Sweet clover: why is it so prevalent on our prairies?

Sweet clover (Melilotus alba; M. officinalis) is probably the most expensive weed that we deal with in prairie restoration work. In contrast to garlic mustard and reed canary grass, the control cost per person hour per acre is quite high, and most infested areas are almost impossible to eradicate. The principal reason is the historical legacy of former sweet clover agricultural use.

As part of my interest in historical ecology I dug into the early agricultural literature on sweet clover.

Sweet clover is native to Central Asia but has spread around the world. The Greeks wrote about it and it has been in Central Europe since the Middle Ages. It spread to North America in the 1600s. (One of the early ways it arrived here is in soil used as ship ballast.) Although an agricultural crop, it spreads well on its own and in the late 19^th Century it was extensively present on roadside, waste lands, and other disturbed areas. In fact, it was so prevalent that in 1893 it was declared an “official” noxious weed in Wisconsin and Illinois.

However, many farmers objected to calling it a weed and a love/hate relationship soon developed.

Here is a report from Illinois in 1916: “When the sweet clover started to come into our state our people rose up in arms as they thought it was a catastrophe that had fallen upon them and they aroused the Legislature and passed a law placing sweet clover on the list of noxious weeds. The law passed and people were compelled to get out on the roads with scythes, grub hoes, etc. to cut down the sweet clover. They tried very hard to destroy it but the more they waged war against it the more the plant would come on and it has gone from the southern part of the state to the northern part of the state and wherever it has gone it has paved the way for the plant which is still better than the sweet clover---alfalfa. [Sweet clover] has carried the proper germs [root nodule bacteria] necessary for the growth of alfalfa. It has taken possession of the roadsides in many parts of the state, but it has been instrumental in crowding out bad weeds along these roadsides….It has performed an important mission and after we woke up and found that the sweet clover was a true friend, they rushed back to the Legislature and had it taken off the list of obnoxious weeds….there is a future for the sweet clover plant in our state.” (Annual Report Vol. 21, Illinois Farmers’ Institute. 1916)

In the period from about 1910 through the early 1940s sweet clover became a widely used crop, especially in the North Central area of the U.S. (In 1929 Illinois had 757,000 acres of sweet clover growing. And in the early 1930s over 50,000,000 pounds of sweet clover seed was produced in the U.S., an indication of the acreage being planted. U.S. Bureau of Agricultural Economics for 1932)

Here were some of the many advantages of sweet clover as a crop:

• Among most productive crops available if handled properly
• Bees love it and it is a great honey plant
• Used in roadside mixes because drought-resistant and hardy (can survive road salt)
• Very useful in the reclamation of eroded and degraded land
• Used as a green manure for soil improvement
• Cover crop for grass
• Hay
• Silage
• Seed production (for market)
• Food plots for wildlife

During the period when sweet clover was widely used, it readily “escaped” and moved not only into roadsides and waste lands, but sunny natural areas such as prairie remnants.

Although presently sweet clover is no longer widely used, we are left with the legacy of earlier times. Listed below are the botanical and ecological characteristics responsible for the problems that sweet clover gives restorationists:

•  Drought tolerance
• Ability to live on poor soils
• Nitrogen fixer
• Prolific seed producer
• Seed characteristics
• Each plant can produce many seeds
• Seeds can remain viable in soil for many years (>100 yrs)
• Hard seeds difficult to germinate
• But heat from prescribed fire brings about germination
• Allelopathic, preventing  the growth of native plants

 It is important to note that sweet clover will only grow in soils with pH 6.5 or higher. This means that for agricultural purposes it often needs lime. In many areas of Wisconsin, especially in the Driftless Area, sandy soils and those that have developed on sandstones (such as St. Peter Sandstone), are too acidic so that they will not support prolific sweet clover growth. Thus, sweet clover may not be a problem in all prairie remnants.

Today, whenever a prairie remnant is restored in our area, fire is one of the first tools that is used. Although fire is obligatory for prairie restoration, it also induces sweet clover seeds to germinate. Especially after the first burn in virtually any remnant, there will be a major “flush” of sweet clover plants. Although these new plants can (and must) be eradicated, they are an “indicator” that likely sweet clover once flourished in this site and left a seed bank that has the potential for lasting many years. If such a sweet clover flush occurs, it guarantees that the restorationist is at the start of a lengthy task.

How about former agricultural lands? It depends. Some of these fields may have been used to raise sweet clover many years ago, and viable seeds may still be present. Although most of these seeds may be well below the surface, scattered seeds may be near the top and may be induced to germinate by prescribed burns. Although sweet clover will likely not be a problem, this is not guaranteed, so that any planted prairie should be monitored. Even without fire, it is inevitable that some buried seeds will germinate. However, it does seem probable that sweet clover eradication will be easier in former agricultural fields than on remnants.

Despite its strong reputation as a highly invasive plant, and the large amount of money spent by restorationists to eradicate it, regulation in North America of sweet clover as a noxious weed is very spotty. Here is a list of what I have found:

Official declaration in the 21^st century established or considered (mostly unenforced):

• New England
• Iowa
• Colorado
• Texas
• Australia
• Quebec
• Alberta
• Various counties, Washington
• Lake Tahoe basin, California
• Town of Middleton: Dane County, Wisconsin
• Others?
• Not listed in Wisconsin’s new regulation NR40 (but neither is crown vetch, bird’s-foot trefoil, or reed canary grass!)