The moment to forage for stinging nettles is early spring, while the tops are young and fresh. Heavy gloves and great respect for the plants are required. A friend on a nearby farm harvested some nettles to help me as I was convalescing after an illness, and later on we gathered a huge batch together. Infusions full of minerals are just the thing to give me strength. The old-timers would take bitters at this time of year, and wild stinging nettles grow just at the moment when we really need some good greens.
To preserve the nettles, I shake them out onto a cookie tray (to keep from getting stung), put my oven on its lowest temperature with the fan on, and pull them out when crispy-dry. Once dried or cooked the sting is removed, happily. Or, if you catch a good sunny day, you can lay them out on a clean sheet and turn them now and then til they are crisp.
Fully dry in a glass jar they will keep for a lot longer than any of them ever last at our house, certainly past the brief autumn harvest and through to the following spring. You can make fresh nettle soup and nettle tinctures too. Foraging and preserving nettles for high-mineral wild infusions and medicinal tisanes is a very old practice. I’m ever so fond of it.