Hydroseeding has emerged as an effective method for revegetation of various landscapes, ranging from public parks to large-scale erosion control projects. A key to the success of hydroseeding is the choice of hydromulch, which plays a crucial role in providing erosion control, moisture retention, and seed protection. Among the different options available, straw-based and wood-based mulch have earned significant attention in the market.

In this article, we will develop a comparative analysis of straw-based and wood-based mulch in the context of hydroseeding. We explore 5 characteristics, including composition, costs, weed control, and binding capacity, to provide valuable insights for professionals in the field of erosion control and revegetation.

Permanent slope protection with grass using the hydroseed method. The grass is used to stabilizes the slope structure and prevent slope erosion. Effective and less maintenance.

Hydroseeding: an effective method for revegetation of eroded areas

Disturbed soils due to industrial activities, construction, or post wildfire are particularly susceptible to rainfall-induced erosion. Due to this issue, there have been many efforts to evaluate the performance of different erosion control techniques including geotextile materials, natural vegetative covers, and hydroseeding.

Hydroseeding consists in the application of a hydromulch that generates an immediate cover in the soil, thus generating a barrier against erosion. This mulch protects the soil directly by providing cover that reduces raindrop impact, prevents soil sealing, promotes infiltration, and slows runoff.

The most popular material used as a matrix in hydromulch is agricultural straw, which has been shown to be effective in reducing soil erosion after fires and other eroded sites. However, this type of mulch has a few disadvantages, for example, it is easily transported by wind, leaving slopes exposed in some areas. Therefore, the market has expanded in recent years to other materials, which we will explain in the next section. Other types of mulch materials such as wood-based mulches have been shown to be effective for erosion and runoff mitigation in laboratory studies.

Types of hydromulch

Hydromulch plays the role of maintaining the appropriate soil conditions for seed germination. It not only protects the soil from erosion but also extreme weather events, it regulates the % humidity in the soil and runoff, and keeps nutrients in balance.

Other than straw-based mulch, other mulches are beginning to rise in importance among erosion control techniques. Other materials used currently for hydromulching include:

Wood chips
Pine bark
Pine needles
Grass clippings
Guano
Foliage remains
Sand
Stones

There are different types of hydromulch, in this case, we will describe two types: straw-based mulch and wood-based mulch. The final choice will depend on the case, the budget, and the conditions for its application.

Wood-Based mulch vs Straw based mulch

Composition

Hydromulches used for hydroseeding are composed of:

Fertilizers
Organic matter
Stimulators
Growth enhancers
Seed mixtures

On one hand, straw-based mulch is mainly composed of shredded straw residues. These residues come from crop residues for example rye, barley, and rice. As a positive side, straw based mulch uses a residue as a raw material, but since straw has been in direct contact with pesticides used for agricultural purposes, straw is very likely to be contaminated with seeds that may act as weeds during revegetation of sites.

On the other hand, wood-based mulch is made from shredded wood chips. Wood mulches have been developed from wood manufacturing waste, wood shreds, or wood chips made from burned trees or forest thinning operations. The wood may come from different types of trees, such as pine or eucalyptus.

As an advantage, woods come from native trees so there is no risk of weed contamination during revegetation.

Costs

Since agricultural straw is generally available from agricultural lands near eroded or damaged sites, it can be less costly than wood-based mulches. However, while straw is lighter, it requires twice the amount by weight compared to wood fibre mulch, leading to higher transportation costs and more frequent deliveries.

On the other hand, although wood is heavier than straw, it requires less material by weight for the same area, potentially offsetting the higher transportation costs and emissions. Additionally, wood-based materials for hydromulching can be more cost-effective in the long term as they are less likely to be blown away by the wind and persist longer in the soil than straw mulch.

From a cost perspective, the availability of materials and the location of the eroded areas can influence the choice between straw and wood-based hydromulch treatments.

Health issues

Due to the direct contact with pesticides during crop production, different studies have stated that agricultural straw has been implicated as a source of noxious weeds in forested watersheds. In addition to this, the fine dust from shattered agricultural straw could act as a respiratory irritant and source of allergens to workers who are involved in spreading straw by hand or machine.

Therefore, unlike wood-based mulch, when using straw-based mulch, workers involved in the spreading process may be exposed to health risks, as straw carries fine dust particles that can become liberated when the straw elements are shattered.

Binding capacity / Runoff

One of the main advantages of wood based mulch in comparison to straw-based mulch is its capacity to immediately bind to the soil and avoid mulch runoffs during heavy precipitations.

Straw based mulch has a low erosion control because it is very light and is easily blown away by the wind. On the other hand, wood based mulch has a high binding because it is more resistant to wind displacement due to its own weight. This allows it to form a dense layer that adheres to the soil and provides instant protection.

That being said, for windy sites, wood based mulch is recommended over the straw option.

A recent scientific study compared the retention of straw based mulch and wood based mulch. The results showed that the wood mulch blankets reduced runoff by 34% and straw based mulch 27%

Another relevant study that compared different treatments for post-wildfire hillslope erosion in southern British Columbia, Canada, established that wood strands have greater resistance to wind displacement as compared to agricultural straw. These findings showed that the agricultural straw mulch cover decreased by nearly 80% during the study period whereas the wood shreds only decreased by about 50%.

According to these results, some of the decrease in the cover area is probably due to:

Vegetation obscuring the treatment mulch during cover measurements.
Rapid decomposition of agricultural straw mulch
Wind redistribution of the straw mulch to a greater degree than the wood shred mulch

Last, the study showed that agricultural straw mulch remains visible for two years or less, while the wood mulches have remained in place for four or more years.

Weed control

In general, the use of hydromulch achieves effective weed control, which reduces the use of herbicides.

This is due to the reduction of sunlight reaching the soil, the suffocating effect, and the chemical characteristics of the mulch. Recent studies have shown that hydromulches containing pine bark and sawdust develop fewer weeds than straw mulches due to the phenols and tannins present in the bark of conifers.

With the increasing use of agricultural straw mulch as a post-fire hillslope treatment, some of the drawbacks have become apparent. These include possible hindrance of native vegetation regrowth, and weed contamination.

Then, using wood mulch ensures that there will be no weed seeds in the mixture, which can happen in straw mulch. The introduction of seeds of other plants is a collateral effect of straw mulch, which is a threat to the recovery of degraded soils.

Other studies show that application in forests may introduce invasive weeds and chemical residues from agricultural pesticides have been reported in composts and straw.

Comparison chart between wood based mulch and straw based mulch

Final Thoughts: Differences between straw based mulch and wood based mulch

Agricultural straw hydromulch is well established as an effective treatment for reducing post-fire sediment yields, but it has to be purchased and shipped to the area of use, may spread seeds of undesirable species, and may not provide adequate soil protection for the length of time needed.

Wood shred mulch shows promise for being as effective as agricultural straw mulch at reducing post-fire hillslope sediment yields, with the added benefit of being created from a native forest material on or near the site where the mulch will be applied.

The correct choice of mulch for the reclamation of degraded soils depends on factors such as the climatic conditions of the site, and the reclamation objectives. Thus, the choice of agricultural straw or wood shreds for a post-fire mulch treatment may be based on the performance characteristics that best fit the needs of the site.