This is quite a recent technique that is based on a simple idea: you apply heat to the plants and this kills them, without needing to use chemicals and without much effort. It is an efficient method but not widely used...
The weeding lance should only lightly touch the weeds.
Thermal weeding is carried out using a flame-producing device like a portable blowtorch. As soon as the plant is exposed to the flame, the plant’s cells burst and the plant quickly dies. The most efficient method does not involve burning the plant but just heating it, so do not insist too much with the flame. This may seem strange but the more heat that is applied the less efficient is the method! If the plant is burnt the remaining parts will tend to heal up and start to regrow but even more vigorously. If you just give the plant a blast of heat, this will damage its tissues and weaken it. If it is only a small plant it will not have enough strength to carry on growing. The danger is carbonizing the weeds, which will then start to grow again but even more vigorously !
To properly weed with this technique it requires that you make several passes over an area, progressively weakening the plants, which are trying to grow after the earlier treatment. It is difficult using this method to destroy weeds with deep roots such as dandelions or couch grass. However, large leaved weeds with shallow roots such as plantain are efficiently controlled by this method. In areas where access to the weeds is difficult such as paving, paths and vegetable patches, this thermal approach can be very useful. Do not use this method in areas where there could be a fire risk such as dry scrubland or on your lawn where it will damage the grass.
The heat should not cook the plants but soften them.
Some definite advantages
Thermal weeding allows you to keep the weeds under control without tiring yourself. You can keep paths and driveways clear without resorting to chemicals, just a quick pass with the blowtorch. In a single swoop, you can destroy all those weeds that are just about to flower and produce thousands of seeds like nettles and thistles. It is a very practical solution when at the end of the weekend you do not have much time to spare!
For large areas such as courtyards and the like, there are special models available where a large gas bottle is mounted on a trolley.
Be aware that passing a flame over the ground does have some undesirable consequences. Firstly, the heat can harm useful animals that live there such as earwigs, wood lice and beetles. The heat can also cause dormant seeds to start growing. Where before there were no weeds, passing the flame might cause large numbers to appear. To prevent this you will need to pass again a few days latter to kill off any new seedlings. Finally, it is worth noting that using the flame on gravel is not very efficient as many of the plants will be protected from the heat and will quickly regrow following the blast of warmth.