My parsley plants always wither very quickly. Why?
If they are young seedlings, I suspect your may have some damping off occurring. This article will help you with that:
If they are more mature plants, they may be underwatered. Check the soil and keep it moist (but make sure the drainage is good).
I have two flat-leaf Italian parsley plants. They are approximately three feet tall and have started to go to seed on the top. How should we be harvesting the parsley, and can we stop this from happening?
You can't really stop it from going to seed. It's a biennial, and goes to seed as a means of repopulating. Once the seeds have been shed, it will die. You MAY be able to prolong it by removing the seed stem. Harvest leaves for cooking from first-year plants only and allow at least one or two second-year plants to fulfill their flowering/seed producing cycles to replenish your garden.
Here is an article that you may find helpful: https://www.gardeningknowhow.com/edible/herbs/parsley/tips-on-how-to-grow-parsley.htm
I have left last year's parsley plant in its pot and it has now flowered. Should I wait until the flowers dry out (or dry them out by picking them), and then can I plant the seeds?
Wait until they dry on the plant, and then harvest them to plant again.
I like to cook a lot with flat leaf parsley and the dill that I have grown in my garden. Whenever I cut some to use, the rest of the stem turns yellow and dies within a few days. Am I doing something wrong? How can I cut the herbs so that doesn't happen?
Trim herbs as you use them or prune as needed. Pruning naturally stimulates plant growth. Pruning herbs can be as simple as pinching back growing tips to encourage branching; removing, or deadheading, spent flowers; or harvesting a major portion of plant growth for drying and preserving. Pinching back herbs throughout the growing season is the best way to maintain healthy, vigorous, attractive plants. Prune branches just above a leaf node or above another branch where you want to force new growth to occur. It is important to deadhead herbs to keep plants growing and to encourage more flowering.
I have parsley, oregano, garlic, thyme, and dill growing in five small (4 inches) pots. What size pots shall I transfer these to?
You should put your plant in a pot one size larger than the one it is currently in. This article will also help: https://www.gardeningknowhow.com/edible/herbs/hgen/growing-herbs-in-containers.htm
Will parsley bolt?
It has Flowered.
what do you mean bolt?
When and how do you dry fresh parsley?
You can dry it whenever you harvest it, which is all season long. This article will help you with drying it: