Starting with the types of covers, I would divide them into two groups: the first group comprises high-density polyethylene covers used as roofs for cherry trees. Additionally, there are macro tunnels, generally made of low-density polyethylene, and greenhouses. The other group involves the use of mesh, initially using Rachel mesh and now utilizing monofilament mesh.
The primary objective of using covers from the first group – macro tunnels, greenhouses, and roofs – has been to protect crops during rainfall, but we have also observed positive side effects in controlling radiative frosts. Furthermore, they have an effect in reducing wind impact, in some cases controlling hail, and in the case of greenhouses and macro tunnels, they are widely used globally for advancing harvests. Additionally, we have seen other comparative advantages, especially in Europe and the United States, in controlling bird damage and Drosophila Suzuki.
Regarding the group of monofilament mesh, they generally tend to mitigate the effects of radiative stress, slightly reduce water requirements, and also act as bird and Drosophila Suzuki control.
One “negative” variable of this technology for cherry orchards is undoubtedly its price, as the implementation costs increase. If we analyze the first group, roofs or fabrics are around US$30,000, macro tunnels range from US$70,000 to US$80,000, and greenhouses exceed US$110,000 per hectare.
Another aspect to consider is that in some windy areas, adapting this technology has proven very complex, especially when orchards are positioned perpendicular to the wind’s direction, causing mesh breaks. The predominant wind direction must always be parallel to the row of planting to avoid such material breakage.
An important issue is that in macro tunnels and greenhouses, there are often problems with bee pollination because bees rely on ultraviolet radiation for navigation, which decreases inside the greenhouse or macro tunnel due to the opacity generated. This causes disorientation in bees. The low-density polyethylene used in macro tunnels and greenhouses contains a significant concentration of additives to increase the plastic’s durability, and these additives generate the aforementioned opacity, reducing the transmission of ultraviolet radiation.
In the case of roofs or fabrics, we generally haven’t had any problems with bee movement. Moreover, as the fabrics are made of high-density polyethylene, they have fewer additives, resulting in at least 70% light transmission within the orchard.
However, in greenhouses and macro tunnels, devices such as bumblebees are frequently used since they work in more extreme temperatures and are not dependent on UV radiation.
Another point to comment on regarding the limitations is related to temperatures, especially for macro tunnels and greenhouses, as they require significant ventilation systems. When temperatures rise significantly, particularly during flowering, these high temperatures can usually lead to a reduced effective period of pollination. This means that the growth rate of the pollen tube increases, and consequently, ovules lose their viability, resulting in a shorter effective period of pollination, i.e., a lower probability of fruit set, all due to high temperatures.
The most relevant point nowadays, which involves all three technologies used in this group – roofs, macro tunnels, and greenhouses – is that the fruit tends to be softer, especially the fruit closest to the roof’s peak.
For instance, in Chile, roofs or fabrics are generally used between August 15 and October 15. Essentially, the goal is to cover the flowering period, which ensures a better environment for bee movement due to the slight temperature increase. Moreover, it prevents the flowers from getting wet in case of rain. In Chilean regions, flowering generally occurs with rainfall, which extends the effective pollination period.
These covers also help reduce frost effects. In Chile, from August 15 onwards, depending on the varieties, we begin to observe slight plant metabolic activity, visually represented by the emergence of phenological stages making the plants susceptible to frost damage. Therefore, in general, if we consider the Chilean industry, orchards should be covered between August 15 and October 15.
At a global level, based on my observations of orchards in Europe, primarily in Italy and Spain, and also in the United States, greenhouses are generally used the least due to their cost. I have seen macro tunnels in Europe, the United States, and even in Chile, although to a very limited extent, used for quite early varieties aiming to advance harvests for better prices, while roofs or fabrics are widely used to prevent “cracking” issues.
In Europe, the United States, and increasingly in Chile, monofilament meshes are being used to mitigate stress caused by abiotic factors, mainly radiation and temperature, which has yielded good results. Nowadays, some orchards also use them during the off-season to reduce the photoperiod, the number of light hours, to induce a more pronounced rest period for the plants.