Keyhole Gardens - the key to fertility

What is a Keyhole Garden?

Keyhole gardens are small composting gardens designed and built to fertilize garden beds (usually for food production) through the direct assimilation of organic waste. They create a synergy between the composting of food scraps and organic biomass and the fertilization of the surrounding plants.

The structure generally takes on a circular shape—similar to the silhouette of a keyhole—with a central container that is permeable to the movement of soil organisms and in direct contact with the surrounding bed. It is in this container that organic waste is placed and where composting occurs, aided by the work of bacteria, fungi, and macroinvertebrates, such as earthworms. These organisms act as a bridge between the container and the plants growing around it.

Where did this idea come from?

This agricultural practice has been widely used for hundreds of years, particularly by communities in Sub-Saharan Africa, as a tool to address both water scarcity and nutrient-poor soils. Direct composting in the ground not only promotes the availability of essential nutrients for plant growth but also helps retain soil moisture.

However, with the rise of permaculture and regenerative agriculture as disciplines, keyhole gardens have gained visibility in other contexts and are now widely used across different regions and geographic settings. In arid or semi-arid climates, such as the Mediterranean region, this strategy becomes especially relevant, as it allows for the circular and efficient use of resources and the optimization of processes in naturally dry or nutrient-poor environments.

How was it implemented at CIÊNCIAS?

HortaFCUL joined forces with the Living Lab for Sustainability and SMARTLEAP.PT to develop the concept of keyhole gardens adapted to an urban context. There are currently two garden modules in operation, built and co-created with the community through two open workshops. Both prioritize the reuse and upcycling of obsolete materials, transforming waste—abundant in cities—into valuable resources. The mobile module (see diagram), designed by SMARTLEAP.PT , offers a unique and innovative design for urban regenerative agricultural systems, integrating both education and sustainability with material upcycling. It repurposes an old waste container from the Lisbon City Council (CML) and is divided into two layers:

• The upper layer consists of soil rich in organic and mineral matter, where the plants and the two keyhole containers are located;

• The lower layer, separated by a geotextile, functions as a water reservoir designed to capture and store rainwater, optimize irrigation, and recover water losses.

This reservoir - which is crucial to increase the resilience of this garden during drought periods — is connected to a pump powered by a photovoltaic panel. The pump, equipped with an automatic timer, delivers water to a small drip irrigation system on the surface of the container, ensuring that plants have access to water during the driest months.

The stationary module was built in partnership with the DOMUS project, which currently manages the Pollinators’ Garden on the rooftop of building C2. Integrated into this garden, the keyhole system was developed from a water cistern and follows the same principles as the first module, although it is technologically simpler.

What is expected from this project in the future?

First and foremost, the keyhole gardens at CIÊNCIAS ULisboa propose a simple and highly scalable approach to diverse urban contexts, with strong potential for replication in other institutions.

Being part of the CIÊNCIAS ecosystem, this initiative is naturally linked to research carried out at the Centre for Ecology, Evolution and Environmental Changes (CE3C), with the goal of generating new knowledge and further understanding the benefits and advantages of this solution.

In partnership with HortaFCUL—which manages this project along with four other areas on campus (Horta do C2, PermaLab, FCULresta, and Bioilhas)—these keyhole gardens are intended to serve as an additional learning tool at CIÊNCIAS, helping us better understand soil, its properties, and the synergies between its many components.

Coordination: HortaFCUL

Project members: António Vaz Pato, Prof. Cristina Cruz, David Avelar, Florian Ulm, François Oliveira, Inês Pereira, Rafael Cruz, Tiago Silva e Ana Cerveira

Curious about this project? Contact us at sustentabilidade@ciencias.ulisboa.pt with your questions, requests, or comments.