Greening highways: reasonable maintenance

The biodiversity of highway greenery must be preserved through sound crop practices and this means reasonable maintenance. Mowing, brushcutting and pruning are the main operations involved for which ranges of specific and efficient machinery and equipment are available

On the green areas associated with roads in tourist areas and highways leading into cities greenery is decorative and is given careful and costly maintenance not much different from the groundskeeping practiced in gardens and now coming to the fore is reasonable maintenance for green areas along highways and expressways in a number of countries, from France to Austria and Switzerland to the Netherlands. This maintenance is aimed at preserving the value of the landscape as well as biodiversity and the ecosystem of the territory. According to thinking gaining recognition within the scientific community, the ecosystem and the territory must be considered together, that is, acknowledging that the biotic population of varieties of plants and animals cannot be disassociated from the land and that a correct balance must be sought for preserving the biological wealth as well as maintaining productive activities. Needed for putting this concept into practice in maintaining highway greenery is the understanding that biodiversity is included in the riches of these areas which must be preserved by good crop operations and reasonable maintenance.

Why protect biodiversity?

Protecting biodiversity means safeguarding the proper function of the natural ecosystem of the land to the benefit of production, the quality of the air and water as well as the stability of the soil. On the production front, consideration must be given to the importance of insects and birds for the pollination of plants without which the production of fruit, legumes and other vegetables would be drastically reduced. Moreover, sound management of the natural environment helps to limit air pollution and water runoff which, in turn, aids in the prevention of flooding and landslides. Greenery on the margins of roads and highways, on slopes and along waterway banks added to the ecosystem, if given reasonable management, can contribute to the improvement of the land and preserving it again hydro-geological risks.

Reasonable maintenance

Reasonable maintenance is the result of the differentiated management of greenery and is carried out through the intervention of mechanical means and leaving out, as much as possible, chemical products. For maintenance operations on road verges what must be considered most is the setting for this work, expressways, national and provincial highways, rural roads, roads on plains or in the mountains, and also of importance is an examination of the value of the landscape and the land on which the road has been built. These various settings, road verges, escarpments and the banks of waterways can be ecological corridors which act as biodiversity reserves. They often account for 20% of the floral diversity of the entire territory. The biodiversity depending on roads and highways is almost always far greater in the context of intensive agriculture than that of fields under crops. In the most widespread configuration, two areas can be distinguished on road and highway margins, one of a grassy bank and the other which ends in an escarpment or a ditch. Maintenance operations involve mowing the grass for guaranteeing safety and visibility for motorists. In applying reasonable maintenance, mowing of the banks is limited to the width providing safety, one to five meters. Moreover, the height of the cut is raised from the tradition 5-8 cm to 10-15 cm and frequency is reduced to two or three times a year. A zone described as pre-flowering is left between the mowed strip and the slope and is mowed only late in the year, in September or October. On the slope or escarpment, mowing or brushcutting is performed only once, in the autumn-winter period. With this type of maintenance, roadsides can become natural habitats for numerous varieties of plants and animals.

There is a degree of diffidence towards this type of differentiated management in farming models which suspect that roadsides become hotspots of diseases and widespread infestations. This means agreement must be reached with farmers through the dissemination of information and communicating the advantages of reasonable maintenance and pointing out that programs of this type require three or four years of experiments. Where these procedures have been carried out, in France for example, the response has been possible and differentiated management has been accepted by the farming world.

For mowing the roadside safety strip, the time taken and vigor of new growth must be taken into account. The operation is performed when grass reaches the height of about 25 cm and in all cased as the height of more than 40 cm. Late mowing in the autumn of the area behind the verge cannot be considered a lack of maintenance because the timing  allows the life cycle of insects and the plants to be completed, that is for the production of their seeds. Experiments conducted in an area close to Paris showed that reducing mowing from three time a year to one autumn cut in September has a positive influence for the wealth of vegetation in that what resulted was an increase in biodiversity and better compatibility among the species present. These results, of course, might differ in relation to differing conditions and must be verified for each single region.

The machinery to use

First of all, a distinction must be made between mowing, which means cutting the grass, and brushcutting, which involves cutting and shredding brush or vegetation which is more or less woody. As stated above, these two operations can both be used for the maintenance of road and highway greenery: mowing for verges and brushcutting for the slopes, ditches and escarpments when the vegetation in these areas is ligneous.

The cutting apparatus used is usually a blade mounted on a horizontal axis or flails mounted the same way. When the grass is high and growing on irregular terrain on which they might be stones the preference is for a mobile Y flail which performs cutting not very different from that done with a sickle. Moreover, flails can be used with no risk of clogging when the grass is wet.

The mower can be self-propelled with a driver on the ground or aboard but more widespread is a cutting head on a telescopic boom mounted on a tractor.

Cutting at the height of 10-15 cm means less power required and therefore reduced fuel consumption, less risk of the blade or flails coming into contact with the ground, especially on irregular terrain, and less wear on the implements. Moreover, compared to cutting at the height of 5-8 cm, the higher cut favors the desired species.

On the matter of autumn operations on the area behind the highway margin, recourse to brushcutting involves the use of a horizontal axis cutting apparatus with flails thicker than those used for mowing grass or hammers when the vegetation is ligneous. Pursuing the concept of reasonable maintenance, the residues of mowing and cutting are collected. These residues can be put to use in various ways. They are usually used in biogas plants for generating energy of value; in terms of energy, one ton of grass is in fact equal to seventy liters of diesel. Exploiting the energy value of mowing residues for the production of compost or biogas had led to combined work operations based on telescopic booms equipped with heads which not only mow but also vacuum up the product. These residues picked up are offloaded directly into a towed trailer. These operations require a certain amount of power and the use of a tractor with an engine rated at 80-100 kW. 

Worksites along road and highways must be clearly signaled to passing motorists. On the level, motorists must be warned at a distance of 200 meters and urged to reduce their speed and warned again as they approach the worksite and progressive lower their speed. 

Reasonable pruning

Trees planted on the margins of roads and highways are a feature which should not be neglected because of their value in ecological terms and for enhancing of the landscape. For this reason, to preserve them drastic pruning such as topping should be avoided because this could compromise their ability to survive and a less severe pruning technique should be used with greater respect for the trees. Also for trees, the concept of reasonable pruning should be applied.

Pruning takes place usually in the autumn when the tree has used up all its reserves and going towards a vegetative shutdown. The important points in pruning are that not too much of the crown should be cut away and branches must be pruned on the branch side of a stem-branch node to allow balanced growth. Also important is that pruning cuts should be made on the branch side of this stem collar. This protects the stem and the other branches that might be growing from it. It also allows the tree to heal more effectively after the prune.  

The worksite usually involves pruning performed by shears or chainsaws used by the operator on an elevated platform and one or more workers clearing away branches and chipping them by using a chipper towed or mounted on the tractor. The chips produced are loaded directly into a towed trailer. Chips produced from pruned branches are widely used for firing wood burning furnaces.

Closing observations

To conclude this brief discussion it would be timely to stress that reasonable maintenance also results in financial benefits. In fact, where this maintenance is practiced fuel consumption reduced by 15% has been reported along with wear and tear on the mower lowered by 20%. In addition is the economic advantage gained with the recovery of residues for the production of energy and compost.