California Department of Transportation
 

Specifying Seed And Plant Species

Erosion Control Planting

This page provides a high-level overview of selecting seed and plant species for Erosion Control purposes.

1. Document Site Conditions

  • Soil Type(s)
  • Exposure
  • Climate/Microclimate(s)
  • Elevation/Growing Season
  • Geographic Location
  • Areas of Active Erosion
  • Trees, shrubs, groundcover, forbs, annuals, perennials and grasses flourishing in each microclimate
  • External (Resource Agency) and internal stakeholder requirements

2. Identify Goals and Objectives

The Role of Vegetation in Erosion Control

The primary goal of effective erosion control is twofold:

  1. Maintain water quality - keep soils in place.
  2. Increase Infiltration - reduce runoff.

Vegetation helps Caltrans meet Stormwater goals by protecting the soil surface from raindrop impact erosion and sheet flow erosion. As plants send roots deep into the soil and as organic material provided by plants restores the soil, infiltration increases and runoff is reduced.

Immediate or Long-Term Solution?

Keep the concept of succession in mind when selecting seed species. On a project site, the plant species most successful will change over time in response to competition from other plant species, changes in soil structure, and other factors. Many disturbed sites are initially "colonized" by annuals and grasses and later support a more diverse cover of perennials, woody shrubs and large trees. Because of the uncertainty of exactly which plants will thrive on a project site, many designers select a plant palette that provides both immediate cover (annuals and grasses) as well as long-term cover (perennials, woody shrubs and trees).

3. Select a Seed Mix

Based upon your site analysis, reference site visit, and identified project goals and objectives, assemble a seed species mix. A typical seed mix might contain the following:

  • A small amount of annual seed species for quick cover.
  • A variety of perennial grasses and forbs indigenous to the area - based upon reference site feedback.
  • Consider using a "nurse crop" such as Barley or Sterile Wheat Grass to provide immediate cover. Understand, however, that the use of a nurse crop species is a controversial topic in certain parts of the state. Contact your District Biologist for feedback here.
  • Forbs or other flowering species for habitat and aesthetics.

Seed Selection Tools

There are a number of excellent online tools available to help identify seed species that meet project success criteria and are well suited to a particular geographic/climatic region. These tools include the US Forest Service Ecological Sub region Map, and Caltrans TransPlant seed selection application.

US Forest Service Eco region Map

USFS Ecoregion MapCalifornia is a large, complex state that can be classified environmentally in a number of ways. For selecting vegetation, one of the best ways to classify the state is the eco region.

Defined as "a geographically distinct assemblages of natural communities and species," eco regions provide a way to "map" the state that captures its geology, physiography, vegetation, climate, soils, land use, wildlife, and hydrology.

Because eco regions are drawn based on environmental conditions, they are well suited for identifying appropriate seed mixtures. Click on the map image to launch a US Forest Service map delineating California Eco region Section and Subsection maps. Select seed species that fit within the plant communities identified for your project site.

TransPlant Seed Selection Application

TransPlant Seed Selection Application TransPlant is another very useful tool for identifying potential seed mixes. Built upon the same eco region classification system as the map above, TransPlant provides a designer with a list of potential seed mixtures, after being provided project District, County, Route and Postmile information.

Click the TransPlant graphic to launch the online Transplant application.

 

 

When all else fails

When all else fails, a designer can always default to seed mixtures that have proven successful in the past. Before relying on this approach, keep in mind that past use of a particular seed species does not ensure that it is necessarily the right plant for your project. Remember to evaluate your seed species with regard to previously identified project goals and objectives. With this caveat in mind, here is Caltrans Most Commonly Used Seed Species List.

Warning!

It is recommended to NOT specify seed species known to have a shelf life of less than six months. While specifying a short shelf life specie may initially make an environmental regulatory agency happy, specifying a seed specie unlikely to germinate is unlikely to meet the spirit of a permit requirement. A good general rule is to avoid specifying short shelf life species. A list of short shelf life species is available on this website.

4. Determine Seed Application Rate

After selecting a seed species mix, the next step is to determine an appropriate application rate for these seed species. In general, the goal here is to determine the appropriate number of mature plants per square foot, and then convert this number into pounds of seed/acre. Refer to “Commonly Used Seed Species” to convert from seeds/ft2 to lb/ac.

The information and exercise below should provide you with enough information to determine an appropriate seed application rate to fill out your erosion control specifications.

Definitions

Dormancy - natural protective mechanism to extend seed life

Germination - % of seed able to grow into normal plants when given favorable conditions. Specifying a higher minimum germination will typically increase seed costs. Specifying a lower minimum germination requirement may lower seed costs by result in increased weed cover. You may want to specify a higher minimum germination requirement for weed-sensitive projects.

Hard Seed - seed with external dormancy from a hard seed coat

Purity - % by weight of desired seed

Pure Live Seed (PLS) - quantity of live seed in a seed lot that will germinate. The remainder may be weed, debris, and non-viable seed. PLS = % germination x % purity/100

Viability = condition of the seed embryo being alive

Total viable seeds = germination + dormant + hard seed

Key Considerations in Determining An Application Rate

  • Primary consideration - the desired number of mature plants/ft2.
    • A commonly used application range is 80 – 100 seeds/ft2. Adjust this number as required by the mature plant size.
    • While 80 monkey flower per square foot may be desirable, 80 giant brush lupine per square foot will lead to vegetation establishment problems.
  • Seed species size & weight.
    • Remember, seed size and weight varies greatly by species.
    • 1 pound of Desert Bluebells = 2,000 seeds.
    • 1 pound of Monkey flower = 54,000,000 seeds.
  • Ease of germination for that species.
  • Seeding method.
    • Drill seeding requires half the application rate as hydroseeding or hand seeding.

Calculation - Example

We want a seed density of 100 seeds/ft2 for our site.  The seeds will be hydroseeded or hand seeded (same recommended application rate, 80 – 100 seeds/ft2).  Calculate the application rate (lb PLS/ac) for the seed mix listed in Table 1.

Given:

Total seed density = 100 seeds/ft2
1 acre = 43,560 ft2

Table 1 - Seed Mix Species and Seeding Density

Scientific Name

Desired seeding density
(seeds/ft2)

Average pure seed weight
(seeds/lb PLS)

Lotus purshianus 11 108,500
Nassella cernua 11 215,200
Bromus carinatus 23 72,600
Festuca rubra molate 22 391,800
Hordeum californicum 22 135,700
Leymus triticoides 11 153,000
Total 100  

Solution:

a. Equation:

lb PLS/ac =          Seed density (seeds/ft2) x 43,560 ft2/ac   
                              Avg pure seed weight (seeds/lb PLS)

b.  Calculations:

Lotus purshianus:       11 seeds/ft2 x 43,560 ft2/ac   =          4.4 lb PLS/ac
                                        108,500 seeds/lb PLS
Nassella cernua:         11 seeds/ft2 x 43,560 ft2/ac   =          2.2 lb PLS/ac
                                         215,200 seeds/lb PLS
Bromus carinatus:       23 seeds/ft2 x 43,560 ft2/ac   =          13.8 lb PLS/ac
                                         72,600 seeds/lb PLS

Repeat for the remaining species (see Table 2 for results).

Table 2 - Seed Application Rates
Scientific name Application rate
(lb PLS/ac)
Lotus purshianus 4.4
Nassella cernua 2.2
Bromus carinatus 13.8
Festuca rubra molate 2.4
Hordeum californicum 7.1
Leymus triticoides 3.1
Total 33.1

Total seed application rate in the Special Provision should be 33.1 lb PLS/ac.

5. Fill Out Erosion Control Specification

Use the seed species identified in Step 3 together with the application rate determined in Step 4 to complete the project Hydroseed, Straw or Drillseed specification.

 

Additional Seed Selection Guidance:

Noxious and Invasive Plant Species:

Before specifying plant species for a specific project site, it is important to be aware of which existing or porposed plant species may be considered noxious or invasive species. More information on this topic is available at the Noxious and Invasive Plant Specie web page.

External Links

Seed Selection Guidance

Seed Supplier Information

Updated 04-15-2014