Section 4: Landscape DesignAnchor: #i1007836
Landscape Design coordinates the Landscape and Aesthetics Assessment, Landscape and Aesthetics Master Plan, and other resources with basic landscape concepts to arrive at a working landscape and aesthetics design for a project. Landscape designers bring together ornamental aspects, functional needs, maintenance and sustainability, special environmental goals, relationships to structures, and other factors.
This section contains subsections on ornamental aspects of landscape design which include:
- The Aesthetics of Right-of-Way Vegetation
- Using Ornamental Grasses
- Non-mow Areas
- Restoration, Habitat Creation, and Naturalization
- Ornamental Landscape Planting Design Guidelines
- Plant Selection Criteria
- Plant Container Sizes
- Using Trees in the Roadway
- Using Shrubs in the Roadway
- Using Groundcovers and Seasonal Color Plants
- Plant Material Relationship to Structures
- Planning and Design for Landscape Maintenance
- Plant Irrigation
- Irrigation System Types
- Designing for Weed Control in Ornamental Landscape Plantings
- Design Guidelines for Weed Control in Tree Plantings
- Design Guidelines for Weed Control in Beds and New Shrub Plantings
The Aesthetics of Right-of-Way Vegetation
The vegetation within the right-of-way, grassy vegetation and wildflowers, is an important part of roadway aesthetics. Grassed rights-of-way in urban areas that are weedy and unkempt give the corridor a neglected appearance and this image may be transferred to the city. Consequently, mowed vegetation that is edged and free of litter will lend the roadway and the city a look of cleanliness and neatness. In most cases, the first step in improving the aesthetics of the roadway should be to bring the existing vegetation to an acceptable level of appearance.
Roadside vegetation is maintained to accomplish specific goals of sight-distance, clear view of obstructions, erosion control, and aesthetics. Consequently, design alternatives should be reviewed to be sure that minimum standards are met for each of these issues. Most roadways are kept mowed to a height that accomplishes the needs of these issues and meets with the public’s expectations for the appearance of the roadside. Public standards should be considered when developing any aesthetic programs that affect roadside vegetation to be sure the proposed enhancements meet with community acceptance.Anchor: #i1007992
Using Ornamental Grasses
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. Plants such as Pampasgrass have been used successfully in some instances where clean-up may be delayed for years at a time.
In the majority of cases grasses for the roadside shall conform as a minimum to the guidelines and specifications given in the TxDOT Standard Highways Specifications for Construction of Streets and Bridges and the Roadside Vegetation Management Manual.Anchor: #i1008010
Non-mow areas are portions of roadside that for reasons of safety, utility, or aesthetics are removed from regular mowing and other management practices such as weeding, pruning, or herbicide control for an indefinite period of time. Sites that are far removed from the travel lanes, are difficult or dangerous to access, are hidden from view, or do not affect drive safety are candidates for non-mow status (see Figure 2-4).
The designer should be aware that the removal of regular mowing might allow weeds previously held in check to proliferate and present an unkempt appearance. The non-mow option is not synonymous with “restoration,” “habitat creation,” or “naturalization.” Likewise, the adjacent land use must be considered. Mowing may be required to control invasive species that could proliferate on the right-of-way and invade the adjacent land. These conditions are most often encountered on rural rights-of-way.
Figure 2-4. Vegetation allowed to attain natural growth may be visually acceptable where it can be set back from travel lanes.Anchor: #i1008037
Restoration, Habitat Creation, and Naturalization
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. These areas of environmental focus may be defined as:
- 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.
- Habitat Creation – Designing and managing 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.
- 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 will usually involve 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. See Figure 2-4 for an example of a natural growth area.
Some portions of the right-of-way may be suitable as part of a re-naturalization project or to remove large areas from routine maintenance. These are usually large areas beyond the minimum distances from pavement edges that do not require regular maintenance and meet aesthetic and management goals. Most often these areas are found in large interchanges. In these projects, plant material that would not normally be appropriate for use in other roadside applications may be desirable as a part of urban reforesting programs, wildlife habitat, or storm water quality programs.
The establishment of naturalized areas in the roadway will often entail specialized management techniques and scheduling that may require special specifications and contracting procedures. These needs should be carefully considered in determining the appropriate use and design of these features.Anchor: #i1008074
Ornamental Landscape Planting Design Guidelines
Plants can be an important addition to the right-of-way as an aesthetic enhancement that blends highway structures with the surrounding environment. Figures 2-5 through 2-8 show examples of effective planting design.
The proper use of plants can help make roadways a positive element in the visual fabric of our cities. The function and management of transportation corridors place exacting demands on the elements within the roadway. These demands must be thoroughly identified and understood in each design situation. The use of plants in the right-of-way must always be considered in the context of the important role of safe, maintainable transportation corridors.
Figure 2-5. Median plantings can add color and visual separation between driving lanes.
Figure 2-6. Bedded plantings in islands can add interest and variety. Height restrictions on plants in these areas are critical.
Figure 2-7. Pedestrian-ways are often good locations for ornamental plantings.
Figure 2-8. Plants are effective for visually softening tall retaining walls or noise walls.Anchor: #i1008130
Plant Selection Criteria
Plants for the right-of-way must 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 (see Plant Material Relationship to Structures subsection).
Plants in the roadside are generally viewed at high speeds and are often only part of the driver’s peripheral vision. Consequently, 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. For ease of management, 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 right-of-way. 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. Understory trees such as Yaupon (Ilex decidua) that are attractive in a forest setting will generally not perform as well in the exposed conditions of the roadside. Additionally, slopes that assure well-drained conditions for roadbeds and structures lead to hot dry soils in the summer. 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.
Plant selection criteria deal with the following areas:
- water requirements
- adaptability to soils and climates
- appropriate size and shape of plants
- insect and disease resistance
- noxious or invasive plants
Water Requirements. TxDOT is mandated to adopt wise water-use techniques associated with landscape developments. Plants selected for use on the right-of-way must be sufficiently hardy to maintain themselves without regular, supplemental irrigation once they have become established. The goal of roadside landscape irrigation is to allow the plants to become established such that supplemental irrigation is no longer required. In most situations, irrigation systems that are three to five years old will not be repaired if damaged. System design should allow for scaling back the system to completely manual operation for plant replacement or during times of severe drought. (See Plant Irrigation.)
Adaptability to Soils and Climate: Plants must be adapted to the climate of the area and to the unique environment of the roadway. The roadway has been engineered to support a paved travel surface. Consequently, the soils are often re-consolidated and compacted substrate materials. These soils are usually droughty and often infertile. In determining the adaptability of a plant, consider also its preferred soil pH, drainage needs, and pollution tolerance. Plants sited close to swales or in low, poorly drained areas should be capable of thriving in wetter soils. Those planted on slopes or near the tops of embankments should be able to withstand drought and high wind conditions.
Appropriate Size and Shape of Plants. Plants selected should fit within their intended location without impairing safety or maintenance access when their mature size is attained. Plants must not obscure any unyielding structure within the 30-foot clear-zone.
Longevity. Plants should be long-lived for their plant type and purpose. In some cases this may be a native plant species but this is not a prerequisite to the consideration of a particular plant.
Insect and Disease Resistance. Avoid using plants that are known to attract and harbor damaging insects that are not easily controlled. Examples of plants to be avoided include (but are not limited to):
- euonymus (scale)
- photinia (aphids)
- non-native holly (aphids and scale)
Noxious or Invasive Plants. Do not use plants that are considered noxious or invasive. Examples of plants to be avoided include (but are not limited to):
- chinese tallow
Pruning. Plants requiring frequent pruning to look or perform well should not be used in the roadway. Plants such as wax myrtle, photinia, or sumac may be more appropriately used in naturalized areas where frequent maintenance is not intended and the plant is free to attain its full, mature size. No plant should be placed where pruning will be required in the future to maintain safe sight-distances.
Due to the open character of the roadway, plants are often exposed to high winds. Trees which are weak-wooded or that routinely generate excessive limb-fall such as pecan and mimosa can provide potential hazards to traffic or become projectiles during mowing operations. Plants that are susceptible to limb breakage should be avoided. Plants that produce large or popular fruits are not suitable for the roadside since these may entice pedestrians into the roadway or generate projectiles. Examples of plants to be avoided include but are not limited to:
- bois d’arc
- fruiting plums
Plant Container Sizes
Plant container sizes will vary according to the type of plant and to the species. Shrubs will usually be nominal sizes of one to five gallons. Trees should generally be installed in sizes large enough to guard against theft yet small enough to be handled without the use of heavy equipment. Generally, 20-30 gallon sizes meet these requirements in most situations. It is recommended that large trees (greater than 3 in (75 mm)) be used very sparingly if at all. Experience has shown that larger trees establish slower and are more susceptible to transplant shock than smaller specimens.
Refer to Item 192, Table 1A and 1B in the TxDOT Standard Specifications for Construction of Highways, Streets and Bridges for a listing of container dimensions and designations.Anchor: #i1008311
The potential for vandalism and theft of materials within the right-of-way is sometimes high. Projects that are most susceptible are those with portions of the site not visible from routine traffic or those which have structures like bridges or culverts that provide shelter in inclement weather. Irrigation systems near areas such as these should have no above ground parts and include lockable covers on valves to discourage vandalism by persons using the water for drinking or bathing.
Plant theft is also 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.Anchor: #i1008326
Using Trees in the Roadway
The size of trees at time of planting should be based on budget, visibility of the plant to mower operators, and susceptibility to theft. Generally, small tree sizes less than 2 inches caliper are more susceptible to being stolen, particularly when the installation occurs near a residential area. Large trees greater than 3 inches caliper are usually slower to establish in the right-of-way and often exhibit some dieback within the first two to three years. In most cases the middle range of sizes, 2 - 3 inches caliper is a reasonable compromise for ease of installation, establishment, visual impact, and costs.
In some cases, the use of large trees, greater than 3 inches caliper, may be desired for the immediate visual impact they provide. Other than this large trees should be used sparingly. History has shown that loss rates are higher with larger specimens, even container-grown materials, and the higher costs associated with purchase and installation are often impractical, given the amount of right-of-way being planted.
It is important to note that trees with mature caliper of 4 inches or greater cannot be planted within clear zone.
Trees will require weeding and this can add significantly to the cost of maintenance of the project. The design should consider the alternative maintenance practices available and make estimates on the anticipated maintenance costs for this item.Anchor: #i1008346
Using Shrubs in the Roadway
Massed shrub plantings are those shrubs that are planted in groups or rows and where each is planted into individual planting pits (no area soil tilling) with the intention of creating a continuous shrub cover over the planted area. This method is most frequently used for erosion control on slopes, filling areas that are difficult for mowers to access, and screening off-site areas (see Figure 2-9). The preferred species for these applications are the larger shrub varieties. Avoid shrubs with leggy growth habits (ex. Photinia) since these invite weed and grass growth that is difficult to access with mowing equipment. Species that carry their foliage close to the ground surface are preferred since this helps reduce weed growth near their trunk and reduces the need for hand weeding and the possibility of damage due to string trimmers or mowers.
Figure 2-9. Massed shrubs on slopes add visual interest in addition to improving mowing efficiency in tight corners.
Bed plantings are those areas of tilled, prepared soil planted with varying plant types, usually shrubs. Beds should always be contained within borders deep enough and wide enough to prevent grass and weed invasion by stolons or rhizomes (see Figure 2-10). A significant portion of a bed is generally open to weed invasion during the establishment period, necessitating frequent weeding activities either by hand weeding or with herbicides. Consequently, the maintenance costs for shrub beds can be a significant part of the overall maintenance budget.
Figure 2-10. Bedded plants in turf areas require substantial edge protection against invasive grasses.Anchor: #i1008385
Using Groundcovers and Seasonal Color Plants
Traditional groundcover plant species such as Asian jasmine, honeysuckle, Virginia creeper, English or Algerian ivy, and similar species are often not suited to the plant management goals of TxDOT due to their general lack of hardiness, specialized maintenance needs, and susceptibility to invasion by weeds. Low-growing evergreen shrubs are a better alternative for situations where groundcovers are desired. Refer to Figures 2-5 and 2-6 for examples of groundcover settings.
The use of seasonal, ornamental plants (i.e.: annual color) is discouraged within the right-of-way due to the high costs of maintenance and replacement. Such plantings should not be included in projects that will be under state maintenance contracts.Anchor: #i1008403
Plant Material Relationship to Structures
- Signs: No plants with the potential of blocking a sign should be placed in front of the face of any sign (see Figures 2-11 through 2-14).
- Retaining and Noise Walls: 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 (see Figures 2-15).
- Storm Water Structures: Trees placed near drainage structures may inhibit mowing equipment and lead to excessive hand maintenance. Keep trees at least 20 feet from headwalls and culverts to allow mower access.
Figure 2-11. Plants must not be placed where they may obstruct any sign.
Figure 2-12. Visibility within intersections is always a primary goal. Plant use in intersection areas must be limited to low-growing varieties.
Figure 2-13. Approach ramps require long, unobstructed sightlines. Do not place plants near merging lanes.
Figure 2-14. Select and maintain plants at intersections that provide open visibility in all directions.Anchor: #i1008482
Planning and Design for Landscape Maintenance
The suitability of any development for Texas roadways will finally be determined by its ability to be managed and maintained within the resource levels as determined by TxDOT. All proposals for development will include estimates of the extent, duration, and costs of maintenance into the future. (Estimating procedures are provided in the following chapters of this set of guidelines.)
Routine Maintenance Activities. Maintenance activities of one sort or another are constantly taking place within the right-of-way. 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. See Figures 2-16 and 2-17 for design considerations related to maintenance activities. Improvements must avoid the creation of unsafe conditions for motorists or maintenance personnel.
Figure 2-15. Trees should not be placed where their mature height or spread will interfere with utilities or encroach on travel lanes.
Figure 2-16. Allow ample room around drainage structures to facilitate maintenance.
Safety. Avoid 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. On frontage roads, allow a minimum of 3 feet clear space between the back of curb and any area to be maintained for maintenance personnel (and their equipment) to stand well clear of moving traffic.
Figure 2-17. Always provide paved setbacks near traffic lanes for the safety of maintenance crews.
Mowing. Mowing roadway vegetation is an important maintenance activity. Landscape development projects must take into consideration the functional requirements of the operation to insure that safety and efficiency are not impaired. The consideration of mowing operations within a corridor must also recognize the relationship between time and equipment. Mowing costs are calculated by the acre for each mowing event and can be significant over time. The rate per acre is a function of the estimated time required to mow an area that is determined by equipment size and obstructions present. Areas that have no or few impediments can be mowed with larger equipment, thereby reducing time and lowering costs. Areas where access is limited will necessitate using smaller equipment, take longer, and increase costs.
The width of the mowing equipment determines turning radius and maneuverability and is impacted by the spacing and arrangement of elements within the site. In general, the design should allow for flowing movements of equipment and avoid sharp turning operations. Avoid the use of isolated obstacles that would impede mowing operations. Consideration should be given to how the equipment will enter and exit the site and how and where turning operations will be required.
Figure 2-18. Trees on steep slopes can impose dangerous turning motions on mowers and often lead to damage of soil-holding grasses.
Weeding, Trimming, and Mowing. The most costly maintenance operations in landscape development projects are weeding, trimming, and mowing. When designing plantings for the roadway, careful consideration should be given to how these may minimize excess maintenance requirements while improving the overall maintenance efficiency of the roadside (see Figures 2-10 and 2-19).
Figure 2-19. Concrete paving in hard-to-reach areas reduces maintenance costs and improves roadway appearance.
Specialized Maintenance Activities. It is important that proposed landscape developments be manageable within the resource capabilities of TxDOT. This includes anticipating 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 municipality or civic organization involved.Anchor: #i1008576
Irrigation of landscape plants is generally recommended both to protect the monetary investment of the project and to help insure the plants’ healthy development. Turf irrigation is not appropriate for the right-of-way except in those situations where responsibility for the maintenance and operation is assumed by other entities.
The conditions of the roadside determine the type of irrigation system suitable for use. Windy conditions can blow sprayed water onto pavement surfaces, possibly creating slick conditions where the driver does not expect them. Therefore, drip or bubbler irrigation techniques are preferred over those that discharge water in the form of sprays. In addition, keeping water on the desired target aids in water conservation.Anchor: #i1008591
Irrigation System Types
Drip system components may consist of barbed emitters in polyethylene tubing or in-line emitter tubing. Spaghetti tubing from multi-nozzle emitter heads should not be used due to ease of damage and numerous parts involved. Bubbler irrigation systems discharge water as a low pressure flow or as streams. Either of these may be appropriate depending on the type of plants and their spacing.
Irrigation systems should be designed so that no parts of the system are above ground in order to prevent vandalism, unauthorized use and to minimize exposure to damage. Valves or controllers that cannot be located below ground should be secured in locked boxes. Brass valves or nozzles should not be used due to their historically high incidences of theft for their salvage value.
Large irrigation systems should consider the use of automatic, electronic controllers. The use of solar power is permissible where costs make electric service impractical. Where possible, equipment selection shall match that of the systems already present on the roadside to create an economy of maintenance. Small irrigation systems may be able to rely on manual operation but the costs and unpredictability of manual operation should be considered. Truck irrigation is an option in isolated situations but unpredictable operators and damage to the site by the vehicles are important issues.
All irrigation systems will require routine inspection and maintenance. This factor should be considered in the development of long-term maintenance cost estimates.Anchor: #i1008616
Designing for Weed Control in Ornamental Landscape Plantings
The most costly aspect of the management of ornamental plantings is weed control. The application of design techniques that limit weed invasion is the first step towards reducing the overall cost of the project and extending its life.
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.Anchor: #i1008631
Design Guidelines for Weed Control in Tree Plantings
Weeds at the bases of trees are unsightly and give the appearance of shoddy maintenance in addition to inhibiting the development of the tree. The following guidelines are recommended practices for controlling weeds in tree plantings.
Soil Amendments. The preferred backfill for new trees is the native soil excavated from the planting pit. Where native soils are rocky or filled with large clods, add compost or other specified material to fill voids in the backfill.
Soil amendments are best applied to the upper few inches of the backfilled pit to improve oxygen availability to the surface roots of the tree. Studies have shown that the addition of soil amendments creates a soil texture very different from the surrounding soils. This texture difference leads to a wicking-away effect on water applied to the plant, resulting in a water stress condition. If soil amendments are added to the backfill, irrigation schedules should be adjusted based on observation of soil moisture in the planting pit.
- Use soil tests to determine fertilizer needs of new tree plantings.
- Apply fertilizer to the surface or top two inches of the planting pit.
- Use a fertilizer drench once every 30 days during the 90-day establishment period.
Weed Control. For control of annual weeds in new plantings, include a granular pre-emergent to the surface of the planting pit prior to mulch installation. For control of perennial weeds, specify liquid post-emergent herbicide as needed and include removal of weed residue.
For control of annual weeds in post-construction or existing plantings, specify water- soluble or granular pre-emergent applied to surface of mulch layers. Follow with hand removal of weed residue.
For control of perennial weeds, specify liquid post-emergent as spot treatment as needed. Follow with hand-removal of weed residue.
Mulch. Pine 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. When using pine bark mulches specify a particle size of one-half to one inch, with an installed depth of two inches, unless otherwise shown on the plans.
Hardwood mulches are suitable for all slopes since they tend to “knit” together and resist erosion. Specify a particle size not longer than six inches at an installed depth of two inches, unless otherwise shown on the plans.
Irrigation. Conduct on-site field test to determine water infiltration rate to set minimum application per plant.Anchor: #i1008701
Design Guidelines for Weed Control in Beds and New Shrub Plantings
The goal of design for weed control in shrub beds or mass plantings is to quickly establish a dense cover of foliage so weeds are not able to effectively compete. Plantings should be designed so that at the end of two complete growing seasons, the plants form a complete, continuous canopy. This goal places strict requirements on the selection of plants for landscape projects. The requirements for shrubs or ground covers are:
- The plant should hold its foliage all year (evergreen); deciduous plants allow invasion by many annual weeds.
- The plant foliage must be dense enough to completely shade the ground surface.
- The plant should have a spreading growth habit rather than upright.
- The top of the mature foliage should be at least 12 in.
The following items highlight the design alternatives to be considered for controlling weeds in shrub plantings.
Bed Preparation. Specify a non-selective post-emergent herbicide to kill all vegetation and remove all existing sod to a depth of three inches. Specify weed-free replacement soil, or compost or other specified material.
Edgings. Beds should be edged with concrete to prevent weed invasion from adjacent vegetation. The edging should be a minimum of 12 inches wide and thick enough to withstand the weight of mowing equipment. Modular materials, such as concrete pavers and fieldstone have joints that allow weed invasion and should be used sparingly. Steel edging is not recommended because this material cannot hold up to mowing equipment.
Soil Amendments. Organic soil amendments such as bark, tree clippings, etc. are recommended for most bed plantings if they have been thoroughly composted and this should be specified clearly, refer to current compost specification. Uncomposted material such as fresh sawdust or tree trimmings should never be used.
Mulch. See mulch subsection for Design Guidelines for Weed Control in Tree Plantings.
Weed Barriers. Synthetic fabrics may be suitable for bed plantings but mulch on steep slopes could be easily dislodged by water runoff. All fabrics should be covered with appropriate mulch.
Weed Control. For control of annual weeds during 90-day establishment period apply granular pre-emergent to soil surface of planting pit prior to mulch installation.
For control of perennial weeds during 90-day establishment period apply liquid post-emergent spot treatment as needed. Follow with repeat application and specify follow-up for hand removal of weed residue.
Irrigation. Beds should be designed for permanent irrigation. Drip or bubbler systems are recommended (see Plant Irrigation) and watering rates should be set according to site conditions.