United States Department of Transportation - Federal Highway Administration FHWA HomeFeedback

How to Work With Native Plants - An ecological approach to native grassland vegetation

An often misunderstood management tool, restoration of native vegetation is likely to have a simplistic yet cost-effective interpretation on highway rights-of-way. Because the engineered roadsides, including slopes and holding ponds do not resemble the original soil profiles before construction; restoring, in the true sense of the word, is impossible. Yet we can aim for a simplified and functional version of what was there before disturbance. We can attain a semblance of an appropriate native plant community that fits the existing site and our need for safety. In most cases, the need for safety will limit us to an herbaceous seed mix of native grasses and wildflowers. Some basic steps are common in this process throughout the nation.

Step 1 - Site Analysis

a. Environment: Determine existing soil type, available moisture, slope aspect and angle. When specifying a seed and/or seedling list, match grasses and flowers to site conditions thoughtfully. Obviously the original soils are gone and the matching will be difficult.

b. Invasive plants: Especially note existing weeds on and off site that will compete with a new planting, and plan accordingly.

c. Context: Actvities on the other side of the ROW fence can affect your project. If the project is in a rural environment, note vegetation management on adjoining fields. If not controlled, those species can out compete your planting. Within the City Limits, note City parks, natural areas, and golf courses. A more natural design and materials will fit there. But within the formal developed city space, a formal design is appropriate to compliment our neighbors. Some horticultural plants, although traditionally used in these locations, have become invasive outside the city limits. Plants like Norway maple, Russian olive, and Tatarian honeysuckle are well-adapted disturbance plants and should be used as a last resort within the City, and never outside the City Limits.

Step 2 - Plant Species List

a. Refer to your state's map: First check your State's natural region map** to determine which plant communities are commonly adapted to your site.

b. Locate a comparable natural site: Visit the preserve. Find an inventory list of native grasses and forbs to be used as groundcover (such a list should be obtained from your Natural Heritage Program for that preserve). Observe the natural associations for clues.

c. Narrow a shopping list: Single out those that match the dry, mesic, or wet microclimates of the project site. More than one mix might be necessary. With plant species list in hand, check early for commercial availability. Aim for as much diversity as the project can afford. Every State has native grasses and forbs that will tolerate harsh roadside environments.

d. Remember native grasses: The result of planting native flowers only, is messy at best. Without their natural relationship with native grasses, the forbs compete with one another. The native grasses add fine texture, fall color, and a backdrop to show off the native wildflowers.

e. Write the specification as tightly as possible to get the result you want. Hold annual consultant and constractor workshops to familiarize bidders with your new methods.

Step 3 - Site Preparation****

a. Minimize disturbance: Disturb the site as little as possible after controlling invasive plants. Remember those invasives have likely added their seed to the soil seed bank. Any tillage will stir them to germinate. A simple method would be 1. mow and rake off dead vegetation, and 2. scruff soil with rake, harrow, but nothing deeper than 1/2 inch. Then plant.

b. Berm project topsoils: Project site soils will have been highly disturbed in the end. However, salvaging topsoils on site can be planned. Winnowing those soils along the edge of the project can be beneficial. These soil berms can prevent runoff better than silt fencing. Then these berms can be spread over the rights-of-way before seeding. This is less costly than importing soils later; especially when the seed bank of imported soils is unknown. Topping a project with weed-laden soils will not yield a success story.

c. Eradicate invasives: It has been proven that weeds that exist before a planting, will continue to plague the planting in the future. Before the bulldozers begin staging, control any noxious weeds found on the project. This small cost of time and effort will prevent the spread of weeds to other projects via construction equipment, as well as save money in roadside management later.

d. Prepare the planting bed: in the least disturbing way. This means minimal tillage, or shallow tillage at a maximum. Specialized seed drills , broadcasters and/or hydroseeders cause minimal disturbance.

Step 4 - Installation Basics

a. Research Seeding Rates: Native seed mixes are typically planted at the rate of 10-20 pounds per acre, usually the lower number. Seeding more will not insure success, but waste valuable seed instead.

b. Decide Seeding Method: Drill native grasses when possible to get appropriate seed-soil contact. By broadcasting forb seed over the top, seeded rows will not be so apparent. The broadcasted seed should be raked, rolled, and/or mulched in. (In some regions, or 1:1 slopes, hydromulching is preferred.) Water only if seasonal rains do not occur. Fertilization or compost only encourage weeds.

c. Accept variable Timing: Many projects do not allow us traditional optimum seeding times. The agricultural approach of spring and fall seedings has been blamed for many failures. Current observations find that most native seedings are successful when planted spring, summer, or fall. Apparently, the native seeds are adapted to germination when conditions match their needs.

Step 5 - Follow-Up
6 to 8 weeks after germination, mow, where possible, at a height of 6-8 inches to discourage shade of early weeds. Only spot spray invasive plants that emerge. Keep the public informed through local media and/or signage. Maintenance crews should understand the project so as to avoid inadvertent mowing. Better yet, invite their input during project planning. If local volunteers are included in the project, let them help with public relations. With an informed public, no one should be questioned about their maintenance work. And no unnecessary mowings should occurÉonly the safety strip along the pavement for which you have planned. Don't forget to add this planting site to your GIS system for future tracking and management. Finally, succession or change over time, will occur. Plan for it!

Planting on the the Edge - Typical roadside scenarios

chart of woodland, wetland ad grasslandHighway corridors are often on the edge of woodlands, wetlands, or grasslands. Or they have created an edge to these plant communities. We can protect those natural areas and meet roadside safety needs by planting a transition zone. Since grasslands are early successional stages to each, maintaining native grasslands as transitions is a natural.

Below are sketches of a woodland, wetland, and grassland concept adjacent to the roadway with a grassland transition included. These grasslands pull duty as erosion control, revegetation and/or landscaping while allowing a safe recovery zone. Contact your State's Natural Heritage Program (experienced in local plant communities) or The Nature Conservancy (experienced in land management) to learn more about grasslands in your area.

WOODLAND EDGE - Here an herbaceous native seed mix can cover the entire site. This allows shrub and small tree seedlings to be planted or to migrate from the adjacent woods. Mowing once very 2-5 years will discourage encroachment by woodies into the clear zone.

WETLAND/ DITCH/HOLDING POND EDGES - A native grassland seed mix can cover all. However, a wet and wetter seed mixes with sedges might be used to border the existing wetland dependent on moisture availability. A shrub-car mix of shrubs and grasses could be added for esthetic and/or wildlife benefit.

GRASSLAND/ OPEN SPACE/ MOWED EDGES - Whether next to a meadow, pasture, or cropland, a native grass seed mix works as a transition here. You need a mix that will tolerate poor soils and full sun, similar to the conditions of a southern glade, shortgrass prairie, alpine meadow or desert grassland.




State Natural Vegetation Maps

map of Early Vegetation of Wisconsin**STATE NATURAL VEGETATION MAPS serve as references for native plantings. This maps are often based on original surveyor notes at the time of European settlement.

Granted, a lot of development and disturbance have followed. However, using a reference that suggests the kinds of plant associations that tolerate the weather patterns and geology of your project is just another ecological insight into chosing hardy plant materials. When designing with native plants, cold hardiness zones are meaningless. Natural region maps are commonly available from your State's Natural Heritage Program. They can also suggest preserves you can visit that serve as references for your planting. Let the natural environment give you some insights about what grows where and with what other species. Every State is different. Delaware has two natural regions, California has twenty-two, and Wisconsin has eleven, per the Kuchler description of native plant communities. New descriptions are being developed.

Sample seed mixes for a hypothetical Wisconsin right-of-way, an overlook, in West Central Wisconsin lies in what once was oak savanna with grassland understory. A small prairie remnant is near the site. The inventory list for the preserved prairie is available from the Natural Heritage Program. Three site-specific seed mixes will be needed: a. dry conditions on the sandy slope or forest edge to the right, b. mesic conditions around the overlook parking lot which is partially mowed, and c. wet conditions in the ditch between the slope and pavement edgeÉ..all zones in need of a grassy solution for safety and stabilization.

Basic to a successful seed mix is knowing the plant species (or accessing someone who does) and species' range of tolerances. One seed mix cannot possibly fit all occasions. Site-specific solutions are encouraged. In this case, for example, Canada Wild Rye would tolerate all three microclimates and serve as a cool-season or immediate cover crop. Little bluestem would survive the dry and the mesic sites but not the wet ditch. And so on. Without that kind of site-matching, our use of native species will be unsuccessful. Site-matching is key to successful establishment. Although the species list in each mix should be as diverse as possible, availability and cost can shorten the shopping list quickly. Aim for a minimum of 10-15 in the beginning.

Ultimately aim for Iowa model mixes of over 100 plant species! Here are sample mixes that should thrive on our pretend project. Although these species are relatively common and available, obtain them from growers close to the project when available.

Dry Seed Mixa. Dry Seed Mix (slope)
little bluestem
side oats grama
Canada wildrye
prairie phlox
blazing star
prairie coreopsis
smooth aster
birdsfoot violet
stiff goldenrod

Mesic Seed Mixb. Mesic Seed Mix (lot)
little bluestem
Canada wildrye
purple prairie clover
blazing star
yellow coneflower
heath aster
showy goldenrod

Wet Seed Mixc. Wet Seed Mix (ditch)
Canada wildrye
mountain mint
common oxeye
New England aster
blue vervain
golden Alexander

****Native Seeding Specification Tips:
1. Eradicate weeds from planting site before planting.
2. Consider a line item for contractor to control weeds and clean equipment.
3. Plant as much diversity as possible, unless an adjacent native seed source exists.
4. Match site microclimates with
distinct seed mixes as much as practical.
5. Most native species will establish more
easily, if you specify a locally-grown or collected source.
6. Order native seed when the contract is let to prevent unwanted substitutions.
7. Limit bids to experienced contractors and approved vendors for these projects.
8. Separate the planting contract from the general contract for best timing.
9. Extend the establishment period to three years and include patience.
10. Learn appropriate seed test criteria and seeding rates to avoid waste.

NOTE: The native wildflower mixes contain both native grasses and native flowers/forbs. They naturally grow together. Each mix has a variety of seasonal colors and textures to please the traveling public. The mixtures are all perennial and will return for repeat performances. While the cover crop performs erosion control, the other plants will slowly establish. Patience might be specified here. A minimum of three years is needed to approach the visual goal of the project.


Table of Contents | Previous | Next

This page last modified on August 4, 2004

FHWA Home | Feedback
United States Department of Transportation - Federal Highway Administration