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Specifying Seed and Plant Species

1. Document Existing Site Conditions

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

2. Identify Goals and Objectives

The primary goals of erosion control are to:

  1. Maintain water quality - source control - keep soil in place
  2. Not increase runoff quantity - maintain existing runoff volume through infiltration
  3. Maintain air quality - control dust by minimizing wind erosion

Vegetation helps Caltrans meet stormwater management goals by protecting the soil surface from raindrop impact erosion, sheet flow erosion, and wind erosion. As plant roots work their way into the soil, and as the organic material produced by plants restores the soil, infiltration increases and stormwater runoff is greatly reduced.

DON'T FORGET POLLINATING SPECIES

In selecting plant material to control erosion, designers should to try to maximize all potential environmental benefits of roadside planting. Because roadside planting is linear, adding pollinator friendly plants to the roadside can help restore transportation corridors for pollinators and other wildlife. For example, adding pollinator friendly plants to roadside erosion control mixes could increase the pollinator population in California by closing gaps in the corridors that connect butterfly breeding grounds along the coast with their winter homes in Arizona, Nevada, Oregon and Washington. A diverse seed mix that can address a variety of environmental problems (erosion control and pollinator propagation) is always preferable to a planting palette that only addresses a single environmental issue.

IMMEDIATE OR LONG-TERM SOLUTION?

Don't forget the concept of ecological succession when selecting seed species. The most successful plant species for a project site will change over time in response to competition from other 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 plants that will thrive on a project site, designers often select a plant palette to provide both immediate cover (annuals and grasses) and 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 may contain the following:

  • A small amount of annual seed species for quick cover
  • A variety of annual and perennial flowers, 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 on this matter

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.

Lists of "pollinator friendly" seed species are available online at:

TRANSPLANT Plant Selection Tool

TransPLANT is useful for identifying potential seed mixes. Built upon the U.S. Forest Service ecoregion classification system, TransPLANT provides a designer with a list of potential seed mixtures, based upon project District, County, Route and Post Mile information.

SHORT SHELF LIFE SEED WARNING!

It is recommended to NOT specify seed species known to have a shelf life of less than six months. Short shelf life species may be required via an environmental regulatory agency. However, species will not germinate and will not meet permit requirements as intended.

4. Determine Seed Application Rate

After selecting a seed species mix, the next step is to determine an appropriate application rate. In general, the goal 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.

PURE LIVE SEED CALCULATOR

This simple calculation will figure the amount of seed to plant 100% pure live seed.

You will need to know

  • % Pure Live Seed
  • % Germination

The information and exercise below should enable you 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 but 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 seed2s/ft. 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 33.0

Total seed application rate on the Erosion Control Legend should be 33.0 lb PLS/ac.

Additional Seed Selection Guidance

References

 

 

Updated: February 12, 2019

 

 

Contact Information

Physical Address:

California Department of Transportation
Division of Design
1120 N Street
Sacramento, CA 95814

Mailing Address:

California Department of Transportation
Division of Design
MS 28
P. O. Box 942874
Sacramento, CA 94271-0001


General Information:

Design Liaison Phone List

Division of Design Org Chart

 

 

 

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