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Weeding even where it seems impossible by hydroseeding

Slopes, ski runs, quarries, landfills, reinforced soil, nature engineering works, as well as sports grounds, golf courses, large public recreational areas and private or common gardens: these are the several areas where hydroseeding is most widely applied. An overview of the machinery used for this green maintenance work

by Domenico Pessina
January - February 2023 | Back

Within the scope of the activities peculiar to naturalistic engineering, the rehabilitation of hydrogeological disruption has always played a key role. The restoration of degraded areas, as they are, for example, subject to landslides, mudslides, erosion, etc., is undoubtedly a priority over other subsequent land redevelopment interventions, such as those subject to landslides, landslides, erosion, etc. In Italy, the problem is becoming increasingly severe, not least because of the evident climate change underway, which exposes the national territory to weather phenomena of progressively increasing intensity and frequency, with an exponential increase in the risk of catastrophic events.

In this far from reassuring context, there is an urgent need to focus efforts on the development of agronomic practices aimed at improving the stability of the slopes of the hillsides, including in their surface layer, i.e., the one most exposed to the mechanical and hydraulic impact of rainfall and wind. Grassing, and more generally, creating an adequate vegetation cover of areas at risk, is an excellent solution to limit surface erosion and stabilize the stand. However, on a high slope and/or rough spots, where there is no suitable roadway, and it is impossible to create one, it is impossible to access the site by land-based means to carry out the necessary work. Hydroseeding is an alternative solution that has long been known and has been interestingly developed for some time.

The hydroseeding system. This is a turfgrass planting technique in which a rather pasty mixture consisting of seed, mulching substrate materials, adhesives, slow-release fertilizers and germination and rooting accelerating principles is applied to the soil.

It is carried out on topsoils, embankments, ski slopes and in nature engineering works. However, it is also profitably applied in the environmental rehabilitation of quarries and landfills. However, the field of application of hydroseeding is even broader. In certain countries, especially in the Anglo-Saxon world, this technique is also usefully applied in much less environmentally critical contexts, such as the creation of extensive flat green surfaces, i.e., sports fields, golf courses, large public recreational areas, as well as private or condo gardens. 

Over time, several hydroseeding techniques have been developed, involving appropriate preparation variations designed to optimize the grassing result. The traditional solution includes a mixture of seeds, fertilizer and glue, but the addition of chopped straw, mulch, wood fibers, cellulose, etc., can significantly increase the water retention capacity and improve the microclimate on site, thus enhancing germination. In addition, anti-erosive matrices, based on water-diluted stable hydro bituminous principles and/or bound fiber compounds, can be added to increase the adherence of the mixture to the soil surface.

Seed management. Typically, a mixture in different proportions of grasses (60-80%) and legumes (20-40%) is used in order to optimize the result concerning the characteristics of the stand, exposure of the area to be treated and its microclimate, slope, etc. The most widely adopted species are Lolium perennial, Festuca arundinaceous and Festuca rubra, Poa pratensis, Dactylis glomerata, Medicago lupulin, Onobrychis vici folia, Trifolium pratense and Trifolium repens, Bromus erectus and Bromus inermis, etc. Specifically, grasses and legumes offset each other well both in terms of the characteristics of their root systems and as the timing of vegetative activity: In fact, grasses have a fascicled root system and, in some cases, the stoloniferous tendency (which favors soil colonization), while legumes have more profound, taproots and contribute to improving the soil's nutritional balance, thanks to the root symbiosis of nitrogen-fixing bacteria.

As previously mentioned, the preparation can be supplemented with a biodegradable mulch, which helps create the best microclimate for germination, a high water-retention component, whi­ch absorbs and gradually releases water; a soil conditioner, which improves the chemical and physical conditions of the soil by promoting germination; a glue, which reduces seed translocation by environmental causes (e.g., by surface ru­noff) and predation by birds; and a dye, which promotes the most uniform distribution of the mixture.

The adjuvants: Glues and dyes. Hence, glue is an essential element of hydroseeding as it fixes the mixture on the soil surface, preventing its leaching by runoff water and dispersion by wind. Depending on the different operating situations (especially the area's slope to be seeded), various active ingredients and concentrations are adopted, although high molecular weight water-soluble anionic polymers and/or natural polysaccharides are frequently preferred. As a matter of fact, the adhesive effectively binds to the substrate (mulch, cellulose, wood fibers, etc.) that is part of the mixture, forming a dense network that also effectively holds the seed and fertilizer in place.

Conversely, dye is added to the mixture to achieve the best visibility of the treated area. For evident reasons, there is a tendency to prefer dyes contrasting with the substrate and, in any case, compatible with the landscape impact, although this is a more or less temporary effect since, in a short time, the growth of the sown species is expected.   Usually, water-soluble dyes are used, also suitable for domestic and industrial detergents, containing preservatives and surfactants, with very varied pHs, from very acidic to very alkaline.

Mechanization of hydroseeding. From a structural and functional point of view, mechanical equipment for performing hydroseeding is basically similar to traditional manure spreaders for field distribution of livestock manure (so-called "tokens"), thus equipped with tanks equipped with mechanical and/or hydraulic agitators, which contain and keep homogeneous the preparation, which is then distributed on-site through pumping and distribution systems, with a product throw of even several tens of meters, thanks to the considerable working pressure. Hydroseeders are coupled with tractors of different types, being able to be installed on dedicated trailers, flatbed trucks, transporters, or even in combination with agricultural tractors, either towed on the hook or carried on the 3-point hitch. Machine operation is often provided by a self-contained endothermic motor, which provides both the main pump and an additional auxiliary hydraulic pump for the remaining drives. However, when combined with a tractor, the equipment makes profitable use of the PTO as a means of operation.

Self-propelled versions, installed on trailers or cargo beds, are characterized by high working capacities and acceptable operating range. They can accommodate on-board mixture tanks of up to 6,000 liters and more, with self-priming positive displacement pumps of up to 600 l/min flow rate, a head of 12-15 bar, and a range that can reach more than 40-45 m from the drop point.   In this case, the endothermic engine is of the diesel type, with a power output of 70-80 Hp.

Obviously, models of smaller capacity are available, which can be profitably installed in the body of pickups and vans:  the tanks have capacities that usually do not exceed 1,000 liters, while the engine, in addition to the diesel type, can also be gasoline, from about 12 to 25 hp. The range, in this case, ranges between 15 and 25 m.

The models intended for coupling with the tractor stand somewhere between these two extremes. They feature tanks of capacities ranging from 500 to 2,000 liters, direct handling from the tractor's power take-off (or alternatively via the tractor's primary hydraulic system), a head ranging from 10 to 14 bar, and a range of 15 to 40 m or more.

The endothermic motor often drives two hydraulic pumps working together. The first is a volumetric type since this type is better suited to properly handle relatively dense and viscous material (to be distributed sometimes even with the help of long pieces of hose). It is also used to draw in through a special kit water, that is at the base of the mixture. Conversely, the second pump is hydraulic and is used for internal re-circulation of the mix in the tank. This is to ensure adequate turbulence to maintain the best uniformity of concentration of the various components. A properly sized cooler maintains the oil temperature at appropriate operating values, preventing overheating and preserving fluid quality over time.

As an alternative to hydraulic agitation (often carried out with the aid of a Venturi device), in several cases, rever­sible paddle agitators are used, made of chromium-plated or stainless steel, for more effective resistance to corrosion of the most aggressive components of the preparation, such as livestock manure, or manure of organic origin.

The distribution lance can have different nozzle conformation, depending on the characteristics of the area to be treated: Narrow fan for borders and seeding in confined spaces, wide fan for more extensive and more regular areas, and narrow jet for reaching areas far from the ejection point, especially if sloping and highly susceptible to erosion.

Of interest is the version for inter-row grassing of the vineyard proposed by Casella di Carpaneto Piacentino (PC), which relies on 3 ejector-distributors that direct the mixture onto baffle plates to best direct the flow of the mixture to the areas to be treated.

Tanks materials. Also, for maximum durability concerning corrosion, tanks are generally made of polyethylene or hot-dip galvanized steel both inside and out. The plastic material of the pressed type is preferred for smaller capacities, while bent and welded sheet metal, treated as mentioned, is used for larger tanks.

Available accessories. If the distribution lance is to be operated at the product deposition area at some distance from the vehicle transporting the equipment, a hose with a length of a few tens of meters is available, which can be unwound and especially rewound on a manually operated hose reel or, in more sophisticated models, also hydraulically.

Alternatively, semi-supported versions for the agricultural tractor are also available. In addition to the connection to the 3-point hitch, these versions make use of ground support via two pivoting wheels.

The rental formula. This is a solution often resorted to for interventions on public and residential greenery rather than for those of naturalistic redevelopment. In this case, companies operating in green maintenance make use of the more or less temporary supply of means offered by manufacturers, using them with seed mixtures and accessory preparations explicitly formulated for the most specific needs.

And there is also heli-seeding

For grassing of inaccessible areas that ground vehicles cannot reach, not even from a distance, aerial means must necessarily be used. In this case, the definitely most suitable aircraft is the helicopter, which is also successfully used in many other operations in the mountain environment. In addition to the slopes of the reliefs, at these junctures, classic cases are the grassing of new ski slopes and/or the grooming of existing ones, as well as the re-vegetation of areas that have been affected by the fire.

The routine involves loading the preparation at the impost in a 100-150-liter capacity container, secured to the helicopter with steel cables and completed with a remote control for opening the distribution holes. When the aircraft arrives over the area to be treated, the product is laid down simply by vertical fall, or by following a predefined path, depending on the conformation of the area to be treated.

Again, an appropriate dye is added to the mixture, which contrasts with the color of the stand, allowing for a sufficiently uniform distribution and minimizing overlaps and unseeded areas.


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