March, 2002

To answer a question posed by an S&S Seeds customer, who wanted to know why bermudagrass returned after herbicide application, we have received permission to reprint an article addressing this topic. The following is excerpted from the Vistas newsletter, a publication of Dow AgroSciences. For more information about this and other topics, visit http://www.dowagro.com.

Harvey Holt, professor of forestry at Purdue University, advises that understanding the importance of translocation, how translocation works within plants, appropriate application, timing and how the environment affects plant physiology are crucial factors in achieving the best results from herbicide treatments.

Most vegetation managers use herbicides to control established perennial plants and the persistent root and stem resprouts from these plants. Defoliating perennial plants provides temporary growth suppression by killing the above-ground plant. However, according to Holt, "Herbicides achieve longer-lasting control because they translocate (move) from the leaves to the roots and underground stems."

There are two groups of tissues responsible for movement of herbicides, as well as water, nutrients, sugars and naturally occurring growth regulators in plantsxylem and phloem.

Xylem tissues move water and nutrients upward from the roots to the leaves. Conversely, phloem tissues transport sugars downward from the leaves to points of active growth, such as root tips, reproductive tissues and storage organs.

In grasses and broadleaf forbs, the xylem and phloem group together into vascular bundles that resemble veins. In woody plants, phloem tissue makes up the inner and outer bark, while the wood, sapwood and heartwood contain xylem tissue.

Most growth regulator herbicides move in the phloem, following the same path as the movement of plant sugars. This process is known as the "source-to-sink" concept

"Plants use metabolic energy to load sugars into sieve elements—primary cells in the phloem," Holt explains. "Sucrose accumulation attracts water, due to the osmotic potential of the sucrose. As water accumulates, hydrostatic pressure builds at the source. Sucrose is then unloaded at the sinks (plant parts using sugars), reducing osmotic pressure and allowing water to flow out of the sieve tubes. Dissolved herbicides move with the water."

Soil-active herbicides can also move through the xylem. These herbicides enter at the root hairs, which is also where most water enters the xylem. Translocation in the xylem is assisted by a "pulling action" caused by moisture evaporation in the leaves. When relative humidity is less than saturated, water, nutrients and herbicides are drawn up to the stems by water evaporation or transpiration.

Proper foliar application timing is essential for taking advantage of translocation, because if plants aren't physiologically active, herbicides won't be, either. Herbicides work most effectively when applied after rapid vegetative growth when the plant is actively storing food reserves in its roots. For best results, applicators should refer to product labels for optimum timing. Mother Nature can be a big influence on results as well.

"For example, during drought conditions, plants undergo growth stress and produce thick, waxy layers on the leaves," Holt explains. "Plants may also close their stomates—small pores in leaf surfaces—and reduce their metabolic rates to protect against excessive moisture loss. Under these conditions, herbicides do not penetrate leaf surfaces or move through plants easily."

When all translocation processes and external factors are working in harmony, herbicides are able to permeate plants and produce the desired control results. Keeping applicators informed of these processes ensures future successes in vegetation management.

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