Houston Arboretum and Nature Center, Phase 1

Challenge

• Shift to a more sustainable savanna ecosystem that is more characteristic of coastal prairie landscapes instead of replanting/reforesting the arboretum in order to prevent tree mortality and the need to repeatedly replace trees.
• Sequester carbon through roots of herbaceous species within the prairie and savanna.
• Retain and absorb stormwater within HANC’s landscape instead of allowing it to run off into Buffalo Bayou to prevent flooding.
• Create and improve diverse habitats for pollinators, birds, mammals, and insects.
• Save on irrigation costs by drawing from stormwater retention ponds and using native plants.
• Control invasive species by utilizing goats and controlled burns to save on long-term environmental management costs.
• Provide environmental education for people of all ages and serve the 10,000 elementary school students who visit the grounds annually to participate in education programs about science and conservation.
• Support an increased number of mission-based educational programs focused on the site’s new resilience to droughts and floods as well as the provision of ecosystem services.
• Increase the number of annual volunteers who help with restoration efforts. 
• Improve equitable access to green space around the site.
• Promote active and passive recreation.

• The arboretum features 25 acres of prairie and savanna ecosystems with an average of 21.6 plant species per square meter that reflect the native ecosystems of the Houston area. These native plants are also able to withstand natural disasters such as drought, flooding, and freezing. Visitors can pass through the savanna ecosystems and reach out to touch the plants as they walk on narrow crushed decomposed granite paths.
• Newly planted native savanna and prairie species include sunflowers, liatris, bluestem grasses, coreopsis, gaillardia, and basket-flower. Plants on site support wildlife, with 52% of species providing fruit and seed sources, 29% serving as nectar sources, 58% providing larval host habitat, 9% supporting biological control, and 15% providing native bee nesting material or structure.
• 7 acres of woodland were restored through the removal of woody invasive species (including early successional trees) to expose the native herbaceous understory layer as well as the integration of new tree species.
• 8 acres of the ravine (which accommodates a tributary to Buffalo Bayou) were restored and stabilized to withstand flood events and reduce erosion. Target species such as graminoids and dwarf palmetto were increased in number, and jute netting was used to better establish plant communities and control erosion.
• The new trail system, including an expanded boardwalk of fiberglass decking and 2 large bridges, gives visitors the opportunity to re-engage with the ravine landscape. 2 bridges were replaced at significantly higher elevations at 2 ravine crossing locations to handle flow velocity and reduce flooding and channel erosion. All materials were selected and engineered to withstand heavy floods.
• 18,548 linear ft of trails give visitors access to areas of the site that were not accessible before the renovation, including the lowest areas of the ravine. This is an increase from the previously existing 7,857 linear ft of trails, but it also represents a reduction in the overall area of trails from 71,417 to 61,042 sf, the result of adjusting the width of the trails to more appropriate sizes for various areas and uses. Trail widths vary from 3 ft wide to the 20-ft-wide display walk which doubles as a fire lane. However, most trails are in the 4- to 6-ft range. Trails in the savanna prairie and around the parking loop are 3 ft wide to allow visitors to directly experience the landscape.
• 2.54-acre and 1.19-acre detention ponds were added as part of a new parking loop and arrival experience. These wetland ponds serve many functions, including the detention and reuse of stormwater for irrigation as well as providing habitat for insects, amphibians, and birds. A plug mix was used in wetland ponds because a seed mix would not have established with water constantly flushing through the system. Coir matting on the slopes holds plugs in place.
• Salvaged materials make up 5.11% of total materials used in order to reduce the need to manufacture and ship new products to the site.
• Invasive plants in the 11.75-acre planted area are controlled by “goat grazing” and controlled burns.

Converting areas of trees damaged by natural disasters to prairie/savanna ecosystems through seeding will save an estimated $2,832,000 in potential replacement costs and water savings over the next 15 years as compared to the original strategy of replanting the HANC with individual trees and lawn. The projected 15-year cost savings on plant materials and installation labor is $934,00 and cost savings on water is $1.9 million when comparing the prairie/savanna seeding and plugs to the original strategy of trees and lawn.

• During the design process (prior to construction), the site experienced floods during the Memorial Day Flood of 2015 and Hurricane Harvey in 2017, which damaged many existing bridges and washed out several of the trails. The design team quickly realized that the light structures with cable rail details that were originally proposed for ravine bridges and boardwalks would not be resilient against the torrents of water produced by regular flooding. The design team engaged a geofluvial morphologist to design a more stable watercourse and reorient and elevate the bridges at two ravine crossing locations. Boardwalks were redesigned with weathering steel and marine-grade fiberglass decking.
• Starting in 2015, the design team used test plots for the savanna seeding to understand the conditions needed for the plants to thrive and inform the development of specifications during design and documentation. This early pilot indicated that a large volume of new soil was not necessary, and that the existing soils with 1 in of amended compost were adequate. This saved a significant amount of money across the site and reduced the amount of imported materials required.
• The design team observed that seeding in the ravine—no matter how steep or gentle the slope—was ineffective without erosion control measures to keep seeds from washing away with strong rains. Jute mesh was used to stabilize the banks and plantings, and seeding was supplemented with plugs which grew more quickly and helped hold the slopes.
• The contractors did not clear the tanks that held the native seed mix before seeding. Due to this, the seed that was initially broadcast on the site was contaminated with turf grass seed. Invasive turf grasses took hold, requiring scraping and reseeding. This highlights the importance of requiring full cleanup before seeding to reduce the potential for invasive species to enter a site.

Ravine Bridges: Pioneer Bridges - Trailblazer
Decking: Aqua Grate fiberglass pedestrian grating by Fibergrate
Bike Racks: City of Houston donation
Trash and Recycling Receptacles: Home Depot
Compost Bins: Custom built
Light Poles: Structura
Light Fixtures, Uplights, and Light Bollards: Bega
Metal: Weathering Steel
Bridge Wood: Theory
Fence: McNichols quality square weave wire mesh
Cable: Keystone Steel and Wire Co.
Fence Hardware: Stay-Tuff Fence MFG Inc.
Filter Fabric: Ten Cate Nicolon

Project Team

Landscape Architects: Design Workshop, Inc. and Reed Hilderbrand Landscape Architects
Other Consultant: Lady Bird Johnson Wildflower Center, Department of Ecological Research & Design
Civil and Structural Engineering: Walter P Moore
Operations and Maintenance: ETM Associates, LLC
Forestry Management: Dr. W. Todd Watson, PhD, BCMA
Historical Research: SWCA Environmental Consultants
Architecture Design Architect (new administrative building): Lake | Flato
Civil Engineering: Frayre Engineering & Consulting, PLLC
Ravine Restoration and Fluvial Geomorphologist: HydroGeo Designs LLC
Irrigation and Soils Engineering: Jeffrey L. Bruce & Company, LLC
Electrical Engineer: E&C Engineers & Consultants Inc.
Architect of Record (maintenance facility and existing administrative building): Kendall/Heaton Associates, Inc.
Design Architect (renovation of existing administrative building): Leslie Elkins

Role of the Landscape Architect

The landscape architects, working in collaboration, administered comprehensive landscape architecture services for planning, design, and construction, collaborating closely with an extensive consultant team and leading coordination with the client and multiple agencies. Site design, grading, and planting design; paving and layout; and materials were completed and detailed by the landscape architects.