Ideally, when maintenance staff takes over a roadside revegetation project, the site is weed-resistant (Section 3.11.4). Maintaining a healthy native plant community thereafter greatly reduces the possibility for future weed invasion. The role of maintenance then, is to prevent or minimize unwanted vegetation from becoming established in a weed-free revegetation project. Prevention is the first line of defense in vegetation management and it is accomplished by maintaining a weed-resistant roadside environment, quickly treating disturbances, and protecting natural areas.
7.4.1 Maintaining a Weed-Resistant Roadside Environment
Roadside vegetation changes over time through successional processes and land uses. If the revegetation project is implemented successfully, the roadside vegetation should continue to be weed-resistant. That can change, however, if the vegetation is disturbed, creating bare soil where weeds can become established. Some practices to maintain a weed-resistant roadside environment are described below.
Minimizing Ground Disturbance
Maintaining a roadside free of ground disturbance is not always possible, but minimizing the amount of disturbance can reduce the area affected. Ground disturbances, and potential solutions, include the following:
- Rutting from vehicle run-offs (repairing roadsides soon after crashes)
- Mowing slopes that exceed 3(H):1(V) often cause rutting and erosion from equipment
- Mowing when soils are too wet, compacting vegetation and soil, and causing erosion
- Landslides (maintaining stable cut and fill slopes)
- Gullies and rills from road runoff (improving road drainage, soil structure, and groundcover)
- Ditch cleaning (limiting ditch cleaning or maintaining a groundcover with occasional mowing)
- Unauthorized trails and off-road vehicle disturbances (controlling access)
- Grazing (controlling animals)
- Vegetation maintenance (performing maintenance activities to prevent soil exposure)
Disposal of Soil in Designated Areas
Road maintenance often necessitates the disposal of soil that comes from maintaining the road. Material from landslides, ditch cleaning, and winter gravelling operations is sometimes pushed over the roadsides or deposited in areas along the roadside, potentially covering roadside vegetation with exposed soils. It is important that excavated soils be removed and deposited in designated areas that have been reviewed for their offsite effects on water quality and other resources.
Controlling Noxious Weeds
If there are no sources of weed seeds, then exposed soils revegetate from the seeds that are sown by the maintenance staff. It is important to reduce or eliminate unwanted seed sources by controlling noxious weeds prior to disturbances (Section 7.3).
Many weeds require full or partial sunlight to thrive (Penny and Neal 2003); therefore, retaining shade from existing native vegetation is one strategy to control some weed species. Cutting trees and shrubs or mowing vegetation can increase light and space for invasive weeds (Schooler and others 2010).
Mulching of Woody Material On Site
Maintenance activities that produce material from processed slash or excess vegetation from right-of-way clearing can be shredded and spread over the roadsides as mulch, especially on areas that have bare soils. Strategic placement of this material can reduce the potential for weed establishment.
7.4.2 Treating Disturbances for Quick Recovery
When ground disturbances do occur, a quick response by maintenance staff to treat these areas can ensure that unwanted vegetation does not become established, as described below.
Limiting or Controlling the Activity Causing the Disturbance
The first response is to assess the activity that caused the disturbance and to fix it before proceeding to revegetate the site. For example, if runoff from a road surface causes erosion on a fill slope, then it is important to fix the drainage before restoring the vegetation. If a large vehicle run-off creates rutting of the roadside, it is recommended that soil filling and regrading of the area occur prior to reseeding. If off-road vehicle use creates bare soil, then access to the area would be limited prior to revegetating the site.
Having an Appropriate Seed Mix Readily Available
Having an appropriate seed mix available prior to a disturbance is important because locating the appropriate seed mixes at the quantities needed can be difficult on short notice. Seed mixes may be available commercially or from the designer of the revegetation project. At a minimum, a list of appropriate plant species in a seed mix can be obtained from the revegetation plan or using the ERA tool. If a seed mix is maintained for these disturbances, keeping the mix in favorable long-term storage environments maintains its viability (Section 5.3.4, see Seed Storage). The disturbance may need to be stabilized until it is favorable to seed.
Sites that have been disturbed often have poor soils because the topsoil has been removed or mixed with the subsoil (e.g., landslide scars, gullies). Compost blankets are a quick way of covering bare soils and increasing the soil productivity at the same time (Section 5.2.3, see Seed Covering).
For the Designer
Seeding at excessive rates can benefit the project to a point. Understanding conditions that contribute to, or limit, seed establishment will help determine maximum seeding rates.
On disturbed sites, environmental conditions for seed germination are often poor for many native grass and forb species. Applying seeds at excessive rates increases the probability of greater plant establishment. It is important to understand the site's environmental conditions that facilitate seed germination and establishment, as all sites have a maximum seed load they are able to support. If seeding is conducted above this rate, a net gain may not be realized.
Applying a Weed-free Mulch
After seeds are sown on a disturbed area, applying a mulch over the seeds ensures that soil erosion is kept to a minimum and seeds have a favorable germination environment. Most seeds can germinate through mulches applied ½ to 1 inch thick on top of the seed. A variety of mulches are available that include wood fiber, hydromulch, straw, hay, erosion mats, and manufactured wood strands (Section 5.2.3). It is important to obtain weed-free mulches so the introduction of unwanted vegetation is kept to a minimum. Having weed-free sources available prior to disturbances expedites a quick recovery.
Using Clean Equipment
Inspecting and cleaning vehicles and equipment that will apply seeds and mulch ensures that unwanted vegetation is not brought onto the site. The equipment is typically pre-washed by the contractor at an approved off-site facility. Washing with high-powered, high-temperature water (steam cleaning) is effective. After cleaning, equipment can be inspected at the wash site or another agreed upon location prior to arriving at the project site. Overlooked areas to inspect are the hoppers and hoses of hydroseeding equipment.
Avoiding Nitrogen Fertilizer the First Year
Consider applying a slow-release fertilizer (instead of fast-release fertilizers) after native plants have established to reduce the nitrogen levels available for annual weed establishment (Section 3.11.4).