3.4 Plant Selection

General Requirements
TxDOT is required to use regionally appropriate plants, both native and adapted, in highway ROW beautification projects.
  • Maintainability:
    Plants for the ROW must also be selected based on their anticipated maintenance needs and their adaptability to the roadside environment. The placement of plant material in the roadway is of critical importance because of its potential effect on driver safety. Plant placement will be discussed in a separate section.
    • Ornamental grasses as part of shrub or mass plantings may be unsuitable in most cases due to the extra maintenance required for cutting and removal of annual litter.
    • Select plants that are sufficiently hardy to maintain themselves without regular, supplemental irrigation once they have become established.
    • Trees will usually require weeding which can add significantly to the cost of maintenance of the project. The design should consider the alternative maintenance practices available and include estimates on the anticipated maintenance costs for this item.
    • Mass shrub plantings in the ROW are frequently used for erosion controls on slopes, filling areas that are difficult for mowers to access, and screening off site areas. Larger shrub varieties are preferred for these applications.
    • Beds and shrub plantings should be designed so that at the end of two complete growing seasons, the plants form a complete continuous canopy.
    • Low-growing evergreen shrubs are a better alternative for groundcovers for higher resistance to invasion by weeds and less need for specialized maintenance.
    • Avoid using plants that are known to attract and harbor damaging insects that are not easily controlled.
    • Do not use plants that are considered invasives in the regions of Texas.
    • Do not use plants requiring frequent pruning to look or perform well. No plant should be placed where pruning will be required in the future to maintain safe sight-distances.
  • Adaptability:
     Plant lists should be short and composed of species that have demonstrated an ability not just to survive but to thrive in the roadside environment.
    • Many ornamental plants that do well in residential or commercial settings do not perform well in the stressful conditions of the ROW. High winds, exhaust fumes, and intense sunlight and heat make establishment difficult for even the hardiest plants.
    • While some native plants are suitable for the roadside, the roadside is very different compared to native environments. Roadside soils are subject to extremes of heat and cold due to the absence of tall grasses or litter layers present in most native plant communities. Many native plants will be able to adapt over time to some of these harsh conditions, but the fact they are native does not indicate any less need for carefully planned establishment programs.
    • Some understory trees that are attractive in a forest setting will generally not perform as well in the exposed conditions of the roadside.
    • Slopes that assure well-drained conditions for roadbeds and structures lead to hot, dry soils in the summer.
  • Perceptibility:
     Plants in the roadside are generally viewed at high speeds and are often only part of the driver’s peripheral vision. The variety of color and textures of complex, multi-species plantings is not appreciated by the viewer as it would be in residential or commercial applications.

3.4.1 Plant Sizing

General Requirements
  • Shrubs
    should be minimum 3-gallon container.
  • Trees
    :
    • 1 ½” to 3” caliper recommended sizes;
    • Typically, 15-gallon minimum size. In “mass tree plantings,” 5-gallon minimum size is allowable; and
    • Trees with mature caliper of 4 inches or greater cannot be planted within clear zones.
  • Groundcovers and seasonal color
    : Groundcovers and seasonal color are discouraged within the ROW due to the high costs of maintenance and replacement. Zone I is the only zone where groundcover and annuals are allowed. Preferred minimum sizing to be 1 gallon. See
    Chapter 2
    for Zone I landscape materials.
Plant theft is an occasional problem, particularly if the project is near residential areas. In such cases, select plant sizes that discourage theft and avoid the use of small shrubs.

3.4.2 Planting Location

Mature size of plant material should be considered in determining its placement. Plant material within the ROW should not obstruct clear sight lines, unyielding structures, or create maintenance difficulties.
Figure 3-6
through
Figure 3-9
show different planting examples.
  • Clear zones:
     Plants may not be placed where they block the ‘unobstructed, traversable area beyond the edge of the highway’. Refer to the
    RDM
    for specific linear distances.
  • Unyielding structures:
     Plants must not obscure any unyielding structure within the clear zone (drainage culverts, etc.). Avoid landscape design situations that would require personnel and equipment to be on the driving lane side of guardrails and concrete barriers or on the shoulders of high-speed, main-lane traffic.
  • Signs:
     Plant material with the potential to block any portion of the sign face should not be placed in front of the sign.
  • Medians:
     Provide 3’ minimum clear setbacks along the median edge for maintenance safety. Plant material height at the median nose should be 30” height maximum. Verify exact heights with specific District.
  • Adjacent to walls:
     Concrete mow strips (minimum 12” width) should be installed immediately adjacent to the wall face. Plants should not be placed any closer to a wall structure than half the expected mature spread of the plant.
  • Elevated roadways:
     Plants should not be placed where foliage may intrude to within 10 feet of the travel lane of elevated roadways and bridges.
  • Utilities:
     Trees should not be located where the mature height or spread will interfere with utilities. Verify tree species with appropriate overhead utility agencies.
Median planting can add color and visual separation between driving lanes.
Figure 3-6: Median planting can add color and visual separation between driving lanes.
Plants are effective for visually softening tall retaining walls or noise walls.
Figure 3-7: Plants are effective for visually softening tall retaining walls or noise walls.
Vegetation with low maintenance properties is a good choice for screening.
Figure 3-8: Vegetation with low maintenance properties is a good choice for screening.
Street trees
Figure 3-9: Street trees

3.4.3 Routine Maintenance Activities

Maintenance activities of one sort or another are constantly taking place within the ROW. These include but are not limited to litter pickup, mowing, trimming, structure inspections or repair, sign repair, guardrail repair, and herbicide application. Landscape development must be undertaken so that access is provided for normal maintenance operations. Improvements must avoid the creation of unsafe conditions for motorists or maintenance personnel.
Project designs should accommodate the physical requirements of the mowing and trimming equipment; safe access and egress to the site, slope, turning radii, and mower width should all be considered in design layout. Areas with linear designs and no obstacles lead to a smooth flow of movement and may be mowed quickly with larger equipment. Limited access with obstacles requires smaller equipment and is less efficient.
3.4.3.1 Weed Control
Cost effective, long term weed control in shrub plantings is dependent on the rapid development of healthy plants. Therefore, intensive and timely procedures should be specified for the early stages of the project, particularly the 90-day establishment period.
Refer to TxDOT’s
Roadside Vegetation Maintenance Manual
for recommended pre-emergent and post-emergent control of annual and perennial weeds
Bark mulches may be used on slopes less than 4:1. They are not recommended for slopes greater than 4:1 because they are easily dislodged and will migrate to the bottom of the slope. TxDOT normally specifies mulch at 3” depth minimum.
3.4.3.2 Plant Bed Edging
Plant bed edging should be installed around beds to prevent grass and weed invasion. Bed configurations should be designed and specified with edge material sufficient to prevent weed invasion from adjacent vegetation (see
Figure 3-10
). Bed Edge Types are:
  • Concrete:
     minimum 12” width x 5” depth, reinforced concrete, thick enough to withstand the weight of mowing equipment.
  • Steel edging:
     not recommended. Use at District’s discretion.
Concrete Landscape Edging
Figure 3-10: Concrete Landscape Edging
3.4.3.3 Specialized Maintenance
  • Planting design along roadways with high-mast lights, signage, and other devices must consider the maintenance area needed for these items. Utility and maintenance vehicles must have a clear path of approach and a work radius around the area.
  • Anticipate the capabilities of the contractors responsible for executing maintenance contracts. In some cases, projects may be maintained by public agencies outside TxDOT. In these instances, the design should be tailored to the maintenance capabilities of the local government organization (LG) involved.

3.4.4 Roadside Pest Management Program

Texas is divided into ten vegetation regions (see
Figure 1-7
) and the State is known for its biodiversity, but at least nine species have disappeared completely. TxDOT, through the
Final Supplemental EIS, Roadside Pest Management Program
, has identified candidate, threatened, and endangered species, as well as vegetation considered to be a targeted species for removal. As part of the aesthetic design, best practices would include:
  • Inventory and identification of existing roadway vegetation species; and
  • Designation of those areas to be preserved and possibly enhanced within the proposed design.

3.4.5 Xeriscape Planting Design Principles

Xeric planting refers to those materials that will thrive under low-water or no-water conditions. For aesthetic design within the roadway, multiple factors must be considered for vegetation to thrive:
  • Plan and design:
     Base the design on zones with different water requirements.
  • Soil improvement:
     Increase the amount of organic material within the soil profile.
  • Select appropriate plant material:
     Use plant material native or naturalized to the region.
  • Reduce grass/lawn areas.
  • Water conservation:
     Avoid overwatering by utilizing drip irrigation.
  • Use mulch:
     Cover the exposed soil to retain moisture.
  • Perform appropriate maintenance:
     Maintain mulch layers, avoid over-mowing and over-fertilizing.

3.4.6 Non-mow Areas

'Non-mow’ areas are defined as sections of the roadside landscape or TxDOT property that have been removed from regular mowing schedules and management practices. The ‘Non-mow’ designation is not naturalization, prairie replication, or habitat creation (
Section 3.4.7.6
). The following areas qualify for Non-mow status:
  • Sites that are beyond the clear recovery zone;
  • Difficult or dangerous to access, hidden from view, not affecting driver safety, and far removed from travel lanes; and
  • All potential Non-mow candidate sites should be evaluated, since removal of regular mowing can have deleterious effects, rather than positive. Invasive species may be allowed to proliferate within the roadway landscape and spread to adjacent properties.

3.4.7 Special Environmental Considerations

The use of the roadside for specialized environmental goals should be carefully considered to be sure that the safety, sustainability, and life-cycle costs of the project meet department goals and resources. The establishment of specialized environmental areas in the roadway will often entail specialized management techniques and scheduling that may require special specification and contracting procedures. These needs should be carefully considered in determining the appropriate use and design of these features. These areas of environmental focus are defined below.
3.4.7.1 Restoration
Restoring a site to the topographic shape, hydrologic function, and plant community that existed in historical times before disturbance by man.
This practice is expensive and requires detailed knowledge and constant management.
3.4.7.2 Urban Reforestation
For several decades TxDOT has encouraged urban reforestation on its state highway system. One example of this is through the
Green Ribbon Program
. Funds for the Green Ribbon Program, as described in
43 TAC §11.101
, are allocated for Districts with non-attainment counties to plant and establish trees that help mitigate the effects of air pollution.
3.4.7.3 Habitat Creation
This is the designing and managing of plant communities for use as habitat by birds, mammals, reptiles, or insects.
Habitat creation involves providing one or all of cover, food, or water to a targeted species and requires detailed planning and development funding.
Where general habitat for wildlife is a goal, the preservation of existing sites is preferable to the development of new habitat.
3.4.7.4 Naturalized Areas
The preservation or establishment of native plant communities either as an aesthetic program or as part of habitat creation.
Naturalization seeks to promote or re-introduce native plants to minimize maintenance or improve the aesthetics of the roadside.
This involves the seeding or planting of desirable plants and periodic management to assist in their survival, or it may focus on preserving threatened or endangered species.
Vegetation allowed to attain natural growth may be visually acceptable where it can be set back from travel lanes.
Figure 3-11: Vegetation allowed to attain natural growth may be visually acceptable where it can be set back from travel lanes.
3.4.7.5 Prairie Replication
Involves replicating critical habitat for pollinators, including monarch butterflies, and enhances the roadway conditions by allowing wildflowers to thrive.
Few prairie areas still exist within the roadway sections due to disturbance, but replication is possible on smaller scales with the right type of installation practices and management.
Once prairie areas are established, these are often less expensive to manage and require fewer resources to maintain as compared to traditional landscapes.
3.4.7.6 Monarch Butterfly & Pollinator Habitat
TxDOT is actively working to keep Texas Monarch Butterfly & pollinator habitats abundant, including the development and implementation of new ideas to expand habitats across Texas. Decreasing mowing and adopting mowing practices that utilize proper guidelines and timing in designated areas contribute to the success of this goal.
TxDOT has funded research to create new native plant species for specific seed mixes, while at the same time it has incorporated pollinator plant species into replanting seed mixes on its construction and maintenance projects.
Native, pollinator hosts and nectar plants are typically specified in planting projects.

3.4.8 Wildlife Crossings

Wildlife communities are typically identified within corridors of roadway development as part of TxDOT’s environmental clearance process for highway projects. Coordination with Division and District environmental specialists, along with familiarization with the related TxDOT Environmental Compliance Toolkit requirements, will guide the landscape architect in designing and planting areas identified for wildlife crossings. The proposed design should reduce wildlife-vehicle conflict, while encouraging wildlife to use designated corridors. Refer to the
RDM
, Chapter 24, Section 6 for additional information on Wildlife Crossings.
3.4.8.1 Plant Materials Selection
  • Use native plants to induce animals to utilize crossing structures; and
  • Avoid use of plant species identified as ‘highly palatable’ plants that may attract animals to cross the road for food and thus affect traffic safety.
3.4.8.2 Fencing