Thursday, June 6, 2013

Trifoliate Broadleaf Weeds Flowering in Turf

Kevin W. Frank
Assoc. Professor & Extension Turf Specialist

The trifecta of common trifoliate broadleaf weeds Black medic (Medicago lupulina), white clover (Trifolium repens), and yellow woodsorrel (Oxalis stricta) are all currently flowering and infesting turf. 

Clover, black medic, and yellow wood sorrel infesting turf.
Black medic and white clover are commonly found growing on low fertility, low maintenance sites, such as roadsides, boulevards, neglected home lawns, and in some cases golf course rough.  Black medic and clover are very competitive in low fertility sites because they host rhizobacteria that fix atmospheric nitrogen into plant available nitrogen.  A long-term strategy to reduce their competitiveness in turf is to ensure adequate fertility levels.  At a minimum I would suggest 2 lbs. N/1000 ft.2 year split over two applications to ensure the turf is dense and competitive.  Depending on the turf use and inherent soil fertility more than 2 lbs. N/1000 ft.2 may be necessary on many sites to produce a turf that is competitive with weeds.  Fall broadleaf herbicide applications are the most effective for controlling these weeds however treatment at flowering should produce results.  The most effective herbicides for controlling black medic and white clover contain the active ingredients fluroxypyr, triclopyr, or quinclorac.

Black medic.
Yellow woodsorrel, sometimes simply referred to as Oxalis, behaves as a summer annual in our climate and often infests voids left following elimination of broadleaf weeds following spring herbicide applications.  In contrast to black medic and white clover, yellow woodsorrel is not a nitrogen fixer and is found in a range of conditions including fertile soils, shady, sunny, and dry, so just about everywhere.  Effective herbicides for postemergence control of yellow woodsorrel include the active ingredients triclopyr and fluroxypyr.

Yellow woodsorrel with distinct heart shaped leaves.
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