Pay close attention to the development of corky, dead outer bark on the portion of the trunk that will be contacted by a contact herbicide. Well applied paint or mouse guards will help if they completely block the spray from contacting the green bark. The most common and serious damage occurs on young fruit trees when unprotected bark is contacted by concentrated doses of glyphosate style products. This is does not mean that these products can not be used in young orchards, you just have to be certain to avoid significant exposure of the trees. Other non-systemic products can also damage the young tree, usually by burning a dead area into the trunk within about a foot of the soil surface.
Young trees have shallow root systems, and most of their roots are under the herbicide treated area.
They may become highly exposed to root active herbicides that leach into the upper foot of soil. A similar exposure of an older tree may not cause symptoms, as most of the trees root would be below or outside of the herbicide strip, and the product may be broken down in the tree tissue without causing damage. Use root active products with special care in young orchards. Organic matter and increased binding sites that come with finer soil texture are major soil qualities that hold potentially mobile herbicides in the upper 2-4 inches of soil, controlling weeds rather than affecting the fruit tree. If the product suggests on the label that you take these factors into consideration, do so.