It is as well to understand a little about stinging nettles before deciding which way to go about killing it. Most will be aware that they can give rather nasty stinging rash, and this will develop into quite a severe problem with repeated exposure. Dock leaves rubbed on the affected areas are said to alleviate the pain. Try it and see! Far far better to avoid being stung in the first place.
Like icebergs, quite a bit of the plant is below the surface of the soil by way of a quite dense mat of roots which spread and then send up new shoots at quite a distance from the parent plant. This is particularly so at the base of garden walls, where the root system can run for several feet.
However, it is this dense mat of roots that make nettles an easy way to control by hand or by the use of a garden fork.
Get will covered with stout gloves and long sleeved jacket/shirt before you attempt this though. The chemical by which nettles sting is in fact the same as those of an ant sting - Formic Acid. Dried nettles also sting, so be careful when moving any nettles that you have either cut down or treated with chemical weedkillers.
Simply hack off the nettle growth either with a bill hook or go at it with a sharp spade. This gets the worse part of the stinging nettles out of the way, so that the foliage and stems are not brushing against you letting fly with its self defence stinging mechanism. Now you are able to tease out the mat of roots - either with a garden fork or simply by tugging the root system of the nettles out of the ground. Bear in mind that the stem parts that are just under the soil surface also have the stinging hairs - so keep the gloves on! You may have to re-visit the area in a few weeks to mop up the young growths that are growing from broken roots that you left in the soil.
If the nettles are in an area that is not too densely populated with garden plants, then it is possible to kill them chemically with either Glyphosate or SBK (obtainable form most good garden shops). Before attempting this, make sure that you fully read the instructions on the container - and adhere to them word by word! It will probably take several applications throughout the growing year to completely kill off the stinging nettles.
I have found that SBK does a better job than the retail strength Glyphosate. Make sure that you do not 'treat/spray' and garden plants, or allow drift from the sprays to reach the plants. Both will kill or at least severely harm garden plants.
Allow around three weeks at least for the top growth to die down after spraying. During this time the chemical will be working through the plant's system and down into the roots.
If you have an area of waste ground where the nettles are, and you do not want to plant any plants for at least a year. Then an application of Sodium Chlorate will also do the job. Be aware that it will also kill off the soil for a full growing season.
By David Hughes - firstname.lastname@example.org