8.3 Plant Selection: Native vs. Non-Native, Turf vs. Woody, Cool vs. Warm Season

8.3 Plant Selection: Native vs. Non-Native, Turf vs. Woody, Cool vs. Warm Season

Quick Reference: Plant selection is the biological anchor of permanent erosion control. Revegetation seed mixes must be tailored to site-specific environmental conditions by balancing photosynthetic pathways: C3 cool-season grasses (optimal 60°F–75°F) provide early spring and autumn stabilization but enter summer heat dormancy, whereas C4 warm-season grasses (optimal 80°F–95°F) provide peak mid-summer growth and exceptional water-use efficiency. Sustainable revegetation relies on deep-rooted native perennials rather than aggressive introduced invasives, incorporates Rhizobium-inoculated legumes to supply biological nitrogen, and respects microclimatic drivers such as solar slope aspect and soil moisture regimes.

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Photosynthetic Physiology: Cool-Season (C3) vs. Warm-Season (C4) Dynamics

Grasses (family Poaceae) are the foundational plant group utilized in erosion control. However, grasses are divided into two fundamentally different physiological groups based on their carbon fixation pathways during photosynthesis: C3 (Cool-Season) and C4 (Warm-Season).

The C3 Photosynthetic Pathway (Cool-Season Grasses)

C3 plants utilize the classical Calvin-Benson photosynthetic cycle. In C3 leaves, atmospheric carbon dioxide ($CO_2$) is initially fixed directly by the enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) into a three-carbon compound (3-phosphoglycerate):

• Temperature Optimum: C3 grasses achieve maximum photosynthetic rates at moderate temperatures of 60°F to 75°F (15°C to 24°C), with active root growth occurring at soil temperatures as low as 40°F to 45°F (4°C to 7°C).
• The Photorespiration Penalty: RuBisCO has an unfortunate biochemical flaw: it possesses an affinity for oxygen ($O_2$) as well as $CO_2$. When ambient temperatures exceed 75°F to 80°F (24°C to 27°C), and particularly under water stress when leaf stomata partially close to conserve water, the internal concentration of $O_2$ rises relative to $CO_2$. RuBisCO begins binding $O_2$ instead of $CO_2$, initiating a wasteful metabolic pathway known as photorespiration. Photorespiration consumes cellular energy (ATP) without producing carbohydrates, wasting up to 30% to 40% of the plant's photosynthetic output. Consequently, C3 grasses experience a distinct summer slump or dormancy during July and August heat waves, ceasing active growth until cooler autumn temperatures return.
• Key Cool-Season Species: Tall Fescue (Schedonorus arundinaceus / Festuca arundinacea), Perennial Ryegrass (Lolium perenne), Kentucky Bluegrass (Poa pratensis), Fine Fescues (Creeping Red, Hard, Chewings), Smooth Brome (Bromus inermis), Western Wheatgrass (Pascopyrum smithii), Canada Wildrye (Elymus canadensis), Orchardgrass (Dactylis glomerata).

The C4 Photosynthetic Pathway (Warm-Season Grasses)

C4 plants have evolved specialized anatomical and biochemical adaptations known as Kranz anatomy and the Hatch-Slack pathway to completely circumvent photorespiration:

• Biochemical Spatial Separation: In C4 leaves, carbon fixation is separated between two distinct cell layers. In outer mesophyll cells, $CO_2$ is fixed by the enzyme PEP carboxylase (phosphoenolpyruvate carboxylase) into a four-carbon dicarboxylic acid (oxaloacetate or malate). Crucially, PEP carboxylase has an exceptionally high affinity for $CO_2$ and zero affinity for oxygen. The four-carbon acid is then pumped into tightly packed, impermeable inner bundle sheath cells surrounding the vascular bundle. Inside the bundle sheath cells, $CO_2$ is enzymatically cleaved off, creating a highly concentrated internal $CO_2$ atmosphere (up to 10 times atmospheric levels) that saturates RuBisCO, completely suppressing photorespiration.
• Temperature Optimum & Water-Use Efficiency: C4 grasses thrive in hot, arid conditions, achieving maximum photosynthetic and growth rates at 80°F to 95°F (27°C to 35°C). Because PEP carboxylase captures $CO_2$ so efficiently, C4 plants can keep their stomata smaller, resulting in a Water-Use Efficiency (WUE) nearly twice as high as C3 grasses (producing $2\text{ to }3\text{ times more}$ dry matter per pound of water transpired).
• Autumn Frost Dormancy: C4 grasses are acutely sensitive to low temperatures. They do not initiate growth in spring until soil temperatures reach 60°F to 65°F (15°C to 18°C), and they enter total dormancy, turning straw-brown, immediately following the first hard autumn frost.
• Key Warm-Season Species: Bermudagrass (Cynodon dactylon), Bahiagrass (Paspalum notatum), Buffalograss (Bouteloua dactyloides), Blue Grama (Bouteloua gracilis), Sideoats Grama (Bouteloua curtipendula), Switchgrass (Panicum virgatum), Big Bluestem (Andropogon gerardi), Little Bluestem (Schizachyrium scoparium), Indiangrass (Sorghastrum nutans), Weeping Lovegrass (Eragrostis curvula).

   Photosynthetic Growth Rate vs. Ambient Air Temperature

   Growth │
    Rate  │            ┌───────┐ (C3 Peak: 60°F - 75°F)
   High   │           /         \                     ┌─────────┐ (C4 Peak: 80°F - 95°F)
          │          /           \                   /           \
          │         /             \                 /             \
          │        /   C3 Grass    \  (C3 Slump)   /   C4 Grass    \
          │       /  (Cool-Season)  \             /  (Warm-Season)  \
   Low    │──────/                   \───────────/                   \──────
          └──────┬─────────┬──────────┬──────────┬──────────┬─────────┬─────
                40°F      55°F       70°F       80°F       95°F     105°F
                                 Air Temperature

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Native vs. Introduced Species & Invasiveness Prohibitions

Historically, transportation departments and civil engineers favored introduced (exotic) turfgrasses and fast-growing perennial legumes due to cheap seed availability and rapid early green-up. However, decades of ecological monitoring have forced a major paradigm shift toward native perennial species in modern CPESC specifications.

Limitations of Introduced Monocultures

• Shallow Root Systems: Popular introduced turf species like Kentucky bluegrass or perennial ryegrass concentrate over 90% of their root mass in the upper 4 to 8 inches (10 to 20 cm) of soil. While effective at stopping raindrop splash erosion on flat lawns, they lack the tensile shear strength to stabilize steep cut slopes against shallow translational sliding.
• High Maintenance Dependency: Introduced turf varieties demand continuous, costly inputs—routine synthetic fertilization, lime applications, and supplemental irrigation during drought—without which they thin out, exposing bare soil to erosion.

The Native Plant Advantage

Native grasses and forbs are genetically adapted to regional soils, rainfall cycles, and climatic extremes over evolutionary timescales:

• Deep, Fibrous Root Architecture: Native warm-season prairie grasses develop massive, deep root systems. Mature stands of Switchgrass (Panicum virgatum), Big Bluestem (Andropogon gerardi), and Indian Grass (Sorghastrum nutans) drive fibrous root networks to depths of 6 to 12+ feet (1.8 to 3.6 meters) into the subsoil. These deep roots physically anchor the soil mantle to the unweathered bedrock, intercept subterranean water tables, extract deep moisture during extreme droughts, and build high root tensile strengths (exceeding $15\text{ to }30\text{ MPa}$).
• Resource Independence: Once established (typically requiring 2 to 3 full growing seasons), native prairie and woodland seedings require zero supplemental fertilizer, no supplemental irrigation, and minimal mowing, delivering true permanent stabilization.

Prohibited Invasive Species

In earlier decades, aggressive exotic plants were widely planted along roadsides and mine reclamations for rapid cover. These species escaped construction corridors, aggressively out-competing native ecosystems and destroying biodiversity. Today, modern environmental regulations strictly prohibit the inclusion of invasive species in seed specifications:

• Kudzu (Pueraria montana): A catastrophic invader across the southeastern US that smothers native forests.
• Crownvetch (Securigera varia / Coronilla varia): Once widely planted on highway slopes; forms dense single-species carpets that suppress native plant regeneration, exhibits severe winter dieback that exposes soil to early spring erosion, and is now classified as an invasive noxious weed in dozens of states.
• Sericea Lespedeza (Lespedeza cuneata): An introduced Asian legume that produces high levels of allelopathic tannins, poisoning surrounding soils to prevent competing species from germinating; strictly banned in sensitive watersheds.

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Life History Strategies: Annual Pioneers vs. Persistent Perennials

An effective erosion control seeding specification is almost never a single plant species; it is an ecologically engineered seed mix combining complementary life history strategies:

Annual Species (Pioneer Protectors)

Annual plants complete their entire life cycle—germination, vegetative biomass growth, flowering, seed set, and senescent death—within a single growing season (or less than 12 months):

• Erosion Control Function: Act as rapid "pioneer" vegetative cover. Annuals typically germinate within 3 to 7 days, producing rapid horizontal canopy cover that shields bare mineral soil from raindrop kinetic energy while slower perennials establish underneath.
• Agronomic Hazard: Annuals possess shallow, weak root systems and die completely within a year. If an erosion control specification consists solely of annual grasses, the slope will become completely bare, dead, and vulnerable to catastrophic rill erosion the following season.

Perennial Species (Permanent Anchors)

Perennial plants persist year after year from dormant root crowns, rhizomes, stolons, or bulbs:

• Erosion Control Function: Provide the permanent, long-term stabilization required for NPDES permit termination. Perennials continually shed and regenerate root systems, continuously pumping organic carbon into the rhizosphere and cementing soil particles into stable aggregates.
• Establishment Challenge: Perennials allocate the vast majority of their early metabolic energy downward into root elongation rather than upward into leafy shoots. Initial top growth is slow (often taking 14 to 28+ days to germinate and several months to form a closed canopy), leaving the surface vulnerable without a companion cover.

The Balanced Seed Mix Formulation

A professionally designed erosion control mix balances these strategies by proportioning:

• 70% to 85% Perennial Grasses and Legumes (by seed count): The permanent biological anchor.
• 15% to 30% Fast-Germinating Annual Companion / Nurse Crops: Providing immediate shade, canopy, and surface erosion resistance without out-competing the perennial seedlings.

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Legumes in Seed Mixtures & Biological Nitrogen Fixation via Rhizobia

Legumes (family Fabaceae) are the ultimate agronomic partner in erosion control seed mixtures. Common revegetation legumes include White Clover (Trifolium repens), Red Clover (Trifolium pratense), Crimson Clover (Trifolium incarnatum), Birdsfoot Trefoil (Lotus corniculatus), Hairy Vetch (Vicia villosa), and Partridge Pea (Chamaecrista fasciculata).

Symbiotic Nitrogen Fixation Mechanics

Graded construction subsoils are notoriously deficient in nitrogen. While synthetic fertilizers can supply initial nitrogen, they quickly leach away. Legumes solve this problem by providing continuous, multi-year biological fertilization through a mutualistic symbiosis with specialized soil bacteria of the genus Rhizobium and Bradyrhizobium:

1. Infection and Nodulation: Plant roots exude chemical flavonoid signals that attract free-living rhizobia bacteria in the soil. The bacteria enter root hairs, initiating cortical cell division that develops into visible root nodules.
2. The Nitrogenase Enzyme: Within the anaerobic interior of these nodules, the bacteria utilize the complex iron-molybdenum enzyme nitrogenase to break the exceptionally strong triple chemical bond of atmospheric dinitrogen gas ($N \equiv N$), converting it into plant-available ammonia: $N_2 + 8H^+ + 8e^- + 16\text{ ATP} \xrightarrow{\text{Nitrogenase}} 2NH_3 + H_2 + 16\text{ ADP} + 16\text{ P}_i$
3. Nitrogen Sharing: The legume provides the bacteria with energy-rich sucrose derived from photosynthesis. In return, the plant assimilates the fixed ammonia into amino acids. As legume roots, leaves, and nodules slough off and decompose, they release 50 to 150+ pounds of pure nitrogen per acre annually into the surrounding soil, directly feeding companion grasses.

The Mandatory Inoculation Rule

Critical CPESC Specification: Rhizobia bacteria are strictly host-specific. The specific Rhizobium leguminosarum strain that nodulates clover cannot nodulate birdsfoot trefoil, alfalfa, or partridge pea! Graded subsoils and deep cuts contain zero viable native rhizobia populations. Therefore, all legume seeds must be inoculated with viable, species-specific Rhizobium bacteria immediately prior to seeding. Pre-inoculated seed must be verified for fresh expiration dates and stored in cool, dark environments. Inoculant exposed to intense heat ($> 90^\circ\text{F}$), direct UV sunlight, or mixed into a hydroseeder tank with high-salt chemical fertilizers for prolonged periods will suffer total bacterial mortality, preventing nodule formation.

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Site Microclimate, Topographic Aspect & Edaphic Adaptations

Plant selection must account for microclimatic variations created by local topography and grading:

Solar Slope Aspect (Orientation)

In the Northern Hemisphere, the azimuth orientation of a slope face dictates solar irradiance, soil temperature, and moisture evaporation:

• South- and West-Facing Slopes: Receive perpendicular, intense solar radiation throughout the hottest afternoon hours. Surface temperatures on dark, bare soil frequently reach 120°F to 140°F (49°C to 60°C), causing extreme vapor pressure deficits and rapid soil desiccation. Cool-season C3 turf species will quickly burn up and perish. South- and west-facing slopes mandate drought-hardy, heat-tolerant C4 warm-season grasses (e.g., buffalograss, sideoats grama, bermudagrass, switchgrass) and taprooted drought-resistant legumes.
• North- and East-Facing Slopes: Receive oblique, indirect sunlight and morning sun only. Soil temperatures remain substantially cooler, and moisture evaporation rates are low. These slopes are ideal for cool-season C3 grasses (tall fescue, fine fescues, Kentucky bluegrass) and shade-tolerant native woodland forbs.

Soil Moisture and Drainage Regimes

• Conveyance Channels and Swale Inverts: Experience intermittent high-velocity storm flows and prolonged saturation. Require flood-tolerant, sod-forming species with flexible stems and rhizomatous root mats (e.g., tall fescue, switchgrass, western wheatgrass, smooth brome).
• Arid Crests and Thin Cuts: Shallow, well-drained, drought-prone soils require bunchgrasses with massive vertical taproots and low moisture requirements (e.g., sideoats grama, blue grama, little bluestem).

Comprehensive Botanical Reference Guide

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Vegetation Strata, Leaf Habit and Root Architecture

Three further plant-biology distinctions are named in the CPESC body of knowledge and drive real specification decisions.

Overstory, Mid-Story and Understory

A mature plant community protects soil in layers, and each layer does a different job:

Restoration planting therefore builds all three strata, but stabilization sequencing always establishes the ground layer first: shrub and vine planting and tree planting are permanent-cover measures layered on top of a seeded and mulched surface, never a substitute for it.

Evergreen vs. Deciduous

• Evergreen species (conifers, broadleaf evergreens) hold foliage year-round, providing winter interception and wind screening when deciduous cover is dormant — valuable on north-facing slopes, along winter-exposed corridors, and for year-round visual and wind buffers.
• Deciduous species drop leaves annually, which builds a protective litter layer and organic matter but leaves the canopy bare through the wettest, most erosive season in many climates. Deciduous litter also acidifies and insulates the surface, which can slow spring green-up of an underplanted grass mix.

The practical mix on a restoration slope combines both: deciduous species for organic matter and rapid canopy development, evergreens for winter function.

Fibrous vs. Taproot Systems

The two are complementary, not competing. Fibrous roots stop surface erosion; taproots resist mass movement. A slope seed mix that is all fescue will hold the surface but do nothing for a shallow translational slide; a planting of only deep-rooted woody species leaves the surface bare for two growing seasons. Specify a grass-and-legume ground layer for immediate surface protection and deep-rooted woody or forb species where slope stability, not just surface cover, is the design objective.