A systemic herbicide is designed to eliminate unwanted plants by being absorbed and moved throughout the entire organism. Unlike chemicals that only affect the part of the plant they directly touch, a systemic product is capable of internal movement from the point of entry to the farthest reaches of the plant structure. This comprehensive action makes this class of herbicide highly effective for managing persistent or deep-rooted weeds, as the chemical travels within the plant’s tissues to target the whole system, including the roots.
The Mechanism of Systemic Action
The efficacy of a systemic herbicide relies on translocation, the internal movement of the chemical to its site of action within the plant. The herbicide must first be absorbed, typically through the leaves, stems, or roots. Once inside, the herbicide enters the plant’s vascular system, which functions similarly to the circulatory system in animals.
The vascular system consists of two main transport tissues: the xylem and the phloem. The xylem moves water and dissolved nutrients upward from the roots to the leaves; soil-applied herbicides often travel this pathway. The phloem transports sugars produced during photosynthesis from the leaves to areas of growth or storage.
Herbicides applied to the foliage are engineered to move with the flow of sugars in the phloem, allowing them to travel downward. This downward movement delivers the herbicide to the plant’s growing points, known as meristems, and into the root structure. By reaching the roots and subsurface storage organs, the systemic chemical ensures the complete and long-term elimination of the weed.
Contrasting Systemic and Contact Herbicides
The main difference between systemic and contact herbicides lies in their method of action. A contact herbicide works by destroying the plant tissue it physically touches, causing rapid, visible damage. Since it does not move beyond the initial point of contact, it is only effective on small, annual weeds that lack extensive root systems.
Systemic herbicides are the preferred choice for controlling perennial weeds, which possess deep root systems or underground storage structures. While a contact product kills the leaves quickly, the perennial weed can easily regrow from its untouched roots. A systemic product travels to these roots, killing the entire plant and preventing regrowth.
The speed of action differs significantly. Contact herbicides show results within hours, offering a quick visual solution. Systemic herbicides are slower because the plant must absorb and translocate the chemical through the vascular system, but this internal process provides more thorough and complete long-term control.
Common Application Methods
Systemic herbicides are applied through several methods designed to maximize absorption within the target plant. The most common technique is foliar application, which involves spraying the herbicide directly onto the leaves of actively growing weeds. This method is highly effective because the chemicals are readily absorbed by the foliage and transported via the phloem.
Soil application involves applying the chemical to the ground as a liquid or in granular form. The herbicide is then absorbed by the plant’s roots and translocated upward through the xylem tissue. Spot treatment is a localized application method applied only to individual weeds to minimize impact on surrounding desirable vegetation.
For woody plants, specific techniques bypass the tough outer bark to deliver the chemical directly to the vascular tissue. These methods include painting the solution onto freshly cut stumps or using stem injection techniques. These direct applications ensure the chemical is rapidly absorbed and translocated throughout the entire plant system.