The effectiveness of the treatment with regard to the weeding operation is influenced by several factors including: the growth stage of the plants, their density on the soil, the presence of protective layers of the leaves (overlapping leaves, presence of hairs or wax, degree of lignification of the outer leaves), time of day, temperature and soil moisture.

The most favorable situation to achieve a weed control treatment with an effective, e fast and low LPG result s to practice it on weeds that are in the juvenile vegetative stage (10-15 days after emergence) and with a low density on the ground. In this situation normally the weeds have thinner leaves, lower biomass and fully exposed meristems (not protected by surrounding leaves) and thus the plants turn out to be much more sensitive to heat exposure.

Another critical factor of plants in the juvenile state is the point of growth of the shoot apex (latent bud), which is normally found unprotected by overlapping leaves and thus easily accessible to the heat transmitted by the hot air generated by LPG combustion.
Juvenile plants also do not possess large amounts of food reserves (soluble sugars, proteins and lipids) in the stems and roots and subjecting them to heat treatment with flame deprives them of the leaf apparatus by inhibiting the chlorophyll function leading to their death by root system stress.

Older or larger weeds have larger, thicker leaves with a larger area and biomass. In addition, these plants have accumulated over time greater amounts of reserve substances in the stems and roots and this provides them with a greater capacity for regrowth. The tolerance of different parts of the plant to flame is also influenced by the protective layers of hair and/or wax, the level of lignification and the general water status of the plant which in an adult weed is more increased. For these reasons, in order to obtain a good herbicidal effect on them, higher temperatures and a longer exposure time to the treatment must be applied than for juvenile weeds.

The so-called “annual” weeds come from seed and generally have a modest root system; they are the easiest to control. Because they have an entire reproductive cycle per year if they die permanently as a result of treatment, they do not recur until the following year.
The “perennial or perennial” weeds are equipped with perennial buds with underground reserve organs

To obtain a good result these weeds must be treated in the juvenile vegetative stage (maximum 5-7 days from emergence, 1 to 10 cm. height) when they have not yet prepared the “latent buds” and therefore will not be able to recover the shock suffered by the treatment. “Polyennial or perennial” weeds have perennial buds with underground reserve organs
For good results, these weeds should be treated in the juvenile vegetative stage (maximum 5 to 7 days after emergence, 1 to 10 cm. in height) when they have not yet set up “latent buds” and therefore will not be able to recover from the shock suffered by the treatment.

These weeds if they are at a fairly developed vegetative stage and thus have prepared so-called “latent buds” that allow them to be able to resume their vegetative cycle over time. Following a treatment they will reappear after about 20 to 30 days depending on the species treated. If subjected to subsequent treatments, root stress will also occur on these plants due to the continued lack of photosynthesis effect of the leaf apparatus.

Numerous studies have shown that plants are more sensitive when the weed treatment takes place in the afternoon than in the early morning. A study compared the effectiveness at 8:00, 12:00, 16:00 and 20:00 and reported better weed control at noon and 16:00.

It was suggested that the differences obtained in weed control could not be explained by differences in temperature, relative humidity or presence/absence of dew in both soil and air. Therefore, in general, weed control is considered to be more effective if conducted during the afternoon period.

The practical consequence is that it is necessary to work with an overabundance of heat and vary the time of exposure to the heat according to the size of the plant on which one intervenes.

The recovery of the vegetative stage of weeds after a treatment can therefore present different aspects depending on both the type of plants treated and the vegetative development of the same. This is very similar to the use of foliar chemicals.

This shows the importance that can have, for the effectiveness of heat treatment with the flame with function of weeding, the possibility of being able to intervene in a timely manner both in relation to the survival of treated weeds and for the reduction of the time spent on treatment and therefore for the decrease in fuel consumption and the consequent reduction in treatment costs.