Hi Jan.
Watering celeriac in dry, hot weather is a major help, although over watering can be an issue – I think Charles experienced this a couple of seasons back? Catch crop of interplanted radish could be possible but I have space elsewhere and find that celeriac is a challenge at the best of times, so I give it all the help I can; space, protection, no competition, watering regimen etc.
Additionally, if I didn’t net them, we’d lose the lot inside 48hrs to pigeons & rabbits – keep an eye open there. Best of luck.