Yellow sweetclover is the tall plant you see growing throughout the county that has trifoliate leaves (3 leaflets) and racemes of small yellow flowers.  According to the book Principles of Field Crop Production, by Martin and Leonard, sweetclover originates from the Mediterranean region and was grown as a crop there over 2,000 years ago.  It was brought to the US for green manure, pasture, seed, silage, and hay the over 300 years ago, and now more than 2 million acres of sweetclover is grown annually in the United States.

Pros

Sweetclover is often planted for its forage and soil quality properties.  As a legume, sweetclover is a nitrogen-fixer; the symbiotic relationship it has with bacteria in the soil allows it to fix atmospheric nitrogen into usable nitrogen.  Because nitrogen is a fundamental element of amino acids and protein, legumes naturally have high protein content.  Furthermore, as an essential nutrient, protein is often used a gauge for forage quality; higher protein content equals higher forage quality.  Thus, sweetclover is good forage for most livestock and wildlife.  It also acts as a great pollinator species and has been known to make delicious honey.  Lastly, nitrogen fixers like sweetclover, contribute to the health of the soil in which they are planted by providing soil stability and reducing soil erosion, encouraging soil biotic activity, increasing soil water-holding capacity, and increasing soil aeration.  Accordingly, sweetclover has been added to reclamation mixes in the past thanks to these soil health qualities. 

Cons

Despite, these positive attributes of yellow sweetclover, there are some cons as well.  As a nonnative, it can reproduce and invade native areas.  Sweetclover is a prolific seeder, producing over 100,000 seeds in a given season, and those seeded remain viable for many years.  It is also drought, fire, and cold tolerant.  Because it is quick to establish on clear and disturbed sites and is easy to disburse, yellow sweetclover can often be found along roadways.  This presents a danger to motorists because it can grow to 6 feet tall and obscure wildlife hiding along the roadside.  Lastly, sweetclover contains a compound called “coumarin”.  When sweetclover gets stressed (i.e. frosted) the coumarin is converted into toxic dicumarol.  Dicumarol is an anticoagulant and vitamin K antagonist, thus decreasing the blood’s capability to clot and increasing the potential for hemorrhaging.  Keep in mind, fresh sweet clover is not toxic; it is sweetclover that has been subjected to stress or a hard frost that is prone to toxicity.
 
So, why then, is yellow sweetclover, so prevalent some years?  According, to Mark Volt, yellow sweetclover when...

• Wet conditions provide great soil moisture and growing conditions
• There is a large bank of seed in the soil that has just been waiting for a good year like this year to germinate. 

Management of Yellow Sweetclover

Currently, yellow sweetclover is not considered a noxious weed in Colorado; however, according to Colorado State University Extension, it is under consideration for addition to the noxious weed list.  If you have yellow sweetclover you wish to control, a factsheet by the US Forest Service suggests that mechanical, prescribed burning, and chemical treatments can all be effective if done properly. 

Also, if you graze livestock, avoid grazing in pastures that have yellow sweetclover in them after the first good frost. 

You may also contact Grand County DNR (970.887.0745) or Summit County Weed Control (970.668.4218) for their specific recommendations as county weed departments.

References:

http://www.gunnison.colostate.edu/agri/forage/forage_docs/sweetclover.pdf
http://www.agriculture.gov.sk.ca/Default.aspx?DN=4b50acd7-fb26-49a9-a31c-829f38598d7e
http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/faq8154?opendocument
http://www.na.fs.fed.us/fhp/invasive_plants/weeds/yellow_sweetclover.pdf