Roadside Revegetation

PLANNING PHASE TWO: ASSESS SITE
5.3.4 ROOTING DEPTH
Rooting depth is the distance from the surface of the soil to the lowest point that roots can
penetrate. It encompasses any strata (e.g., topsoil, subsoil, and parent material) that can be
accessed by plant roots. The deeper the rooting depth of a disturbed site, the greater the total
available water storage and the higher the productivity of the site.
Rooting depth is affected by restrictive layers that block root penetration to lower strata (see
Section 5.7.2, Restrictive Layer). For example, the rooting depth of a post-construction site is
estimated at 6 feet deep. However, further investigation finds that there is a highly compacted
layer at 12 inches, which would limit most, if not all, root penetration below that point. The
rooting depth under these conditions has been reduced to only 1 foot of soil instead of 6
feet. Restrictive layers also include soils with very high or very low pH, toxic materials, or a
high water table.
Rooting depths vary by plant species and age of the vegetation. Most mature tree species
have deep root systems that access subsoil and parent material; roots of grasses and forbs
are predominantly limited to surface soils. Annual grasses and forbs require less rooting
depth than perennial grasses and forbs, with the roots of these species growing in the upper
surfaces of the soil. The age of the vegetation also determines the abundance and location of
roots. Newly established seedlings have shallow roots but, as the plants mature, root systems
expand to access moisture deeper in the soil.
Rooting patterns and root morphology play a role in how plants access soil water. Some
species have finer-textured root systems that access tightly held soil moisture; other species
have aggressive root systems that can penetrate deeply into cracks between rock fragments.
Grasses, for instance, have shallower root systems than trees and shrubs, but their small size
and high density in the surface soil gives them an advantage in shallow soils.
Rooting Depth – How to Assess
Rooting depth should be estimated from reference sites during planning and post-construction,
but it is not always an easy parameter to measure. Observing road cuts is often the best means
to determine rooting depth. Rock type (e.g., granite, sandstone, and schist), fracturing patterns,
rock weathering, and the degree of rock fracturing will provide an indication of rooting depth.
Observing the amount and type of roots in the fractures of existing road cuts will give a good
idea of rooting depth.
Fracturing and weathering of rock can also be determined from geotechnical analysis. If the
bedrock has been drilled, the drill log report can provide an indication of degree and depth of
rock fracturing and weathering. One way that rock quality is assessed is through a classification
called the Rock Quality Designation Index (RQD). This system rates the bedrock by how much
fracturing is observed in the cores. It is calculated by measuring the pieces of rock in the core
sample that are longer than 10 cm, summing the length of these pieces, and dividing by the
total length of the core (Deere and Deere 1988). A small RQD means that the bedrock is highly
fractured whereas a high RQD means the bedrock is massive. A RQD may be poor from an
engineering standpoint because of the high fractures, but favorable from a revegetation
standpoint because cracks will hold some moisture and allow root penetration. A RQD rated
as “very poor,” “poor,” and even “fair” should be somewhat favorable to root penetration.
Rooting depth is also affected by the presence of a restrictive layer caused either naturally
or by compaction. How to determine the presence of these layers is addressed in Section
5.3.3.1, Soil Structure – How to Assess, and Section 5.7.2.1, Restrictive Layer – How to Assess.
The literature contains many references to defining the depth of soil needed to support
different plant communities. For example, 18 inches of soil has been shown to support simple
grassland ecosystems, but more diverse native grassland communities are reported to require
up to 4 feet or more (Munshower 1994). These figures can be misleading if they are not put in
Roadside Revegetation: An Integrated Approach to Establishing Native Plants and Pollinator Habitat
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