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RIYADH: Saudi cities are exploring water-wise tools to cut heat, clean air, and capture carbon on hardscapes where trees cannot easily grow.

Land-based moss — able to dry out and revive with just a trace of moisture — offers a low-maintenance, climate-active layer for facades and roofs that aligns with Vision 2030’s Quality of Life goals and the Circular Carbon Economy approach.

“Moss can switch off and on with moisture — exactly what harsh climates demand,” Joshua Van Alstine, co-founder of the ��������n Botanical Society, told Arab News.

He explained that when integrated into facades, rooftops, and green walls, moss can lower surface and ambient temperatures while passively trapping particulate matter and heavy metals, contributing to energy savings and cleaner air in dense districts.

Ahmed Al‑Jameel, an energy and climate policy expert, told Arab News: “Trees are the lungs; moss can be the skin — covering walls, bridges, and courtyards where trees can’t grow, adding a layer of cooling and carbon capture that complements tree-planting.”

Because it needs no soil and only micro-pulses of water, moss can transform heat-radiating concrete into climate-active skin without straining scarce water resources, said Al-Jameel.

“Earlier national checklists recorded around 119 moss species in ��������, but the most recent research in 2025 has confirmed 135 species,” Van Alstine noted.

He highlighted two natives as particularly suited for cities: Bryum argenteum, valued for its heat and pollution tolerance and reflective foliage, and Tortula atrovirens, which forms durable mats in exposed, arid habitats.

“A key opportunity in Saudi cities is air-conditioning condensate, a constant, free source of distilled water in summer,” said Al‑Jameel.

Redirecting that condensate to moss panels, pairing with shaded orientations and roughened substrates, and adding light night‑time misting can sustain growth with minimal additional water, he explained.

Van Alstine added: “Known as the ‘sidewalk moss,’ Bryum argenteum thrives in some of the harshest conditions — from rooftops to pavements — and is remarkably tolerant of heat, drought, and pollution.” 

Tortula atrovirens, with its wide distribution across the Kingdom and resilience, is also a reliable choice for shaded public areas where long service life is critical, he said.

Designers can improve moss establishment by prioritizing north- and east-facing walls, gentler in sunlight, while Al-Jameel noted: “Moss can also do well under shaded arcades, in courtyards, or on the undersides of bridges where direct solar exposure is limited.”

Spaces with moss mats act as natural cooling systems, reducing surface and ambient air temperatures and lowering energy demand for air conditioning, Van Alstine said.

Because mosses absorb moisture through their leaves, capillary wicking mats and water-retentive substrates such as felt or mineral wool with hydrogels can maintain thin moisture films without continuous irrigation, he explained.

“Moss is one of nature’s most resilient forms of greenery,” said Al‑Jameel.

Attachment improves on textured concrete or limestone; on smoother surfaces, roughening, applying mineral slurries, or installing pre-cultured mats can accelerate coverage, he added.

“Research has shown they are highly effective at capturing dust, particulate matter, and even heavy metals,” said Van Alstine.

In traffic corridors and construction zones, that dust-binding improves pedestrian-scale air quality and reduces resuspension, complementing thermal relief.

“This approach reflects the CCE principle of resource efficiency,” said Ahmed Al‑Jameel.

Using condensate and reclaimed water rather than new supplies aligns moss projects with Circular Carbon Economy and Saudi Green Initiative priorities, he said.

Among the options, Van Alstine said, Bryum argenteum stands out as the most promising for urban cooling thanks to its high reflectivity and rapid rehydration after dry spells, while Tortula atrovirens adds robustness across regions and building types.

“Moss is often described as a ‘carbon sponge,’ and while that’s true, it’s important to set realistic expectations,” said Al-Jameel.

He noted that sequestration per square meter is modest compared with trees, but the cumulative impact across walls, roofs, and underpasses can be meaningful — adding carbon capture where none exists today.

Native moss species, he added, can directly support ��������’s wider sustainability goals under Vision 2030.

Van Alstine said that micro carbon sinks such as moss mats capture measurable CO2 annually, and when combined with reduced building heat gain, they lower air-conditioning demand — one of the Kingdom’s largest electricity loads.

“The challenge is how to bring that success into Riyadh, Jeddah or Dammam, where summers are hotter,” said Al‑Jameel.

He suggested shaded orientations, light-colored backings, winter installation, quarterly rinsing with reclaimed water or condensate, and slightly angled panels to shed dust as ways to keep performance stable with simple maintenance.

Another innovation is pairing moss systems with treated greywater recycling, Van Alstine added. In regions like Asir and the Eastern Province, where fog and dew harvesting are being explored, passive collectors could feed moss panels with low-pressure trickles that mimic natural inputs.

All these solutions are low-tech and cost-effective, making moss especially attractive for municipalities and developers.

According to Al-Jameel, modular, lightweight panels are well suited for pilots on public buildings, transit corridors, campuses, and highway underpasses — with monitoring for temperature reduction, particulate capture, and energy savings.

“Another valuable species is Tortula atrovirens, which is the most widely distributed moss in ��������,” said Van Alstine.

Combined with Bryum argenteum, it broadens species choice for vertical greening while creating micro‑habitats that lift urban biodiversity on previously sterile surfaces.

Saudi cities present predictable but manageable hurdles for urban moss, the experts agreed.

Al-Jameel explained that moss can enter dormancy under heat stress, browning before reviving with moisture. Solutions include shaded orientations, reflective backings, and establishing during cooler months.

Another hurdle is dust deposition. “Dust storms can smother thin moss layers,” he said. “Quarterly rinsing with reclaimed or AC condensate water, slightly angled panels to shed dust, and selecting cushion-forming species improve resilience.”

On polished concrete, attachment is difficult. “Roughen surfaces or apply a thin mineral slurry; pre-cultured mats can accelerate establishment on facades and bridges,” Al-Jameel added.

These measures, he stressed, keep costs and complexity low, making moss suitable for municipal pilots and private retrofits.

“Under the Saudi Green Initiative, we don’t just need more trees; we also need smarter, water-efficient greenery. Moss can cool walls, bind dust, and improve street-level comfort with minimal irrigation,” Al-Jameel said.

He added that Saudi municipalities could accelerate adoption by piloting moss projects on shaded facades and public buildings. Embedding moss in green codes, awarding credits for condensate reuse and native species, offering expedited permits or fee reductions tied to SGI recognition, and partnering with universities to standardize substrates and plumbing details would all speed uptake.

For ��������, moss offers a dual climate solution — mitigating the urban heat island effect while cleaning the air — at low cost and with minimal water use.

Van Alstine concluded: “Starting in favorable microclimates and scaling with native species, capillary mats, and reclaimed moisture streams can deliver cooling, carbon capture, and biodiversity across the vast urban surfaces beyond the reach of trees.”