Celery growing reasonably

I'm only growing 5 plants in the garden (and another half dozen up the allotment) as I only need 2 for Sturton way ahead in October. I've never been a great celery grower and lost all my plants last season to some strange rust disease that rendered them hopeless for showing. So far I have no sign of this showing, nor celery leaf miner which is a pest that burrows into the leaflets and makes them quite unsightly..

I have very loose collars on them....actually 6" bottomless plant pots but I'll be putting a 10" loose collar on them soon.