The terms hydro-seeding and hydro-mulching are often interchanged but in reality they refer to two different processes.
Hydro-seeding refers to the hydraulic application of seed or soil ameliorant, such as fertiliser, gypsum and lime using truck or trailer-mounted purpose-built machinery.
A seed carrier, typically shredded paper or wood fibre, is used to help mix the seed, protect it through the pump and help the seed stick to the soil surface.
Hydro-seeding is normally undertaken on steep or inaccessible slopes where conventional earthmoving equipment cannot operate.
Some of the largest hydro-seeders can apply seed 37 m away from the truck via the cannon.
Because hydro-seeding is simply another means of applying seed to the soil, it doesn’t provide any additional erosion protection until the vegetation establishes.
Hydro-mulching is the application of mulches, glues and soil ameliorants using identical equipment to hydro-seeding.
Best practice involves hydro-seeding the area first to ensure the seed has intimate contact with the soil and then applying the hydro-mulch over the top.
Unfortunately, due to the competitive nature of the industry and the low profit margins, the seed and the mulch are generally applied at the same time.
While this isn’t so critical with the light hydro-mulches ranging from 1,500-2,000 kg/h, it can be a significant problem with the heavy hydro-mulches, which range from 3,500-6,000 kg/ha, as the seed can be suspended in the mulch and not make contact with the soil.
It is important to apply the seed first for the heavier hydro-mulches.
There are three major different types of hydro-mulches:
- Hydro-mulch;
- Bonded fibre matrix (BFM)
- hydro-mulch; and,
- Hydro-compost.
Conventional hydro-mulches are suitable for low rainfall erosivity environments, such as southern New South Wales, Victoria, South Australia, or where slopes are shallow or intensive erosion protection is not required.
This is because hydro-mulches have low amounts of organic material (1,500-2,500 kg/ha) and low amounts of glue (40-60 kg/ha Guar Gum or 10 kg/ha of PAM).
I personally have never used a conventional hydro-mulch, always preferring to spend a few more cents per square and have a much higher quality product with improved erosion protection and revegetation success.
Typically wood fibre or milled sugar cane toppings are used as the organic material.
Some of the sugar cane mulches are hydrophobic and have approximately 20 per cent shredded paper added to the mix to help retain water.
In the last year or so, a heat-treated, straw-based hydro-mulch has been developed in Australia.
Straw has excellent thermal protection and moisture retention properties.
It also is great for protecting the soil from raindrop splash and is easier to pump than the sugar cane products.
This is good news as the wood fibre mulches are imported from the United States, whereas the straw products are grown and manufactured in Australia.
Recent times have seen a significant increase in the use of PAM glues as the price of Guar has risen substantially, primarily due to its use in the hydraulic fracturing industry for coal seam gas extraction.
It is not possible to achieve 100 per cent soil surface cover with hydro-mulch due to the low organic matter application rates.
This, combined with low glue rates, leaves the soil susceptible to raindrop splash erosion and erosion from flowing water.
BFM hydro-mulches are heavy duty hydro-mulches designed to offer increased erosion protection, longer term soil surface cover and more effective vegetation establishment.
The organic matter application rates range from 3,500-6,000 kg/ha and glue rates ranging from 75-300 kg/ha depending on slope steepness and rainfall erosivity.
Providing the soil surface is not too rough, 100 per cent soil cover can be achieved with an application rate of 3,500 kg/ha.
Hydro-mulches and BFM hydro-mulches require suitable topsoil for vegetation growth.
They are incapable of applying and storing sufficient soil ameliorants to modify poor topsoils or subsoils.
In these instances, the topsoil needs to be ameliorated prior to being applied to site, or in the case of subsoils, a compost blanket should be used.
Recently, compost has been added with wood fibre or straw fibre mulch (approximately 60 per cent) to create a hydro-compost.
The purpose of the hydro-compost is to improve organic carbon levels in soils and help improve the soil biology.
While anecdotal evidence demonstrates that the hydro-composts are effective at improving vegetation establishment on poor quality topsoil, there is not sufficient evidence to demonstrate an ability to increase organic carbons levels in subsoils to a sufficient level to sustain vegetation (given that soils labs typically recommend compost application rates of 20 per cent by volume to increase organic levels to above 2 per cent).
It is important not to confuse hydro-compost with pneumatically applied compost blanket.
Specifications for hydro-mulches, BFM hydro-mulches and hydro-compost are generally poorly written and of little benefit.
A typical specification should include:
- Mulch type and application rates per hectare (based on 10-15 per cent moisture content);
- Glue type(s) and application rates;
- Soil ameliorant types and application rates;
- Seed type and application rates; and,
- Desired soil surface cover percentage.
Accurate specifications are important if competitive quotes are going to be sought for hydro-mulching works.
It is equally important to inspect hydro-mulching works to ensure the specified products are actually being applied to the site.
Michael Frankcombe is a current Director of the IECA and is also a Principal Environmental Consultant, Newcastle Office Manager and a Director with Landloch Pty Ltd with more than 25 years practical environmental management experience across a wide range of industries.
He is one of Australia’s most experienced erosion and sediment control specialists with specific pipeline, road construction and mining experience across New South Wales, Queensland and the Northern Territory.
He is the past President of the International Erosion Control Association (Australasian Chapter), is a Certified Professional in Erosion and Sediment Control (CPESC No.1351) and is an International CPESC Liaison to the Envirocert International Board of Directors.