Roadside Revegetation

PLANNING PHASE TWO: ASSESS SITE
Mitigating for Road Drainage
Species Selection
In areas below culverts, soil moisture is typically higher than surrounding areas after rainstorms
or snow melt. These areas may be suitable for more moisture-sensitive plant species that
require increased soil moisture. When planted with plant species that support pollinators,
these sites will increase pollinator habitat.
Large Wood
Obstacles, such as large wood, can be placed at the base of culverts or perpendicular to the
slope to slow concentrated water and increase soil moisture in these areas. Large wood also
provides nesting habitat or shelter for a number of pollinator species.
Biotechnical Slope Protection
Gullies can form below culvert outlets and, for this reason, these sites are often armored with
rock. Moisture-loving vegetation, such as willows, sedges, and rushes, can be integrated into
the hardened surfaces, such as live silt fences, as shown in Figure 5-9 and as discussed in
Section 10.3.3, Installing Cuttings.
Water Harvesting
Road surfaces, shoulders, and to a lesser extent, cut and fill slopes are impermeable surfaces
that create runoff water during precipitation. Utilizing this water for plant growth, as shown
in Figure 5-5, is a form of water harvesting.
5.3 AVAILABLE WATER STORAGE AND ACCESSIBILITY
The previous section discussed how water enters the soil surface. This section describes how
water is stored in the soil and how soil water is accessed by roots. Where precipitation is
low or infrequent during the growing season, the amount of water a soil can hold between
rainstorms is important from a plant survival and growth standpoint.
The total available water-holding capacity (TAWHC) is the sum of all water stored in the soil
profile that is available to plant roots. The amount of water that a soil can store is primarily a
function of the following:
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Soil texture
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Rock fragments
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Soil structure
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Rooting depth
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Mycorrhizae
Figure 5-9 | Live silt fence
In gullies, draws, intermittent streams,
or below culvert outlets, live willow
stakes (see Section 10.3.3.2, Live Stakes)
are placed in rows, creating what is
referred to as a live silt fence, to slow
water velocities and catch sediment and
debris (Polster 1997). The stakes root and
establish into plants over time.
The amount of water a soil stores and how easily it is accessible by roots determines the types
of species and the amount of vegetative cover a site can support.
5.3.1 SOIL TEXTURE
Soils are composed of minerals of varying sizes, ranging from clay (the smallest) to stone (the
largest). Each mineral particle in a soil sample can be grouped into six categories depending on
its size: clay (8 in [20 cm]). The fine soil fraction is composed of a combination of sand, silt, and clay size
particles. The proportion of these size groups in a soil is called the soil texture.
Figure 5-10 shows 12 soil textural classes by their proportions of sand, silt, and clay as defined by
the U.S. Department of Agriculture classification system (Soil Survey Staff 1975). Two other soil
Roadside Revegetation: An Integrated Approach to Establishing Native Plants and Pollinator Habitat
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