An extension client asked why waterhemp that survive a postemergence herbicide often develop into a bushy plant with an abnormally thick stem. The short answer would be the plant was too big when it was sprayed, but I trust this person wanted a more in-depth response.
Most erect plants such as waterhemp produce axillary buds at leaf nodes. During early stages of growth these buds remain dormant due to apical dominance. The growing point at the top of the plant (the apical bud) controls the axillary buds by closely regulating growth regulator concentrations within the plant. By keeping these buds dormant the plant allocates resources to upward growth, reducing competition for sunlight with neighboring plants.
As the season progresses, if the plant has abundant resources available the dormant axillary buds will be released. This results in a large plant with many lateral branches. However, if resources are limited the axillary buds remain dormant. Most late-emerging waterhemp that broke through the soybean canopy in the past three to four weeks have few lateral branches due to intense competition from the soybean.
When waterhemp is exposed to a sublethal dose of herbicide (it could be due to too low of rate, too large of plant, resistance, etc.) the apical bud might be killed since it is very tender, but the remainder of the plant can survive. The loss of apical dominance releases the axillary buds and the result is a plant with many lateral stems. The plant can’t grow taller since it lost its primary stem, thus it has excess resources and produces a much thicker stem than normal.
While several factors may be involved in the formation of these ‘bushes’, in most cases spraying too big of waterhemp is a primary issue. If waterhemp is sprayed when less than 3” tall, in most instances the lateral buds won’t have matured sufficiently to reinitiate growth if the apical bud is killed. Prior to glyphosate resistant waterhemp, glyphosate was ‘immune’ to this survival mechanism due to translocation of glyphosate to the lateral buds if the apical bud was killed.
Beating the waterhemp epidemic requires an integrated approach. Timely herbicide applications are a critical component of diversified management systems.