I've never had parsley winter over, yet this year 3 of my plants are luxurious already. Are they useful to harvest parsley from? or should I buy new ones? I imagine these will flower and go to seed?
Yes, they will flower and go to seed. But, if you are into using herbs for medicinal purposes, the second year roots of parsley can be harvested in the spring. Parsley roots will benefit your liver and kidneys, and are good for kidney stones and bladder infections. Parsley root is also good for flatulence and digestion problems. But it is a diuretic, so it'll make you pee a lot.
The roots are also edible, but, depending on the type of parsley you grew, the roots may not be very big.
Let it grow and go to seed. You can use the leaves as usual, but there might not be as many. If you usually plant started plants I would do that too, to make sure you have enough for the kitchen, but next year, with luck you won't have to do that as you should/might have a good stand of both first and second year plants.
My mother -- of pennsylvania dutch extraction, she considers parsley a major vegetable, not just an herb or garnish -- always had two years' worth of parsley plants going. In late winter she would start a few seeds (from the previous year's flowered-and-gone plants) but for the first half of the season all the parsley for the kitchen came from overwintered plants in their 2nd year. By the time they started to bolt to flower, the seedling ones would be big enough to start harvesting from. When the 2nd year ones set seed she'd save a little for starting next February and when the foliage got too nasty they got retired to the compost heap and all further parsley for the kitchen for the season came from the 1st year plants.
I didn't explain that well, it is not complicated at all, hopefully you can figure out what I mean
Let them go to seed. Parsley seeds take forever to sprout, but if you have your own seeds from your own plants, they sprout a lot faster. We now have parsley growing all over the place, like weeds, and we like it that way.
Do go and buy more plants for this year and let them go to seed next year, if they overwinter. Then you will hopefully always end up with parsley growing in your garden without having to plant it or buy new plants every year.
I just sprinkle the seeds in places that I think it will grow well. When you have lots of seeds, it doesn't hurt to just spread it everywhere. They also distribute themselves nicely and when they are still tiny, I sometimes transplant them to places where I want them.