting mine out til next week at the earliest. We're forcast 3oC for Friday night. My books (pre-global warming) say plant out end of May, beginning of June. It's a lot easier than celery. my celery was a complete failure 2 years ago. My celeriac last year was also terrible, but the amount of effort I had to put in to get nothing was a lot less. Result! :D