By Mark Myrowich
The most important element of long-term vegetation is its ability to withstand erosive forces, such as stormwater runoff and wind over open areas. This is true for road-building, capping closed landfills, brownfields remediation, and other applications.
The security and health of vegetation rests in the health of its topsoil.
Suitable topsoil is rarely available onsite, however. Replacing eroded or weak soils can be expensive. When applied or secured poorly, the results can be even worse. Consider that an inch of good-quality topsoil can take upwards of 100 years to develop — but can be lost easily in a year if the vegetation coverage cannot handle wind, water flow, and other site challenges.
It's possible, however, to improve site soils with hydraulic growth mediums (e.g., hydromulches, hydroseeds, and other hydraulically applied, vegetation-promoting cover materials) rather than trucking in new topsoil.
Topsoil hosts many organisms, all of which help promote growth: bacteria, fungus, microorganisms, etc. Regardless of where the soil is located and what organisms it supports, it's made up of four primary components: air, water, minerals, and organic matter. The fertility of soil is rooted in the interaction of these organisms and the balance of the topsoil's basic components. The hydraulic growth medium, when properly formulated, increases microbial activity and the formation of micro- and macro-pores necessary for topsoil building.
Protected by the right tackifier, which binds the product to the soil, the hydraulic growth medium can be just as secure as a stapled erosion control blanket, even on various slope configurations. (No single product is, of course, suitable for all sites; due diligence in the selection process must still be performed.) The tackifier should allow rainwater to infiltrate and nourish growth and microbial activity, while simultaneously protecting the surface from eroding.
As the activity in the soil increases, the vegetation it supports becomes stronger. Thriving vegetation provides better coverage, better survives erosive forces, and decreases maintenance.
Additionally, biotic-enriched hydraulic growth mediums can be mixed with project-specific seed mixes for particular vegetation, such as regionally or historically important wild-flowers and grasses. The organic nature of biotic materials stimulates the natural processes of top soil formation. Commonly, biotic elements are part of rolled erosion control products (e.g., wood, straw, coconut). These materials are your site blankets and turf reinforcement mats. From a topsoil-forming perspective, the advantage of hydraulically applied products is that they can incorporate materials with higher, more active biotic levels, like peat.
Choosing a hydraulic growth medium with topsoil-forming biotics is both more economical — cutting fuel and labor costs by 40% or more — and healthier for long-term vegetation.
Thriving in harsh climates
For a number of years, the Canadian energy company Hydro-Québec sought to reclaim roadsides along its northern construction sites and company-owned land, but salvaging the topsoil wasn't possible. Suitable topsoil borrow sites are approximately six hours away — making the long haul too expensive. Complicating these projects, the subsoil is highly erodible sand and the growing season is extremely short. Vegetation needs to be established within one season to make it sustainable without new topsoil.
The company secured a hydroseed contractor to test a biotic-enriched hydraulic growth medium called Biotic Earth. The product combines sphagnum peat moss, wheat-based straw mulch, and other elements. The rate of application was 1,786 pounds/acre — a decent speed at which a great deal of space can be covered quickly.
The seed mix consisted of a project-specific blend of creeping red fescue, timothy, bent grass, birdsfoot trefoil, white clover, barley, and reed canary grass. Adding these plantings helped meet the company's goal to “re-naturalize” the land.
To promote the right setting in of growth, a 12-18-12 (12% nitrogen, 18% phosphorus, 12% potassium) fertilizer was used; and 25% of the fertilizer was slow release. The tackifier was applied at a rate of 8.5 gallons/acre on the steeper slopes to firm up those sections and make sure they wouldn't lose cohesion during storms that might occur prior to full vegetation.
Grass grew within the first season, and the birdsfoot trefoil bloomed within the second. This is a typical pattern with hydraulic growth medium systems: strong, thick-rooted grasses grow at the outset to get the site through the first winter, followed by more advanced growth of additional vegetation in the next season.
It's been more than six years since that first installation and the roadside reclamation work still thrives. Hydro-Québec continues to specify the straw-and-peat mix for its annual northern projects.
— Myrowich (firstname.lastname@example.org) is CEO of Verdyol Plant Research in Cookstown, Ontario.