Old leaves contain particles called cystoliths which act as an irritant to the kidneys, this is why we only use the young nettle tops!
Stinging Nettle Urtica dioica
I don’t feel I need do much other than utter the words ‘stinging nettle’ for everyone to know what I mean and what the look like. There are a couple of different types in Great Britain, but they can all be used in the same way.
For me, the humble nettle has to be one of the most useful of all wild plants (not weeds), so before you dig them up from your veg patch, please bear several things in mind. Nettles play an important role for our wildlife, not just for insects, but for birds and small mammals too. For our benefit they can be used a food source, such as soups, spinach substitute, nettle pasta, and of course the famous nettle beer / ale. They are also very handy in the garden, for they can be turned into natural insect repellents and plant foods, all of which are easily made (I may run a tutorial on this in the future). They are also the source of an exceptionally strong fibre which has been used for odd jobs in the garden.
Most people are afraid of getting stung, so rarely use nettles for anything, but if treated correctly they can be handled without the need for gloves.
Picking stinging nettles without gloves
LEAVES – the fresh young tops of nettles are what you need if you want them for food or drinks. Never use old nettles as they can cause irritation to the kidneys (see notes above). Young nettles are not only available in the spring, check under hedges and among tall grass in the summer or winter months, you will find plenty of young growth.
STEMS – the stems are the source of the fibres, which you can use a string. Find out how here
ROOT – the root can be boiled with alum to yield a yellow dye