Flame weeding has been used for many decades, particularly in organic farming situations for pre-emergent weed control in carrots and other slow-germinating row crops, but also for general weed management on hard surfaces in urban areas.
Best results are obtained when follow-up weed control is undertaken 4-6 weeks after treatment. In addition, weed control should be conducted periodically after that. For example, to control weeds over a period of a year it is likely that between 3-5 applications will be necessary, depending on rainfall and the extent of the weed seed bank.
Flame and radiant heat do not penetrate into the crown of the plants efficiently, often requiring more frequent interventions. Exposed flame weeders pose a significant fire risk in dry conditions and on mulches, and cannot be used on rubber paving, near litter, debris or irrigation lines, and fittings.
Flame weeders can be used for total vegetation removal or for selective removal of unwanted plants. Selectivity may be achieved by timing the application to kill weed seedlings before the crop emerges (pre-emergence flaming). For best effect, flaming requires a level soil surface. In addition, flame weeders have the advantage that they can be used when the soil is too wet for mechanical weeders.
They work directly by damaging the waxy cuticle that protects the cells in plants’ leaves and indirectly by subsequent desiccation. The weed flaming process does not require the weed to be burnt: it raises moisture temperatures to above 212°F (100°C), at which time the moisture turns to steam and ruptures the plants’ cells. Small herbs are generally more susceptible to flaming than large ones. Species with an upright habit and thin leaves are also more sensitive than species with a low stature and protected growth points.
All information taken from: http://byronshirechemicalfreelandcare.org/techniques-2/general-weed-management/