White Sweet Clover

Invasive plant species control can be practiced year round.  No matter when you decide to tackle the job of eliminating these nasty invaders, there’s a species that is vulnerable to your efforts.  At Blue Jay Barrens, June and July constitutes Sweet Clover season.  The target is Yellow Sweet Clover, Melilotus officinalis, and White Sweet Clover, Melilotus alba.  Both of these species are a problem on the prairies, but a little effort can greatly reduce their numbers.

Yellow Sweet Clover usually produces its yellow flowers in June.  It produces a shorter, more compact plant than the White Sweet Clover.  I’ve discussed the Yellow Sweet Clover before and am happy to report that it has been eliminated in many areas.  White Sweet Clover, which blooms several weeks after the Yellow, was here prior to my purchase of the property and is also slowly losing ground due to my control efforts.  Both species are effective competitors and can reduce the number of native species on a site.

The blooms are miniature nectar factories and attract a large number of insect species.  Beekeepers often plant Sweet Clover to support their Honeybee hives.  The plant was also once highly recommended by government conservation agencies as a high quality conservation cover.  Fortunately, native plants are now more often recommended for that purpose.

Sweet Clover is a biennial that produces a deep taproot during its first year.  During year two it flowers, produces seed and dies.  Its ability to utilize atmospheric nitrogen and draw moisture from deep soil areas gives it an advantage in dry, low fertility soils.  The most effective control method is to physically pull the plants from the ground.  After a couple of years, the amount of viable seed left in the soil is greatly reduced and the work required to keep the site clover free is much easier to accomplish. 

A deep tap root is particularly advantageous in the drought conditions we are currently experiencing.  This is normally a tough plant to pull when the ground is dry, but our extremely dry conditions have proven to be an exception to that rule.  These soils have not seen any penetrating rains for two months and have dried to a much deeper level than normal.  The presence of clay in the soil profile has caused the drying soil to shrink and pull away from the Sweet Clover roots.  This soil shrinkage is the same phenomenon that causes cracks to form in the ground.  As a consequence, the roots have lost most of their anchorage and easily pull from the ground.

As hot as it’s been, these pulled plants will be dry and crispy in just a few days.  Pulling out the root eliminates regrowth of that plant.  Even if a pulled plant separates at ground level, the ability of the remaining root to produce flowers and seeds is severely compromised.  The only negative I’ve found to performing Sweet Clover control is the fact that it coincides with Chigger season at Blue Jay Barrens and Chiggers seem to thrive in the vicinity of Sweet Clover.