Mowing requires management to be effective

July 13, 2016 1:56 PM
Blog Post

The current epidemic of wild parsnip has garnered much attention due to the toxins present in the plant.  Since wild parsnip is a biennial, mowing is often recommended as a control option.  The theory is that by preventing seed production, mowing can eliminate future infestations.  In practice, its a little more complicated than simply mowing when convenient; mowing needs to be timed properly in order to be effective.

While returning from the field this morning I encountered an example of poor mowing timing resulting in ineffective control of wild parsnip seed production (Figure 1).  I suspect the mowing of this area was not targeting the wild parsnip, but it does illustrate the importance of timing.  In addition to the seedheads, there was a healthy crop of new rosettes in the area that benefited from removal of the grass canopy (Figure 2).  These rosettes will provide an ample crop of flowering plants next year.  This area is an ideal setting for a herbicide application this fall to control the abundant crop of new rosettes.

Figure 1.  Wild parsnip flower heads produced after poorly-
timed mowing.

Figure 2.  First year wild parsnip basal rosette benefiting from
mowing that removed competing grass canopy.

To be effective, mowing should be made after the flower heads (umbels) have emerged, but before seeds have matured.  Mowing too early will allow secondary flowering stalks to develop, mowing too late will result in the formation of viable seed.  Many situations will require multiple mowings to completely prevent seed production due to differential maturation of plants and the ability of biennials to send up secondary flowering stalks.  

In an earlier article I speculated on the reason for the increase in wild parsnip populations across the state, implications for next year's infestation, and provided a brief review of control tactics.

Figure 3.  Wild parsnip in adjacent unmowed area.