11.4 Mulch and Rolled Erosion-Control Products

1. Mulching (12.3.5)

Definition, Purpose, and Material Types

Mulching is the application of a protective layer of organic residue—such as dry cereal straw, wood chips / shredded bark, pine needles, or a $2\text{- to }4\text{-inch}$ compost blanket—onto bare or newly seeded soil. Mulch serves five critical agronomic and erosion-control functions:

1. Absorbs raindrop splash kinetic energy to prevent soil detachment and surface crusting;
2. Increases surface hydraulic roughness to slow overland sheet flow;
3. Conserves soil moisture by reducing surface evaporation around germinating seeds;
4. Moderates soil surface temperature extremes (preventing winter frost heaving and summer heat scorching);
5. Suppresses weed competition (and adds organic matter as it decomposes).

Standard Application Rate, Thickness, and Anchoring Methods

• Application Rate & Ground Coverage: Dry cereal straw mulch is applied at a standard rate of $1.5 \text{ to } 2.0 \text{ tons per acre}$ ($3,000 \text{ to } 4,000 \text{ lb/acre}$, or approximately $70 \text{ to } 90 \text{ lb per } 1,000 \text{ ft}^2$—roughly $2 \text{ to } 3$ standard bales per $1,000 \text{ ft}^2$). This rate produces $75\% \text{ to } 90\%$ uniform ground surface cover at a loose depth of $1.5 \text{ to } 2.0 \text{ inches}$.
• The Over-Mulching Trap: Some bare soil specks ($10\% \text{ to } 25\%$) should remain faintly visible through properly spread straw so sunlight can reach germinating seedlings. If straw is applied too thickly ($> 3 \text{ inches}$) or left in dense, unspread bale clumps, it blocks sunlight, traps excessive dampness (promoting damping-off fungal rot), and physically suffocates emerging grass shoots!
• Mandatory Immediate Anchoring: Loose dry straw blows away in light wind and floats into storm drain inlets during rain. Immediately after blowing or hand-spreading, straw must be anchored using one of three methods:
  1. Mechanical Crimping (Straw Punching): A tractor pulls a heavy frame equipped with dull, notched coulter disks ("crimper") along the contour (perpendicular to slope flow), punching the middle of the straw stalks $2 \text{ to } 4 \text{ inches}$ deep into the soil every $8 \text{ to } 12 \text{ inches}$. The upright ends stick up like stiff "whiskers" that trap windblown dust and slow sheet flow. (Limited to friable, non-rocky soils on slopes $\le 3\text{H}:1\text{V}$.)
  2. Liquid Tackifier Application: Spraying an organic plant gum (guar or psyllium), starch, or copolymer emulsion over the straw to glue fibers together.
  3. Biodegradable / Photodegradable Netting: Stapling jute, coir, or netting over the straw on windy or steeper slopes.

2. Rolled Erosion Control Products — RECPs (12.3.6)

Rolled Erosion Control Products (RECPs) are prefabricated rolls of natural organic fibers and/or synthetic polymer filaments engineered to shield steep slopes ($\ge 3\text{H}:1\text{V}$) and concentrated-flow drainage channels where loose mulch would wash away. RECPs are divided into two major functional classes:

Temporary Degradable Erosion Control Blankets (ECBs) vs. Permanent Turf Reinforcement Mats (TRMs)

Six Mandatory RECP Installation & Inspection Rules (Part B Section 2 Photo Favorites)

1. Smooth Seedbed Preparation: Grade the slope or channel smooth and remove all rocks, dirt clods, sticks, roots, and debris $> 1.5 \text{ to } 2.0 \text{ inches}$ before unrolling. Protrusions lift the blanket off the soil.
2. Seed and Fertilize Before Unrolling ECBs: For standard Temporary Degradable ECBs, apply seed, lime, and fertilizer directly to the prepared soil before unrolling the blanket so seeds rest in moist contact with the soil underneath the protective fiber layer. (Note: Certain thick 3D open-weave TRMs are anchored to the bare subgrade first, then soil-filled with $0.5 \text{ to } 1.0 \text{ inch}$ of topsoil and seeded into the 3D web per manufacturer specifications.)
3. Top-of-Slope Anchor Trench ($6 \text{ in.} \times 6 \text{ in.}$): Excavate a minimum $6\text{-inch deep by }6\text{-inch wide}$ ($6 \text{ in.} \times 6 \text{ in.}$) anchor trench set back $2 \text{ to } 3 \text{ feet}$ behind the crest of the slope (or across the upstream head of a channel). Lay the top edge of the RECP into the bottom of the trench, drive a row of staples every $12 \text{ inches}$, backfill with soil, compact firmly, and fold the blanket back over the compacted trench and down the slope. Failure Mode: If a crew merely staples the top edge flat on the slope face without a trench, overland runoff dives underneath the leading edge and carves deep gullies completely hidden beneath the blanket!
4. Shingle-Lap Overlaps in the Direction of Water Flow: Whenever joining two rolls end-to-end down a slope or along a channel, always overlap the upslope (upstream) roll ON TOP OF the downslope (downstream) roll by $4 \text{ to } 6 \text{ inches}$—just like roof shingles—and secure with a staggered double row of staples. If the downslope roll is placed on top of the upslope roll ("reverse lapping"), flowing water catches the lip of the lower roll and dives underneath! Overlap adjacent side-by-side longitudinal edges by $2 \text{ to } 4 \text{ inches}$ and staple along the seam.
5. Loose Unrolling & Proper Staple Length/Pattern: Walk the roll gently down the slope so it drapes loosely into micro-depressions (never pull or stretch an RECP tight!). Drive $11\text{-gauge}$ steel wire U-staples or biodegradable stakes flush to the soil surface at the manufacturer's required density ($1.5 \text{ to } 3.0 \text{ staples/yd}^2$ on slopes; $3.0 \text{ to } 6.0 \text{ staples/yd}^2$ in channels). Use minimum $6\text{-inch}$ staples in firm cohesive clay/loam soils and $8\text{- to }12\text{-inch}$ staples in loose, sandy, or friable soils.
6. Diagnosing "Tenting" (Bridging): When an RECP is stretched tightly across depressions or laid over unremoved rocks, rills, or dirt clods, it hangs in the air above the ground ("tenting"). Tenting causes double failure: (a) runoff flows freely underneath the suspended blanket, scouring rills out of sight, and (b) the air gap shades and dries out germinating seedlings so grass stems cannot push up through the netting.