By choosing only the oldest, largest plants you will also be using your energy wisely. Any seasoned root digger will tell you that it is not worthwhile to dig up small plants. The time involved to obtain a root that weighs only a few grams does not justify the effort of digging, washing, and drying. So a selective harvest of root crops is both smart and a good conservation practise. If you are harvesting a foliage or leaf crop such as stinging nettles or horsetail, you can be more aggressive.
Stinging nettles are a very therapeutic and nutritional herb that grows best in moist areas.
Indications are that there is a fairly ready market for this herb. As with many other medicinal plants, you should know its life cycle. If you wait until seed has developed and cut it down to the base, it's likely that the rootstocks will not produce again. Of course to avoid being stung by the little hairs that grow on the underside of the nettle leaf, you will need to wear gloves. Once the leaves are dried on the stalks they can be easily rubbed off.