Weeds compete directly with fruit trees for water and nutrients throughout the growing season. This is especially important for young trees.

In cherry orchards, during the initial stage, weed control may be more important than even plant training and other management efforts. This is generally due to the fact that soils are treated with physical preparation, nutritional plans, and more efficient irrigation control, all of which are essential for weeds to benefit from these programs.

The intense competition exerted by weeds can cause stunted growth in young trees, reduced size, and reduced fruit production, which can result in significant economic losses.

The use of a program based on soil-active herbicides, applied during the fall and winter months, plays a fundamental role in an adequate weed control program, often accompanied by supplements for specific objectives in each case.

Soil Herbicides: Active or Residual
The term “residual” or soil-active herbicide applies to long-acting herbicides in the soil. These herbicides are applied directly to the soil before weed emergence (pre-emergence).

All soil-active herbicides degrade in the soil. However, their degradation requires time so that, with adequate soil moisture and temperature levels, a basic population of microorganisms that degrade the herbicide molecules can establish itself.

Some herbicides decompose quickly, avoiding any problems in crops established the following season (CarryOver). Others take longer to decompose and, as a result, persist into the seasons following the year in which they were applied.

These residues can damage sensitive crops established in subsequent seasons. The dosage used plays a key role, as the degradation rate depends on it. A herbicide with a half-life of 30 days means that if 1.0 kg/ha is used, theoretically 0.5 kg will be available in the soil 30 days after application.

Application Considerations
Mobility of the product in the soil. Herbicides used as soil-active or residual herbicides are generally very poorly mobile in the soil. Therefore, they must be positioned in an adequate layer thickness through irrigation or tillage. The low solubility of these products in water makes them poorly mobile, and their selectivity, in some cases, depends on their avoidance of contact with the roots or rootlets of our crops (positional selectivity).

The retention of this type of herbicide in the soil will depend on the physical and chemical properties of the herbicide and the soil, which can be measured by adsorption coefficients (Kd or Koc). The higher the Kd value, the more strongly the herbicide is retained by the colloids, and therefore, the less available it will be in the soil solution.

Soil moisture. Residual herbicides are highly dependent on rainfall following application, as they are solubilized through rainfall, passing into the soil solution, from where they are adsorbed by soil colloids and then released back into the soil solution, where they can be transported and absorbed by weeds.

Soil organic matter / clay content. All soil-active herbicides, once in the soil, are retained (adsorbed) by the soil surface colloids: clay and organic matter (carbon). The higher the organic matter and/or clay content of a soil, the more strongly the herbicide will be adsorbed or retained by it, and therefore, less herbicide will be available to act on weeds. In these heavy soils with high organic matter content, higher doses will be used. If the soil is also dry, the adsorption force will be even greater, since water is the means by which herbicides “detach” from the colloids (desorption). This is another reason for making this type of application in moist soil. The doses of active herbicides in the soil must be adjusted strictly according to the colloid, organic matter, and/or clay content of the soil in each plot.

Residues from the previous crop on the surface or emerged weeds. The presence of plant debris or larger weeds can act as a physical barrier, preventing the residual herbicide from reaching the soil. Therefore, if sprinkler is available, irrigation should be used to “wash” these residues and ensure that as much of the herbicide as possible reaches its destination. Similarly, in these cases, it is recommended to consider a larger application volume.

Herbicide incorporation into the soil. Herbicides applied to the soil rather than mechanically incorporated are called pre-emergence herbicides, meaning they are applied before the crop and/or weeds emerge. In these applications, the herbicide forms a film on the soil surface, and when seedlings pass through this area, the herbicide penetrates and kills the weeds. Removing or breaking this herbicide “film” will leave areas without the product, so it is advisable not to walk on or use machinery during the first weeks after application. Heavy rain or irrigation can prevent this period.

When the herbicide is applied after the crop and weeds have emerged, these treatments are called post-emergence treatments. Some typical pre-emergence herbicides can also be applied to newly emerged weeds, in what is known as early post-emergence. Post-emergence itself is a practice aimed at controlling weeds at a more or less developed seedling stage that have escaped a pre-sowing or pre-emergence application.

These herbicides usually have foliar action, either contact or systemic. However, there are some that, in addition to this foliar effect, also have residual action, such as oxyfluorfen, flumioxazin, and various ureas, sulfonylureas, and triazines.

For this reason, it is possible to use tank mixes of two or more independently formulated herbicide products and, in some cases, other pesticides. The benefits of tank mixes include time savings and reduced machinery usage.

A common example is using a full-action foliar herbicide such as glyphosate and taking advantage of this application to “incorporate” a soil-active agent that will begin to control weeds once it is in the soil. With weed coverage above 50% of the soil, two separate applications are recommended, but below this limit, herbicides can be mixed, keeping in mind that the use of a good adjuvant is key to ensuring the products reach the foliage and soil.