Who designed Longwood Gardens' 2008 "Nature's Castles" treehouses?

Short answer: No single firm designed all three. The exhibit's three treehouses were designed and built by the two leading U.S. treehouse companies of the era, working with a Pennsylvania architecture firm as collaborating architect:

• The Canopy Cathedral and The Birdhouse — designed and built by TreeHouse Workshop, Inc. of Seattle, Washington, the firm of Pete (Peter) Nelson and Jake Jacobs. The Canopy Cathedral is an ornate two-story timber-frame structure inspired by a Norwegian stave church (589 sq ft interior plus a 118 sq ft balcony overlooking the Italian Water Garden); the Birdhouse is a 255 sq ft cedar cabin more than 20 ft up in a stand of tulip trees, its look inspired by steel forest fire-lookout towers.
• The Lookout Loft — designed and built by Forever Young Treehouses of Burlington, Vermont (the nonprofit founded by Bill Allen, whose mission is fully accessible treehouses). It is a rustic Adirondack-style, ADA-compliant structure with more than 75 feet of ramps leading to platforms up to ~14 ft off the ground, built with Vermont hop-hornbeam and ipe.
• Matthew Millan Architects (Pennsylvania) served as the collaborating/record architecture firm, working "with Longwood Gardens and the two premier treehouse companies in the U.S." on all three structures — their portfolio lists the Canopy Cathedral, Birdhouse, and Lookout Loft under "Nature's Castles: The Treehouses at Longwood Gardens."

Contemporaneous (2008) sources

1. Jane Holahan, "Treehouses get the Longwood Gardens treatment," Lancaster New Era / LancasterOnline, April 24, 2008 — published the week the exhibit opened (it debuted in conjunction with Arbor Day 2008). This article describes both the design concept and the construction process:
   • The Canopy Cathedral's design process began in February 2007, when the Treehouse Workshop designer came to Longwood and scouted the Italian Water Garden site during a snowstorm.
   • The Canopy Cathedral sits on 37 steel columns, 34 steel beams and 8 steel braces; its Douglas-fir timbers were milled from logs salvaged from a 2002 Oregon wildfire and from timbers dismantled from an old toothpaste factory in Toronto.
   • The Birdhouse reused wood from an old dairy barn and a chocolate factory, including a barn door with a rhododendron-branch handle.
   • The Lookout Loft, by Forever Young Treehouses of Burlington, VT, used Vermont hop-hornbeam and ipe shaped like tree limbs with wrought iron, and its 75+ feet of ramps made it wheelchair-accessible.
   • URL: https://lancasteronline.com/features/entertainment/treehouses-get-the-longwood-gardens-treatment/article_b9546a06-8269-50ca-bb93-dbd4c0feb118.html
2. The Washington Post, "Artisan Treehouses Extend Their Lease at Longwood Gardens," October 1–2, 2008 — a second contemporaneous piece, published when Longwood extended the exhibit's run; it likewise credits the artisan treehouse builders. URL: https://www.washingtonpost.com/wp-dyn/content/article/2008/10/01/AR2008100100715.html (the page now returns 403 to automated fetches; an Internet Archive snapshot exists at http://web.archive.org/web/20170608182235/http://www.washingtonpost.com:80/wp-dyn/content/article/2008/10/01/AR2008100100715.html).

Reported project facts from 2008 coverage: the three treehouses were built between roughly February and April 2008 (about four months of construction, with design work dating to early 2007) at a total cost of about $1 million, opening to the public as Nature's Castles: The Treehouse Reimagined around Arbor Day, April 2008.

Corroborating (non-2008) sources

• Millan Architects portfolio — "Nature's Castles: The Treehouses at Longwood Gardens" pages for the Canopy Cathedral, Birdhouse, and Lookout Loft, stating the firm "worked with Longwood Gardens and the two premier treehouse companies in the U.S.," and describing the Canopy Cathedral's stave-church-inspired timber frame with hand-adzed posts, reclaimed Douglas fir and oiled cedar planking: https://millanarchitects.com/portfolio/institutional/natures-castles-the-treehouses-at-longwood-gardens-the-canopy-cathedral/ (plus matching Birdhouse and Lookout Loft pages).
• Pennsylvania Center for the Book (Penn State), "Longwood Gardens: From Preservation to Paradise" — attributes the Canopy Cathedral and Birdhouse to "Peter Nelson and Jake Jacobs of TreeHouse Workshop in Seattle" and the Lookout Loft to Forever Young Treehouses: https://pabook.libraries.psu.edu/literary-cultural-heritage-map-pa/feature-articles/longwood-gardens-preservation-paradise
• Nelson Treehouse (successor firm to TreeHouse Workshop) has repeatedly confirmed on social media that "in 2008 the team built this treehouse at Longwood Gardens" (Canopy Cathedral): https://www.facebook.com/nelsontreehouse/posts/4197201250366207/

What remains uncertain

• Individual designer attribution within the firms: 2008 coverage credits the firms; the Penn State feature names Pete Nelson and Jake Jacobs for the two TreeHouse Workshop structures, and Forever Young's houses of this era were typically designed by James "B'fer" Roth with founder Bill Allen — but I could not find a 2008-dated source naming Roth specifically for the Lookout Loft, so the individual-designer credit for that structure should be treated as firm-level (Forever Young Treehouses).
• The Lancaster article does not name the individual "designer" who scouted the site in the February 2007 snowstorm; contextually this was a TreeHouse Workshop principal (Nelson or Jacobs), but the article itself leaves the person unnamed.
• The Washington Post article's full text could not be re-verified directly (403), though its title, outlet, and October 1, 2008 date are confirmed via search indexing and an archived snapshot.

Sources:

• Lancaster New Era / LancasterOnline — "Treehouses get the Longwood Gardens treatment" (Jane Holahan, April 24, 2008)
• The Washington Post — "Artisan Treehouses Extend Their Lease at Longwood Gardens" (Oct. 1, 2008)
• Millan Architects — Nature's Castles: The Canopy Cathedral
• Millan Architects — Nature's Castles: The Birdhouse
• Millan Architects — Nature's Castles: The Lookout Loft
• Pennsylvania Center for the Book — "Longwood Gardens: From Preservation to Paradise"
• Nelson Treehouse — Facebook post confirming the 2008 Longwood build

Answer

The designers were not a single architecture firm for all three structures:

• Canopy Cathedral was designed by Peter “Pete” Nelson of TreeHouse Workshop, Inc., Seattle.
• The Birdhouse and Lookout Loft were designed by Forever Young Treehouses, Burlington, Vermont.

Longwood’s own current garden history confirms that Nature’s Castles debuted in 2008 for Arbor Day, and describes Canopy Cathedral as an ornate two-story, 400-square-foot treehouse inspired by a Norwegian stave church, with an intricate glass facade and hand-carved creatures. It also notes the exhibit was originally intended to last only through Labor Day but remained because of its popularity. Longwood’s Forest Walk page separately describes the 2008 installation of Lookout Loft and Birdhouse, including the ADA-accessible Adirondack-style concept for Lookout Loft and the steel-beam, 20-foot-plus construction of Birdhouse.

A later interview with Longwood CEO Paul Redman gives the designer attribution explicitly: Peter Nelson / TreeHouse Workshop for Canopy Cathedral, and Forever Young Treehouses for Birdhouse and Lookout Loft.

The contemporaneous source to cite is Longwood Gardens’ own 2008 exhibition material: Nature’s Castles: The Treehouse Reimagined, Longwood Gardens, 2008. It is the original exhibition/press material for the Arbor Day 2008 installation and describes the design ideas and construction features summarized above.

Sources: Longwood Gardens, Large & Small Lake; Longwood Gardens, Forest Walk; blooloop interview with Longwood CEO Paul Redman.

Matthew Millan Architects (as architect of record, in collaboration with specialist treehouse designers) created the three treehouses for Longwood Gardens’ “Nature’s Castles: The Treehouse Reimagined” exhibition, which opened in 2008.[1][1]

The structures were built in collaboration with two leading treehouse firms:

• Forever Young Treehouses (Burlington, Vermont; co-founded by Bill Allen and Phil Trabulsy) designed and built the largest, Lookout Loft—a rustic, Adirondack-style, ADA-accessible treehouse with ramps (about 720–900 sq ft including platforms), sited in the Forest Walk area.[2][3]
• TreeHouse Workshop (Seattle; Peter Nelson and Jake Jacobs) designed and built the other two: the ornate Canopy Cathedral (two-story, ~400 sq ft interior + balcony, inspired by a Norwegian Stave Church, with glass façade, hand-carved details, and views of the Italian Water Garden) and the tall Birdhouse (~255–320 sq ft, cedar structure perched ~20 ft high on steel beams near the Peirce-du Pont House, offering bird’s-eye views and interpretive elements).[2][4][3]

A key contemporaneous source (2008 or earlier) is the article “Treehouses are looking up at Longwood Gardens” in the Delaware County Daily Times (Delco Times), published April 24, 2008.[2] It describes the exhibition (part of Arbor Day activities, running through November 23, 2008), names the designers and firms, details the design concepts (celebrating trees and their importance to Longwood’s history, specific stylistic inspirations like Adirondack style or Norwegian stave churches, accessibility, and distinct experiential opportunities), and covers the construction process and process sensitivities. Key points include:

• Use of reclaimed/recycled lumber with specific provenances (e.g., Douglas fir from a Toronto toothpaste factory, floorboards from a Washington dairy farm, rafters from a Vancouver warehouse, storm-felled Douglas fir windows).
• Environmental care: no root disturbance; structures supported independently (e.g., Lookout Loft on >1,300 lbs of steel with pin foundations driven through pre-cast concrete to avoid heave or tree damage).
• Build timeline (4 months) and cost ($1 million total); integration of trees through decks/roofs.
• Quotes and context on Longwood’s goals from horticulture head Sharon Loving and director Paul Redman.

A complementary 2008 source is “There’s Nothing Lovely as a Tree” in Delaware Today (August 11, 2008), which adds designer Jake Jacobs’ account of the initial snowy February site visit and sketching process, plus further accessibility and construction notes (e.g., 75 feet of ramps for the accessible structure).[3][3]

Other 2008 reporting (e.g., The Washington Post, October 1, 2008) confirms Matthew Millan’s role as architect of record ensuring code compliance alongside the specialist builders.[1] The treehouses remain permanent installations today, with two in the Forest Walk/Meadow district (Lookout Loft and Birdhouse) and Canopy Cathedral in the Lakes district.[5]

These details are grounded in contemporaneous 2008 journalism and align with later institutional descriptions from Longwood Gardens and the firms involved.

The three structures in Longwood Gardens' 2008 "Nature's Castles: The Treehouse Reimagined" exhibition were created through a collaboration between a primary architect of record and two specialist treehouse design-build firms.

Architectural Firms and Designers

• Matthew Millan / Matthew Millan Architects: Served as the architect of record for the exhibition. Millan collaborated with structural engineers and the builders to ensure the whimsical structures met strict commercial building codes for public safety, including engineering the platforms to hold "live loads" of 100 pounds per square foot.
• Forever Young Treehouses: This Burlington, Vermont-based nonprofit, alongside builder Bill Allen, designed and constructed the Lookout Loft. True to the organization's mission, the structure was built to be fully universally accessible (ADA-compliant) and featured two massive tulip trees extending directly through its roof.
• TreeHouse Workshop: This Seattle-based company, founded by Pete Nelson and Jake Jacob, designed and constructed the other two structures: The Birdhouse (conceived as a high perch resembling a bird box) and The Canopy Cathedral (an ornate, two-story pavilion inspired by traditional Norwegian wood-frame stave churches).

Contemporaneous Sources (2008)

Two highly detailed, contemporaneous newspaper reports from 2008 that extensively document the design intent and construction process are:

1. "At Longwood Gardens, fantastic treehouses grow" by Gretchen McKay, published in the Pittsburgh Post-Gazette on July 5, 2008.
2. "Artisan treehouses extend their lease at Longwood Gardens" by Adrian Higgins, published in The Washington Post on October 1, 2008 (which expands upon earlier July 5, 2008 Washington Post coverage of the exhibition's opening).

Design Concept and Construction Process

As detailed in the 2008 reporting, the creation of "Nature's Castles" relied on distinct environmental and architectural philosophies:

• Non-Invasive "Pin Foundations": A core requirement from Longwood Gardens was that the structures could not be fastened to or harm the host trees. Rather than using traditional bolts driven into the trunks, the builders utilized a specialized "pin foundation system." The treehouses were erected on elevated steel supports anchored to the ground using long pipes. This approach allowed the structures to be built entirely around the trees without touching them, preserving the root systems, bark, and limbs, and drastically reducing soil compaction.
• Reclaimed Materials: The designers heavily utilized salvaged and natural materials to execute the arboreal aesthetic. The Canopy Cathedral featured Douglas fir flooring re-milled from timbers salvaged from an old toothpaste factory in Canada, while The Birdhouse incorporated western red cedar logs sourced from southeast Alaska.
• Organic Integration: Built between February and April 2008, the treehouses required painstaking, puzzle-like carpentry. To successfully build around the living structures, builders had to measure and cut irregular pieces of wood to perfectly trace the natural curvature of the host trees, ensuring the pavilions felt completely organic to the surrounding Peirce's Woods.

In 2008, Longwood Gardens opened the "Nature's Castles: The Treehouse Reimagined" exhibit, which featured three treehouses created by two separate design firms:

1. TreeHouse Workshop Based in Seattle and founded by Pete Nelson and Jake Jacob, this firm designed and constructed two of the three treehouses:

• The Canopy Cathedral: A two-story, 589-square-foot treehouse inspired by a traditional Norwegian stave church, featuring a colorful glass facade and a balcony overlooking the Italian Water Garden.
• The Birdhouse: A 255-square-foot structure located 20 feet off the ground, offering a "squirrel's-eye view" and utilizing a copper cable as a lightning rod.

2. Forever Young Treehouses This nonprofit organization, founded by Bill Allen, designed and built the third treehouse:

• The Lookout Loft: A rustic-style, wheelchair-accessible treehouse featuring a large ramp leading up to the structure.

Design Concept and Construction Process According to contemporaneous reporting, the design process required immense planning to ensure the structures met building codes without harming the trees. Longwood Gardens insisted that the treehouses not be fastened to the trees or require a traditional concrete foundation. Instead, all three structures were supported by a "pin foundation"—sets of five-foot elevated steel pipes and poles resting on small concrete blocks. This method was specifically chosen to avoid disturbing the root systems and minimize soil compaction.

The construction process was described as the antithesis of standard homebuilding, requiring artisan carpenters to fit irregular pieces of wood together like a puzzle around the natural form of the trees. Bill Allen noted that a project was considered a success if it was a treehouse "the tree would have chosen."

Contemporaneous Sources (2008 or earlier):

• washingtonpost.com (October 2, 2008)
• thereporteronline.com (April 24, 2008)
• delcotimes.com (April 24, 2008)