Crownvetch's Role in Vegetation Management

By John T. Whaley, RLA, C.A. - PennDOT Bureau of Design

Many naturalists, conservationists and environmentalists have jumped on the invasive plant species bandwagon and are waging war against many plants including Crownvetch (Coronilla varia L.) as an exotic, invasive. It has become a whipping boy plant even at the national level in many circles. The campaign against crownvetch (sometimes indicated as two words) has been promoted at the national level of the Federal Highway Administration without full realization of Crownvetch's role in PennDOT's vegetation management program.

Unfortunately, many people will see a plant's name on another state's invasive or noxious weed list and naturally assume that it must be invasive everywhere else. They immediately want the plant's use banned and the plant eradicated wherever it grows. To assume that a plant is automatically 'bad' by association to another state's list gives gross injustice to the plant. Extensive research that would examine all characteristics of the plant over the plant's life should always be conducted before a decision is made on the invasive quality of the plant. We know that some sunflower plants are considered invasive in one state and listed in another as it's state flower.

An invasive plant utilizes many unique growth and propagation characteristics that help the plant become dominant in a plant community and therefore enables it to spread to adjacent areas. Some of these invasive plant characteristics include: 

Exhibits strong and rapid growth 

Has the ability to spread over a large area 

Has the ability to out-compete nearby plants 

Produces abundant seeds 

Has increased seed germination rate 

Has long-lived seeds 

Rapidly maturates to a sexually reproductive stage

All plants have the ability to reproduce themselves but these invasive characteristics help the invasive plant out-compete other plants, especially some less competitive native species, to form dense, single specie vegetation stands.

Crownvetch exhibits several of these unique traits and that is precisely why it has been developed for roadside slope applications. It has been promoted for over 50 years in Pennsylvania and has performed admirably as a plant that can help prevent soil erosion and can help impede, but not prevent, the woody brush invasion and natural plant succession on highway roadside slopes.

The invasive species executive order signed by former President Bill Clinton was basically concerned with native plants in nature (natural) areas on federal lands being adversely affected and eventually replaced by more dominant, non-native (exotic) plants. Natural native plant areas are often prone to invasion and conversion to other plant communities since there is little intervention by humans. The nature areas are generally allowed to respond to the many whims of natural plant succession without human interference. An anti-invasive, anti-exotic plant crusade as also been the resultant outcome at the state and local level. Often this crusade is directed against plants that some people do not like for various preferences or prejudices.

Agricultural land and highway roadside slopes are also highly prone to invasion and degradation by invasive plants since these plants can readily move and adapt to new habitats, particularly those where the land has been disturbed by man or nature. The roadside slopes created by highway construction present a difficult challenge to the establishment of any kind of vegetation.

Looking at natural plant succession, the crownvetch plant, PennDOT's previous history of establishing highway slope vegetation, and PennDOT's vegetation management program is necessary to help understand, evaluate and appreciate the use of crownvetch along Pennsylvania's highways.

Natural plant succession 

Natural plant succession is a process by which a plant community slowly evolves into a collection of species best suited for a location. The successional stages can vary greatly with climate, time and soil condition.

Pennsylvania was almost entirely covered by a mixed hardwood forest prior to European settlement. This forest was the result of previous natural plant succession since Pennsylvania was also covered to a large extent by glaciers in ages past. The hardwood forest climax vegetation tries to re-establish itself whenever the land is altered by clearing away the vegetation. All vegetation management programs are an effort to stop the natural succession or maintain the current plant community. Natural succession can occur rapidly in the absence of a management program.

PennDOT's roadside vegetation management program is a constant attempt to halt the gradual reversion of woody vegetation from growing too close to the highway travel lanes while maintaining the desired lower growing plant communities. These lower growing plant communities are usually comprised of turf forming grasses such as fine and tall fescues, and legumes such crownvetch and birdsfoot trefoil.

Plant Succession Stages 

Differing stages of plant growth will occur on bare ground or lightly vegetated areas as each step in the successional stage modifies and prepares the soil for the next incursion of plants.

Annual weeds are the first bare soil or lightly vegetated ground invaders and are usually already at the site within the existing soil bank. These weeds can also be easily transported into the area by wind or carried to the site on the coats of animals passing through the area. Many of these plants are very durable and remain viable in the soil for many years. Foxtail, dandelion, crabgrass, pigweed, lambsquarter, panicum and other grass like plants are only a few of the annual weeds that can first appear. Some biennials and herbaceous perennials such as wild carrot, burdock, teasel, hawkweed, common pokeweed, native goldenrods, and many others may also appear at the same time as the annual weeds since their early development is similar to the annuals. The perennials can colonize an area for longer periods of time than the short-lived annuals. The perennials can live for three or more years and reproduce by seed, but some can also reproduce vegetatively by rhizomes, tubers or small sections of root. Once perennials become established, they are hard to control because of their extensive root system. The increasing number of perennials slowly fill the space available to the annuals and the annuals then decrease and become a minor part of the new plant community. Spotted knapweed (Centaurea maculosa) is an aggressive biennial or short lived perennial that has successfully invaded bare soil areas and older stands of crownvetch and is particularly noticed along central Pennsylvania highways during the month of July when it is in flower. Canadian thistle, a noxious weed in Pennsylvania, seems to readily invade stands of crownvetch in certain portions of the state.

Woody perennials such as blackberry, raspberry, and dewberry type brambles, wild grape, poison ivy, sweetfern, multiflora rose and others also start to appear and compete for growing space claiming less area than the annuals and perennial herbaceous weeds. The woody plants however, will grow much taller and will eventually shade out the lower growing herbaceous species. Taller growing woody tree types such as sumac, ailanthus, pin cherry, Virginia red cedar, boxelder, black locust, aspen, birch, ash and other trees will eventually outgrow and shade out the lower growing woody plants and become dominant creating a re-established forested area on the areas once exhibiting bare ground or lightly vegetated conditions. Of course, the soil must be conducive to plant growth or little or no successional growth will occur. Older agricultural fields that have laid fallow for many years will start to slowly evolve into a wooded field. We can also see this readily occurring along most older highway routes in Pennsylvania as the woods line slowly 'creeps' down toward the edges of the highway pavement.

The course of natural plant succession also depends on the plant species contributing to the local seed bank and plants inhabiting the local vicinity through which the seed can be transported by various methods into the site boundaries. Since many areas of the state have been repeatedly disturbed by human activities such as cultivation, grazing, fire, lumbering and various types of construction practices, the seeds of many native plant species are no longer readily available as a source of plants to participate in the natural succession evolution.

Usually plants that invade and inhabit highway roadsides (the area from the edge of the highway shoulder to the right-of-way line and the median areas between the highway lanes) are plants that can adapt and are well-suited to the adverse growing conditions of infertile and droughty soils and can take full exposure to sun and drought conditions.

The type of plants covering an area is also of prime concern in the eventual re-growth of the forest climax community. Grasses and legume plants forming dense sod or exhibiting dense soil coverage will help to delay or suppress the natural succession process. Maintaining these existing plant communities by cutting or spraying with selected herbicides will further delay or curtail any invasion by other unwanted plant species. Short-lived or bunch forming grasses, wildflowers and other forbs that do not form densely populated communities leave bare soil voids that will allow a more rapid germination, invasion, and establishment of other plant (either desirable or undesirable) species.

Crownvetch 

Crownvetch is a herbaceous, perennial legume with strongly branched, coarse stems ranging from 2 to 6 feet in length. One plant could cover 75 to 100 square feet (10'x10') within 3 to 4 years as the plant matures. The leaves are dark green and pinnately-compound with an odd number of leaflets ranging from 15 - 25. The top growth forms a mounded, upright (1-1/2 to 2 ft) plant mass having a trailing or reclining appearance. It is not a true vetch and does not have tendrils or other holdfasts for climbing. It has a multi-branched root system similar to bromegrass and can spread by seed, as well as by its fleshy, rhizome like roots. Each growing season, new foliage shoots out from the multi-branched creeping root system. As winter approaches, the plant leaves and stems become dormant and change to a golden brown color until spring again brings the plant out of dormancy and it 'greens' up.

Since it is a legume, it's roots help to fix nitrogen, a major necessity for plant growth, to the soil benefiting itself as well as other plants. The plant develops a symbiotic relationship with nitrogen fixing soil bacteria thus storing nitrogen in the soil. Estimates of the amount of nitrogen added to the soil by crownvetch is 50-100 lbs/acre each year after the stand has been established.

Crownvetch produces a pleasing array of pinkish-purple to white flowers that bloom in various parts of Pennsylvania from May through August. The mounded spreading habit provides good ground cover for ground nesting birds, rabbits and other small mammals as well as providing forage for deer and elk. Deer are known to paw away a winter snow layer in order to get to the crownvetch to feed. The plant stays green during the growing season even in hot, dry weather. Moisture retained in the coarse top growth actually helps to retard fires burning along the roadside.

Three cultivars have been developed (Chemung, Emerald, and Penngift) although 'Penngift' was the cultivar specified for use on Pennsylvania highways. 'Penngift' is the lowest growing in height, is finer stemmed, and is the most attractive looking of the three cultivars.

Crownvetch has other growth characteristics that help it to succeed where other plants fail. It grows well on infertile, sandy, gravely-rocky, clay or shale soils that range in acidity from 5.5 to 7.0 and lack organic matter. These soil conditions are often encountered on highway embankments. It is cold hardy although young seedlings can be winter killed. It is fairly drought tolerant once established and it is reported to be disease tolerant although research has shown that it has disease problems as well including various stem, root and crown rots. Crownvetch grows best in full sun and becomes stunted and thins out under heavy shade. The natural decay of the old dead plant matter also adds organic matter to the soil.

It is however, not the plant for every soil condition since it is not recommended for wet, poorly drained soils or where soil toxicity is apparently present. Although used extensively along Pennsylvania highways where salt is used for winter snow and ice melting, crownvetch is described as intolerant of salt contaminated or alkali soils.

It is noted as a prolific seeder although it is slow to establish itself by seed. The seed takes as long as two growing seasons to become established. When excessive vegetation competition exits from other grasses or forbs, the crownvetch establishment is delayed. It is generally seeded with a nurse crop of annual ryegrass that will germinate quickly and help hold the soil in place while the slower growing crownvetch germinates and sets seedlings. The seeds have an extremely hard seed coat and have to be treated before they can be used in seeding operations. The seeds, like other leguminous seeds, germinate best when inoculated with bacterial cultures that help to break the seed coat. The seeds are produced in pods and mature about three weeks after the end of the flower bloom. Crownvetch can spread by seed, but since the seeds have difficulty breaking the hard seed coat, the plant can't spread as aggressively as some other invasive plants or as quickly as some literature would have you believe. The plant is said to 'creep' since the roots will also put up new plants during each growing season.

Many sources of crownvetch literature from other states, indicate that it readily invades prairie and other open communities. The term 'readily' is not a very time specific description. Crownvetch can be a vigorous grower and can spread to adjacent sparsely vegetated areas if the conditions are right. Crownvetch will spread to other areas over a period of time if they are not being actively farmed, grazed, mowed or otherwise tended or maintained. We know that crownvetch establishment in Pennsylvania may take several seasons before it can successfully compete with other vegetation for the available growing space. The term 'tended or maintained' is a key factor in crownvetch's reputation for invasiveness. The plant can be controlled and contained in a certain location by regular mowing maintenance, agricultural operations like tilling and the use of herbicide treatments. Continual close mowing will drastically reduce the plant vigor and eventually kill it.

Two other noted characteristics have helped to firmly establish crownvetch as a prime plant for highway embankments in Pennsylvania:

1. The established plant provides a spreading, heavy, top growth soil cover that breaks the impact of rain droplets hitting the soil, reducing the capacity of the droplets to dislodge soil particles that lead to erosion. The strong branching root system also holds the soil together preventing the probability of soil erosion.

2. The spreading top growth forms a fairly deep and dense mat that helps to shade out other plant seedlings trying to invade the crownvetch. This is highly desirable from PennDOT's perspective since this suppression of growth of other vegetation, especially woody species, delays the eventual maintenance necessary to keep this unwanted vegetation (native or non-native) away from the highway travel lanes.

Crownvetch Proclamation 

On June 17, 1982, Governor Dick Thornburgh signed Act 1982-150, Laws of Pennsylvania, declaring 'Penngift' Crownvetch as the official State Beautification and Conservation Plant of the Commonwealth of Pennsylvania.

The Act states: WHEREAS, 'Penngift' Crownvetch has provided Pennsylvania with worldwide recognition as the "Crownvetch Capitol"; and WHEREAS, Penngift Crownvetch was first discovered in Pennsylvania by Dr. Fred Grau in June 1955, extensively researched by the staff of the Pennsylvania State University and was named Penngift; Penn for Pennsylvania and Gift for the farmer on whose farm the legume was discovered; and WHEREAS, the Pennsylvania Department of Transportation has successfully utilized this plant along roadsides of every interstate highway throughout Pennsylvania since 1958 and receives hundreds of congratulatory letters from the motoring public on the roadside beauty created by Crownvetch; and WHEREAS, Penngift Crownvetch has successfully stabilized and prevented erosion on over 50,000 acres of erodible soils and is recognized by conservationists as a major plant species in the re-establishment of vegetation on subsoils; and WHEREAS, Penngift Crownvetch is well recognized for it's agronomic value as a livestock feed and use as a ground cover for erosion control, nitrogen feeding, and soil insulation in no-till corn production. The General Assembly of the Commonwealth of Pennsylvania hereby enacts as follows: Penngift Crownvetch (Coronilla Varia L. Penngift) is hereby selected, designated and adopted as the official State Beautification and Conservation Plant of the Commonwealth of Pennsylvania.

History of PennDOT roadside plantings 

Prior to the adoption of Penngift Crownvetch in 1958 as the dominant plant species to be planted on steep highway slopes, other plant species were often planted and gave varying results in providing ground cover and control of erosion on the bare soil embankments. They also required varying amounts of maintenance care to keep the plants alive and growing. Hall's Japanese Honeysuckle and other vine type plants like American Bittersweet were planted. Over the years, these plants developed their own invasive characteristics and were not highly successful in preventing erosion. Other plants including Multiflora Rose, Coralberry, Acacia, Rugosa Rose, Memorial Rose and Daylily were used with limited success. Daylilies were one of the more successful plants used in providing erosion control once they were established, but the costs involved with planting individual plant/root divisions for long stretches of embankment areas could not be justified and they also took several years to grow together into a dense soil cover mat where they could hold the soil against erosion. There are still plantings of daylily on steep slopes surrounding inlets along Rt. 322/22 (Lewistown Narrows) and other locations across the state where these plants are still growing and providing erosion protection and lovely flowers to enjoy.

Bunch and sod forming grasses were also used and in most cases they were successful in helping to stem erosion but they were quickly invaded by undesirable woody species after a few years. Keeping the unwanted woody vegetation out of the grassed slopes required an expensive maintenance program of hand and mechanical cutting and later herbicide treatments since the steeper slopes could not be mowed. Sod forming grasses can provide an initial competitiveness and delay against herbaceous weed and woody vegetation invasion but without a continuous maintenance regime, natural succession will take place. It is interesting to note that almost all of the cool season grasses that we use around our homes are exotic plants brought to North America by the early settlers.

The grasses used for ground cover along highways have varying growing conditions but do best in fertile, well drained soils. Prior to the use of crownvetch, many of the steep subsoil slopes proposed for grasses were topsoiled in order to provide a layer of 'good' soil for the grass to establish. Since crownvetch establishes and grows well on infertile soils, this topsoil expense was drastically lowered on highway construction and less topsoil needed to be brought in from agricultural sites off of the highway right-of-way.

Crownvetch adoption

In 1935, Dr. Fred Grau, a young extension agronomist from Penn State University, noticed an unusual plant growing on a shale hillside on a farm near Virginville in Berks County, Pennsylvania. Nothing else seemed to grow on the slope. Dr. Grau collected the plant and reproduced it from seed on his farm near Pine Grove Mills in Centre County. The plant was identified as crownvetch and was probably imported from Europe in alfalfa seed sown on the farm around 1905. After nearly twenty years of research on establishing and collecting seeds, the plant was released by the Pennsylvania Agricultural Experimental Station in 1954.

The former Pennsylvania Department of Highways was asked to perform planting trials to investigate it's use and success for erosion control along Pennsylvania roadsides. In 1954, highway embankments were seeded to 'Penngift' Crownvetch near Port Matilda in Centre County. The planting trials investigated the following topics: ü Did the plant establish a relatively permanent vegetative ground cover ü Did the plant effectively reduce soil erosion and hold erosive soils in place ü Could the plant reduce follow-up maintenance procedures ü Did the plant provide an inexpensive method of establishing vegetative cover on steep slopes that could not be easily mowed ü Did the plant retard encroaching development of woody vegetation.

The Department of Highways studied the results of the trials and wrote a favorable report.

In 1955, crownvetch became used as a standard plant for erosion control along Pennsylvania highways. In the 1960's, 70's and 80's, the development of the interstate highway system crossing through Pennsylvania required large amounts of excavation through rugged hills and mountain sides and filling across valleys and depressions with high and long embankments. These construction efforts left large amounts of exposed subsoil that needed to be re-vegetated. The 313 mile length of I-80 necessitated the use of 6,475 acres of crownvetch seeding and 2,867 acres of various grasses.

Crownvetch use guidelines were established and all slopes 3:1 (slope ratio of 3 ft. length to 1 ft. rise) or steeper were to be seeded with crownvetch. As the benefits of crownvetch were noted over the years, many maintenance mangers realized that grass mowing costs could also be drastically reduced if flatter areas were also seeded to crownvetch. Therefore, more crownvetch was planted beyond the original 3:1 slope criteria. It also became evident that the crownvetch stands were repressing and delaying the eventual natural plant succession along the roadsides to an extent that very little brush/tree removal was necessary close to the travel lanes for many years. This was especially important in the wider median areas that separated the directional travel lanes. The crownvetch helped to delay the natural succession but did not prevent it.

The criteria for placing crownvetch on highway slopes has been revised in recent years so that slopes 2:1 or steeper, rather than 3:1 or steeper, are now designated for seeding with crownvetch. There has also been an attempt to reduce or eliminate using crownvetch on other areas outside of the standard mowed areas and steep highway slopes where grasses, wildflowers, and other herbaceous plants could be seeded and allowed to return to a more natural state of plant development and natural succession. Descriptive information has also been added to PennDOT's Design Manual to inform highway designers that crownvetch has certain growth characteristics that need to be carefully considered before specifying it's use on a highway project.

Sometimes crownvetch is found growing in other grass stands along the highway where it was not supposed to have been planted. Many people jump to the conclusion that the crownvetch has naturally spread from another nearby location. This may be the case in some areas, but we also believe that the crownvetch may have been seeded into its 'unintended' location because the seeding contractors did not thoroughly clean-out their seed tanks between seed mixtures.

Over the years, crownvetch use was firmly entrenched in PennDOT's construction and roadside maintenance programs. Public acceptance over the years has been high and the Department still receives requests for information on the name and use of that 'pretty' pink roadside flower along the highway. As the years pass, newer generations of people begin to notice and want to learn about our roadside vegetation.

Crownvetch invasion and complaints from adjacent farmland owners have been almost non-existent over the nearly 50 years it has been used. The plant could be easily plowed under by the farmer adding nitrogen and organic material to the soil and it did not recover quick enough to be troublesome. Crownvetch planted along highway corridors also did not seem to invade adjacent land bordered by thick brush or forests since the shade produced by the taller growing plants reduced the plant's spreading vigor.

Anyone traveling along a Pennsylvania highway with construction slopes originally planted in crownvetch should be able to observe that the crownvetch is not readily moving 'off' the highway right of way but is steadily being invaded by other plants over the years. The so called 'invasive plant' has itself been invaded by other plant species and without a maintenance program designed to eliminate the woody invasives, the crownvetch will eventually disappear from the roadside environment. Getting 20-30 years of useable coverage from a well established stand of crownvetch seems to be the average life performance along our roadsides.

Crownvetch is certainly not the 'cure-all' plant that was originally desired and promoted since it does have limitations just like any other plant, native or non-native. Many other plants and plant combinations have been researched over the years and are still being investigated (including various warm season grasses and forbs) as a replacement plant for crownvetch as a ground cover on the harsh growing conditions found on roadside embankments, but nothing has been found to date that has the previously mentioned varied benefits of providing dense ground cover, longevity, durability, and the ability to grow in so many adverse soil conditions.

References:

'Weeds Gone Wild' - website, National Park Service, Alien Plant Working Group

Roadside Vegetation Management Manual - PSU Report #PA 92-4620+85-08, March 22, 1992

USDA/NRCS - Plant Materials Program website, US Dept. of Agriculture, Natural Resources Conservation Service

John C. Harper II, U.Ed. 8-464, PSU/College of Agriculture, Extension Service, University Park, PA 'Crownvetch for Erosion Control and Beautification'

'For Slopes the Word is Crownvetch', Grounds Maintenance, August 1971, Intertec Publishing Corp. Kansas City, MO

'For Erosion Control, No Maintenance and Beauty: Crownvetch', Weeds Trees and Turf, September 1970, The Harvest Publishing Co.