Defoliation of Grasses and Broadleaves
Defoliation is the removal of above ground plant material. A few ways that defoliation can be accomplished is by mowing, burning, grazing, hail, frost, or applying herbicide. In many cases defoliation is done intentionally. What happens to a plant once it has been defoliated?
There are three main questions to address in order to determine how the plant will respond after defoliation.
1. Where is the growing point located?
The location of the growing point throughout the season is critical in predicting the response of a plant after defoliation. During the first part of the growing season, the growing point in grasses is located at the base of the plant, but as the season progresses the growing point moves to a higher positioned along the tiller. This is important in determining how quickly regrowth occurs. If the grass plant is defoliated above the growing point, regrowth will occur rather quickly. This is because the growing point is still intact and growth can continue from the same tiller. However, if defoliation happens below the growing point, regrowth will be slower. Once the growing point is removed, growth no longer occurs from that tiller. Therefore, new growth must begin from dormant buds rather than continuing from the original tiller. Forbs have many growing points all along the shoot. The main growing point is at the tip of the shoot. When this growing point is removed, secondary growing points along the shoot are activated and lateral growth occurs. Even when individual, secondary growing points are removed growth will still occur. It is only when all plant material is removed below all growing points that growth ceases and a dormant bud must initiate new growth.
2. What part of the growing season was the plant defoliated?
The time during the growing season when plant tissue is removed determines the response of the plant. Removing tissue during the spring and summer, with growing points still intact, will most likely not affect the ability of the plant to grow actively for most of the season. This is when growing conditions are optimum (i.e., good soil moisture, high temperatures, and good light conditions) and the photosynthetic rate is high; therefore the plant can easily replace the removed tissue. There is also still enough time remaining in the growing season for adequate regrowth and storage of carbohydrates. However, if the plant is defoliated in late summer-fall, when the focus is on storing carbohydrates, this will have a negative effect on the initial growth next spring. Without sufficient storage of carbohydrates in late summer and early fall, dormant buds in the spring will not have the energy required to initiate growth.
3. How much green tissue is left after defoliation?
The amount of residual leaf area remaining after defoliation must be adequate for continuing photosynthesis in order to get significant regrowth. Photosynthesis can be greatly inhibited with the removal of leaf tissue because green tissue is required for photosynthesis. This directly affects the source-sink relationship in the plant. Without the green tissue to perform photosynthesis there is no production of carbohydrates to supply the sinks with energy.
Removing leaf tissue causes immediate changes in plant structure and function. The timing and intensity are critical in determining plant response to defoliation. Addressing these three questions prior to defoliation will allow maximum benefit to the plant and producer.