There is considerable potential for the improvement of the slow and irregular germination of celery seeds (Apiurn graveolensL.) by pre-sowing hydration treatments, but poor germination performance as a result of drying seeds after treatment is a recurrent problem. Over the temperature range 5–20°C the onset of seed germination is determined by a thermal time relationship, but a pre-sowing treatment temperature of 15°C was identified as the most effective for the two celery cultivars—‘Tall Utah 52–70’ and ‘Green Giant Hybrid’—when the seeds were only surface-dried before sowing. However, conventional air-drying of treated seeds at 20°C caused severe delays in germination and decreased uniformity. Higher treatment temperatures reduced the adverse effect of drying on the seeds but resulted in little or no germination advantage because of thermodormancy, which was particularly evident at 25°C. Deleterious effects of drying-back were avoided, and in some instances further germination enhancement gained, by slowing the rate of drying which was achieved by exposing the treated seed to a series of controlled humidity atmospheres at 80, 68, and 48% RH for various times.