When should I harvest celery? Also, when should I harvest green onions?
Harvest celery as soon as it's large enough to use. Either cut off individual stems as they develop color or pull the entire plant and cut off the roots. Celery should be harvested when the petioles (stalks) from the soil line to the first node are at least 6 inches long.
Once the tops begin to lay over, usually by late summer, onions are ready to be lifted. If it's immature onions (scallions) you are referring to, then this article will help: https://www.gardeningknowhow.com/edible/vegetables/onion/harvesting-scallions.htm
What diseases should I be looking for outside of the normal ones like aphids, scale, rust, black spot, etc. ? Have I planted too late in the season?
Celery is normally planted once daytime temperatures are around 50-60 degrees F. Also, the plant needs at least a soil temperature of around 59-60 degrees F. This article will help you with growing celery: https://www.gardeningknowhow.com/edible/vegetables/celery/tips-on-how-to-grow-celery.htm
Here is info on both growing this plant and common issues/treatments: http://harvesttotable.com/2009/06/celery_growing_problems_troubl/
Celery plants, can they be dug out and replanted before harvesting?
If you mean, that you need to move them to a new spot before you harvest them, they typically do not respond well to that and your crop can suffer if you attempt to move it.
My garden soil does well for growing celery. However, at age 70 I need to simplify! This year I mulched my two rows with rice hulls. I know rice hulls are safe for flowers, but what about vegetables?
Yes, rice hulls are safe to use with vegetables. As with most grain hulls, they add few nutrients to the garden and are somewhat slow to decompose, but rice hulls do make an excellent soil conditioner for areas with poor drainage. Because they are light, you may want to add a layer of straw on top to help keep them from blowing away in the garden.
Why has my celery bolted, and is it still edible?
Unlike many other crops, which bolt due to heat, celery will bolt prematurely if plants are exposed to too many days with temperatures below 55°F., especially if planted too early. Growing celery works best at daytime temperatures of 60° to 85°F.
Once a plant has fully bolted, the plant is normally inedible. It tends to be tough and woody, or even bitter tasting.
When harvesting my celery, do I leave its root on the soil for the next one?
This article may help: https://www.gardeningknowhow.com/edible/vegetables/celery/harvesting-celery.htm
I am growing celery for the first time. It has gotten quite tall and leafy.
The celery that is grown at home typically does not look like the kind you buy in the store. Celery in the store is grown under very specific conditions that most areas cannot copy. Home grown celery tends to be darker, with thinner stalks and more leaves.