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Guideline S.4 Vegetation

Intent

To optimize the ecological function of project sites by restoring Minnesota’s native vegetation, protecting natural areas, conserving existing site features, and selecting vibrant and appropriate vegetation to ensure the optimum functioning of ecosystems and human uses on the project site.

Required Performance Criteria

Guidelines are required for New Construction and Major Renovation projects that include at least 1,000 square feet of land disturbance, or at least ten cubic yards or more of cut and fill of the site, excluding the land disturbance and cut and fill within or below the building footprint. Exterior scope related only to ADA improvements or utility connections may be excluded from contribution toward these amounts.

  1. New construction projects shall not select sites containing any of the following critical site characteristics:
    1. Prime farmland (as defined by the NRCS WSS).
    2. Farmland of state significance (as defined by the NRCS WSS).
    3. Former municipal, township, or county parkland.
    4. Former federally protected lands.
    5. Areas covered by a conservation easement.
  2. The following tree conditions shall be established:
      1. Minimum combined trunk areas of all trees (including deciduous and coniferous) as evaluated at 10-year estimated maturity post-construction and measured by calculating the trunk area at 4.5 feet above the ground (i.e., DBH) for the site’s tree plant community. Tree trunk area must follow the minimum guidelines below:
        1. Tallgrass aspen parkland and savanna: two square feet. of trunk area per acre minimum.
        2. Southern and southeastern hardwood deciduous: four square feet. of trunk area per acre minimum.
        3. Northern deciduous: three square feet of trunk area per acre minimum.
        4. Boreal conifers: four square feet of trunk area per acre minimum.
      2. Required minimum of tree diameter per acre as evaluated in DBH to trunk area at ten years maturity, and number of large, medium, and small trees needed to achieve this requirement. Note that the maximums shown below are suggested only. Landscape Architect shall determine appropriate maximums.
        Type of Tree Plant CommunitySq. Ft. (144 sq. in./1 sq. ft.) Trunk Area Per AcreNumber of Large Trees Only (0.66 sq. ft. ea/12-in. DBH)Number of Medium Trees Only (0.39 sq. ft. ea/7-in. DBH)Number of Small Trees Only (0.20 sq. ft. ea/3-in. DBH)
        Prairie0000
        Tallgrass Aspen Parkland and Prairie Savanna 2-43-65-1110-20
        South & Southeast Hardwood Deciduous4-76-1111-1820-35
        Northern Deciduous3-65-108-1615-30
        Boreal Coniferous4-76-1111-1820-35
        Type of tree plant community shall be established based on the DNR Ecological Provinces Map.

        1. Sample Ratios of Trunk Areas at DBH
          DBH InchesArea Square InchesArea Square Feet
          10"78 sq. in.0.55 sq. ft.
          15"177 sq. in.1.23 sq. ft.
          20"314 sq. in. 2.19 sq. ft.
          25"491 sq. in.3.41 sq. ft.
        2. Annual Tree Growth Diameter Increase in Inches DBH with trees planted at ~1-in. DBH/2.5-in. Caliper:
          Tree Type SizeAnnual Rate Increase in Inches of DBHDBH in Inches at Ten YearsArea of Trunk at Listed DBH in Square Inches at Ten YearsArea of Trunk at DBH in Square Feet at Ten Years
          Large>1"~11"95 sq. in.0.66 sq. ft.
          Medium>3/4"~8 1/2"57 sq. in.0.39 sq. ft.
          Small>1/2"~6"28 sq. in. 0.20 sq. ft.
      3. Adequate tree soil volumes shall be achieved. Where trees are surrounded by hard surfaces (e.g., sidewalks, patios, driveways, car parks, plazas, parking islands), suspended pavement techniques, structural soils, or other comparable methods such as larger tree openings to provide adequate rootable soil volumes shall be used. Minimum volume of rootable soil volume per tree shall follow the requirements below:
        1. Small trees (e.g., Serviceberry, amelanchier): 400 cubic feet
        2. Medium trees (e.g., Ironwood, ostyra): 800 cubic feet
        3. Large trees (e.g., Hackberry, celtis): 1,200 cubic feet
          If using structural soils, multiply the total soil volumes above by five (x5) to obtain equivalent volume of soil usable by the tree. If above soil volumes cannot be met, trees species requiring smaller soil volumes shall be selected. Where applicable, suspended pavement or comparable methods shall be utilized to allow tree roots under hard surfaces to access adjacent open space.
      4. For the minimum number of trees required on a site, adequate tree diversity shall be achieved to limit susceptibility of site to disease and increase ecological resilience: Tree genera of like form shall be planted in large single species tree plantings, such as allees or formal groupings. For example, Kentucky coffeetree (Gymnocladus), honey locust (Gleditsia), and black locust (Robinia) are similar in form, structure, and leaf texture but are three different genera not susceptible to the same pests and diseases. The following numbers of tree generas shall be achieved:
        1. Three genera on sites less than two
        2. Five genera or more on sites of two to five
        3. Seven genera or more on sites of five to ten
        4. Nine genera or more on sites of ten to 15 acres.
        5. Twelve genera or more on sites of 15 to 20 acres.
        6. Fifteen genera or more on sites of 20 to 40 acres.
        7. Eighteen genera or more on sites of 40 to 100 acres.
        8. Twenty genera or more for sites of greater than 100 acres.
      5. Tree planting requirements: At time of tree planting, the following criteria shall be met:
          1. All soil/mulch/media covering trunk flare removed.
          2. Point of stem/root union exposed at original ground surface elevation (see UMN Extension).
          3. Caliper size of all trees limited to 2.5 inches at planting.
          4. All stem girdling roots (SGR) on trees rejected or removed (see UMN Extension).
          5. Metal baskets and burlap removed from balled-and-burlapped (B&B) root balls to 12 inches below soil level.
          6. Trees shall not be planted deep in the planting hole to stabilize them.
          7. Mulch shall not be placed against tree trunk deeper than 1-inch deep; tree stem/root union shall be planted one to three inches higher than surrounding ground plane elevation.
          8. Newly planted trees shall be watered at a rate of 1.5 gallons per caliper inches per three calendar days from May through September, at the following intervals. Watering bags are recommended.
              1. Year 1: At least every three days.
              2. Year 2: At least weekly.
              3. Year 3: At least every two weeks.

            See soil section for acceptable drainage and bulk density rates for soils in planting areas.

          9. Crowns shall not be pruned at planting to balance root and crown volumes.
          10. Broken branches shall be pruned and removed to develop a single central leader. Codominant branches that exceed half of tree trunk diameter at branch/trunk attachment point shall be removed.
          11. Trees in containers that are rootbound shall be box cut (see UMN Extension).

        The following shall be evaluated and implemented if feasible:

        1. Contracting growing trees for orders exceeding 20 trees total.
        2. Growing bare root stock in Missouri gravel bed nursery for half the growing season prior to planting out to create a large vigorous tree root system (see UMN Extension).
        3. Using arborist’s wood chips, mulch to a depth of up to six inches deep, over tree root systems but not against trunks. If trees are unstable in their planting hole reject trees or require one year of tree staking.
        4. Grade all landscaped areas to slope towards tree plantings.
  3. The vegetation selected shall be subject to the following and selected in coordination with the animal and vegetation requirements listed under S.1 and animal habitat requirements listed under S.5:
    1. Existing, noninvasive, nonnative vegetation shall not be removed solely in order to achieve the amount of native vegetation required under S.1A.
    2. The selection of herbaceous plantings for prairies, wetlands, savannas, parklands, and forests shall use the methodology (steps 1–5) found in MnDOT’s Native Seed Mix Design for Roadsides (2014): www.dot.state.mn.us/environment/erosion/pdf/native-seed-mix-dm.pdf.
    3. The required strata are a ground layer less than 48 inches tall and a tree canopy greater than 78 inches
    4. Keystone species shall be selected according to the following restoration goals:
      1. Where savanna, south and southeastern hardwood deciduous, and northern deciduous are being restored, one of the selected trees in the cohort population shall be burr oak (Quercus macrocarpa).
      2. Where prairie and savanna are being restored, one of the selected trees in the cohort population shall be American hazelnut (Corylus Americana) or beaked hazelnut (Corylus cornuta).
      3. Where prairie, tallgrass aspen parkland, and savanna are being restored, two of the selected grasses in shall be selected from the following two cohorts (one from each):
        1. dry/xeric grasses: little bluestem (Schizachurium scoparium) and side oats gramma (Bouteloua curtipendula);
        2. medium/mesic grasses: big bluestem (Andropogon gerardii) and Indian grass (Sorgastrum nutans); wet/hydric grasses: switch grass (Panicum virgatum) and prairie cord grass (Spartina pectinata).
      4. Where south and southeastern deciduous hardwood and northern deciduous and boreal conifers are being restored, one of the selected trees in the cohort population shall be serviceberry (Amelanchier canadensis).
      5. Where boreal conifers are being restored, one of the selected trees in the cohort population shall be white pine (Pinus strobus).
    5. The coefficient of conservancy for all B3 sites with wetland hydrology at planting must be greater than four as calculated by the floristic quality assessment (FQA) method (per Wilhelm 1977). In areas adjacent to water bodies or wetlands or rivers or streams, a series of exploratory holes 12 inches deep shall be dug between April and November. If water appears and persists in the hole for more than two hours after excavation, an FQA must be performed.
    6. Invasive species on site shall be determined using the Invasive Species County Weed Guideline. If the site does contain invasive species, a mitigation and maintenance plan as defined by the Minnesota Department of Agriculture shall be created or implemented.
    7. All vegetation must be selected in accordance with the correct local USDA hardiness zones.
  4. Pollinator-friendly vegetation:
      1. Neonicotinoid-free sites: All project plantings must use a written chain of custody method to verify neonicotinoid-free claims. Reject plants that have been neonicotinoid treated, or that do not have a clear, verifiable chain of custody indicating that they are neonicotinoid-free. This requirement also applies to trees, shrubs, and vines.
      2. Site plantings shall be selected so that at least 50% by quantity of all trees, shrubs, groundcovers, vines, and herbaceous perennials are insect pollinated, and rich in pollen and/or nectar.
      3. Blooming pollinator plants shall be provided for all three seasons of blooms (spring, summer, fall), with at least two different species blooming during each season.
      4. Coincidently blooming pollinator plants shall be clustered in large groupings to reduce expended energy of insect pollinators.
      5. Abundant human and natural structural enhancements shall be provided for insect pollinators (e.g., dead tree snags, downed tree logs, sand baths, bee skeps, solitary beehives, mason bee houses, green roofs, and green walls).
      6. Cultivars with double and triple petal flowers that do not produce pollen or nectar shall be minimized.

    Note: This guideline has been developed in response to Minnesota Executive Order 16-07, which also outlines requirements of specific state agencies, departments, boards, and committees not listed here.

  1. Site shall be designed so that the entire site albedo is at least 0.25 as evaluated using the B3 Albedo Calculator.

Recommended Performance Criteria

  1. Achieve biomass target according to major ecosystem of site, as measured in biomass per area of vegetated site area, estimated at ten years post-occupancy according to the major ecosystem characterization from DNR:
      1. Prairie: 1.1 kg per square meter or 2.03 pounds per square yard.
      2. Tallgrass aspen parkland and savanna: 0.9 kg per square meter or 1.66 pounds per square yard.
      3. South and southeastern hardwood deciduous forest: 0.7 kg per square meter or 1.29 pounds per square yard.
      4. Northern deciduous and boreal conifer: 0.6 kg/per square meter or 1.11 pounds per square yard.

    These biomass targets do not apply to the restoration of specialized NPCs installed pursuant to S.3K: Atypical Soils.

  1. Use a diversity of native plants to express multiple design styles (do not exceed 500 of any single herbaceous perennial species or cultivar; 50 of any single shrub species or cultivar; ten of any single tree species or cultivar). DNR’s 49 designated terrestrial invasive plants shall not be
  2. Subject sites with existing NPCs shall be protected from development, and DNR regional ecologists consulted for restoration of existing native plant areas.
  3. Achieve an entire site albedo of at least 0.3, as evaluated using the B3 Albedo Calculator.

In the predesign and early design of the project, determine ideal spatial needs for existing or new development. Critical sites for preservation or restoration on the project site as defined by the B3 Guidelines should be identified, and the type of buildings and related infrastructure that will be required for the developed areas and their spatial requirements should be determined. After a site has been selected, create potential spatial footprints that preserve any critical sites identified during the project’s Predesign Phase (PD).

Select a site where the proposed building and infrastructure will have minimal disturbance on the existing vegetation and on the supporting soil and hydrologic conditions. Areas of vegetation or high-quality areas for restoration should be identified for protection or restoration during the design and construction process.

Through the design process, techniques should be used to minimize negative impacts on soil, water, and vegetation on the site and on adjacent sites that are to be preserved or restored. Develop details and specifications that support the use of native plantings, maintain existing biodiversity, and promote enhancement of site conditions per the B3 Guidelines.

A preconstruction meeting should be held to identify requirements for protection/preservation of vegetation during and after the construction process. Submittals should be monitored for compliance with plans and details. Bidders should be made aware of specific responsibilities for integrating the onsite vegetation management with connections to vegetation on adjacent sites. Existing plants and trees that will remain should be protected, and soil and water conditions maintained or improved to promote and improve vegetation growth.

An O&M manual should be created to protect and maintain onsite vegetation. The existing conditions of the vegetation should be documented, as should the reason the vegetation was preserved or enhanced, and its ability to function in its current capacity. The necessary enhancements needed to accommodate a different building type in the future should be noted, as well as what the enlargements or reductions in spatial area would be.

Pollinator-friendly plantings should be selected from the lists below for compliance with S.4H:

To calculate the site albedo: On the project site plan, measure all areas in square footage that are exposed to sun during noon on June 22nd and determine the albedo value of the area according to its color. For living plants, use expected growth in ten years after the project is completed for square footage.

Calculate the total proposed site albedo using Appendix S-4: Site Albedo Calculator. The total site albedo as determined by this calculation must be at least 0.3 (or 0.25 if pursuing S.4N). Submit a site plan with each site material and albedo values identified and the completed Appendix S-4.

Using https://www.pca.state.mn.us/water/floristic-quality-assessment and the available FQA calculator and instructions, ensure that the site achieves at least a score of 4.0.

Predesign

  • 4A: Identification of any critical site conditions.

Design:

  • 4B: Verification of intended compliance and preliminary calculations of tree trunk areas, tree soil volumes, tree soil diversity and preliminary specifications outlining tree planting methods.
  • 4C: Verification of intended compliance and preliminary calculations for all required vegetation conditions, including planting plan for each plant category identified in the Predesign (PD) matrix for genera, species, and structural diversity of planting guidelines, updated as needed.
  • 4D: Verification of preliminary selection of pollinator-friendly plantings.
  • 4E: Identification of major ecosystem and preliminary intended method of compliance of biomass target.
  • 4F (and S.4I if pursuing): Completed preliminary Appendix S-4 Albedo Calculator demonstrating anticipated design compliance with albedo limits.

Final Design:

  • 4B: Verification of language mandating compliance in construction documents for tree trunk areas, tree soil volumes, tree soil diversity, and specifications outlining tree planting methods.
  • 4C: Verification of compliance method in construction documents for all required vegetation conditions, including planting plan for each plant category identified in the predesign matrix for genera, species, and structural diversity of planting guidelines, updated from prior iterations.
  • 4D: Verification of selection of pollinator-friendly plantings and specifications prohibiting using neonicotinoid products during the establishment, maintenance, and operation of the site.
  • 4E: Identification of major ecosystem and construction documents requiring compliance with biomass target.
  • 4F (and S.4I if pursuing): Completed Appendix S-4 Albedo Calculator demonstrating compliance with albedo limits. A site plan showing the location and size of areas with different reflective characteristics and their assigned albedo values should be included.
  • 4G: Planting plan demonstrating diversity of native plant species.
  • 4H: Documentation of NPCs, including site plan and documentation of correspondence and implementation of recommendations of DNR Regional Ecologists.

Closeout:

  • 4B: Verification of language mandating compliance in construction documents and guidelines requirement compliance if trees are surrounded by hard surfaces. Final planting plan for each plant category identified in the final design matrix for genera, species, and structural diversity of planting that meet required genera diversity, updated as needed and verifying that substitutes were not implemented for more than 5% of trees, shrubs, vines, or perennials.

Appendix S-4 Site Albedo Calculator

Coffin, Barbara and Pfannmuller, Lee. 1988. Minnesota’s Endangered Flora and Fauna. Minnesota: University of Minnesota Press.

Dirr, Michael. 2009. Manual of Woody Plants. Illinois: Stipes Pub.

Hightshoe, Gary. 1988. Native Trees, Shrubs, and Vines for Urban and Rural America: A Planting Design Manual for Environmental Designer. New York: Van Nostrand Reinhold.

International Society of Arboriculture

MacDonagh, L. Peter, 2005. Minnesota Soil Bioengineering Handbook. Minnesota: MnDOT.

Minnesota Department of Agriculture, Invasive Species

MN DNR, Invasive Species

MacDonagh, L. Peter, Nathalie Shanstrom. MnDOT Native Seed Mix Design for Roadsides, May 2010

Smith, Welby. 2008. Trees and Shrubs of Minnesota: The Complete Guide to Species Identification. Minnesota: University of Minnesota Press.

Wilhelm, Gerould and Rericho, Laura. 2017. Flora of the Chicago Region: A Floristic and Ecological Synthesis. Indiana Academy of Science.

Sutton, Richard K., ed. et al. 2015. Green Roof Ecosystem. Springer International Publishing. (See Chapter 11, MacDonagh, Shanstrom.)

Urban, James. Up by Roots, International Society for Arboriculture 2008

Albedo:

Exterior surface reflectivity index, or the capacity of a surface to reflect back light as evaluated across a specified range of frequencies.

Biomass:

The standing dry weight of all vegetation, typically measured in kilograms per square meter.

Coefficient of Conservancy/Floristic Quality Assessment (FQA):

An ecological integrity evaluation tool for NPCs, pioneered by Wilhelm (1977) and later refined by Swink and Wilhelm (1979, 1994). Currently, all 50 states use FQA systems. All plants within a subject community are scored from 0–10, from most to least invasive (e.g., in Minnesota: common buckthorn Rhamnus cathartica = 0; pink and white ladyslipper orchid Cypripedium reginae = 10).

Cultivar or CV:

Named variety of a hybrid plant species e.g., Iris “Caesar’s Brother” is an Iris siberica crossed Iris sanguinea hybrid. CVs are generally more vulnerable to pest infestations and usually lack pollen or nectar in their flowers.

Ecosystem Provinces:

Major ecosystem zones with distinctive physical structures and groupings of plants (trees, shrubs, vines, herbaceous) with unique soil orders, precipitation, climate, and specific stochastic disturbance regimes, such as winds, fire, or floods.

Historical Context or European Presettlement:

Refers to native landcovers in Minnesota prior to 1840.

Tree Sizes:

Large, medium, and small refer to the ultimate mature dimensions of that tree species, not tree size at time of planting. For example, hybrid elm, burr oak, hackberry, American linden, white pine, etc. are considered large trees.

Tree Species:

Primary taxonomic classification, ranking below genus.

Tree Genera:

Second-level of scientific species classification, (e.g., maples: Acer, oak: Quercus, elms: Ulmus are all tree genera). Note that because multiple species (cultivars) may be of the same genera, if multiple species or cultivars belonging to a single genera are selected for a site, only a single genera is represented by those selections.

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