Celery will frequently become bitter during warm and hot weather. The best celery is grown in cooler climates. It’s best harvested right before frosty nights set in. Stress can also make it bitter, especially uneven moisture, with dry periods.
Though loving cooler weather (60–75°F, 16–24°C), celery can tolerate some heat for short periods, especially with light shading. Blanching by hilling or paper tubes/wrapping can help improve eating quality. I’d use a combination of brown paper inside, with white paper facing out (cooler). Blanching is unlikely to reduce bitterness in areas where it’s normally hot, above 80°F, 27°C, for long periods.
Despite favoring cool growing, if seedlings are set out when it’s too cold <50°F, 10°C, plants can bolt. It’s recommended that post harvest, celery get chilled quickly, in cold water.
To give some perspective, Michigan produces a lot of celery, as do cooler parts of California (often as a late season/winter crop). If you have long, hot summers where the heat extends into fall, it’s not worth trying. If you’d like to experiment with one of the more heat tolerant types, try Chinese White or Chinese pink. Some overhead shade (about 50%) during the hottest parts of the day may help. Chinese celeries are less fussy, but still require evenly moist, rich soil. Their flavors are stronger and the leaf stems, petioles are thinner. Since celery can tolerate light frosts, it can be harvested after growing in cool-cold conditions.
Celery herb, often marketed as Par-Cel, is likely the most tolerant of heat. One can avoid harvesting until cold weather mellows it, and by picking younger leaves which grew during cool weather. This is a leafy, vs. petiole type, with stronger flavor. I’ve had it overwinter, here in Virginia, which surprised me. For soups, stews and potato & tuna salads, less is needed to impart that celery taste. It looks a lot like the giant Italian parsleys, and is equally mowed by rabbits!
