Cultivando en la ciudad. La revolución de los huertos urbanos

Growing in the city has evolved from a marginal activity into an urban revolution with quantifiable impact on food security, public health and environmental sustainability. According to the FAO (2022), approximately 800 million people practice some form of urban and peri-urban agriculture worldwide, producing between 15% and 20% of the global food supply. The global urban agriculture market reached 267 billion USD in 2023 and is projected to reach 382 billion by 2028 (Allied Market Research, 2023). Urban gardens occupy an estimated area of 67 million hectares globally, equivalent to the territory of France. This revolution is not new: Victory Gardens during World War II produced 40% of the fruits and vegetables consumed in the United States with 20 million household gardens in 1944; German Schrebergärten, established in 1864, today total 1 million plots across 900,000 hectares; and the Aztec chinampas of Mexico City fed populations of up to 200,000 people from the fourteenth century onward.

The contemporary evolution of urban gardens shows an acceleration since 2008, driven by the global food crisis, environmental awareness and municipal resilience policies. In Europe, the cities with the largest urban garden area per inhabitant are Berlin (3.2 m²/capita), Vienna (2.8 m²/capita), Amsterdam (2.1 m²/capita) and Barcelona (0.4 m²/capita, with a program of 600 municipal plots). In Spain, the number of municipal urban gardens rose from 120 in 2006 to 630 in 2023 according to the Network of Cities for Agroecology, with a total area of 95 hectares and more than 40,000 plots managed by neighborhood communities, associations and educational centers. The COVID-19 pandemic accelerated the trend: Google searches for "huerto urbano" (urban garden) in Spain tripled between March and June 2020, and sales of seeds and seedlings for home gardening increased by 200% to 500% according to specialized distributors. The average waiting list for a municipal urban garden plot in Spanish cities of over 100,000 inhabitants exceeds 2 years.

Urban garden yields depend on the technique employed, the substrate, the climate and management intensity. Gardens on soil using agroecological techniques produce between 3 and 6 kg/m²·year of mixed vegetables in a Mediterranean climate, reaching 8 to 12 kg/m²·year with optimized crop associations and rotations. Raised bed gardens with improved substrate (a mix of 40% mature compost, 40% topsoil and 20% perlite) boost production to 10 to 15 kg/m²·year thanks to higher planting density and the absence of foot traffic compaction. Hydroponic techniques in urban greenhouses achieve yields of 30 to 60 kg/m²·year for lettuce and 40 to 80 kg/m²·year for tomato, multiplying soil-based cultivation productivity by 10. The company Gotham Greens (New York, founded in 2011) operates 50,000 m² of hydroponic greenhouses on urban rooftops producing 2,000 tonnes annually of leafy greens, with water consumption 95% lower than open-field farming and zero food miles for local consumers.

Agronomic techniques adapted to urban settings include container farming in 40 to 200 liter vessels (fabric pots, wooden crates, grow bags), vertical gardens with recycled felt pocket systems at densities of 20 to 40 plants/m² of wall area, and aquaponics combining fish farming (tilapia, trout) with hydroponic growing in a closed loop with 98% water efficiency. On-site composting of organic waste from the garden and community closes the nutrient cycle: a 300-liter domestic composter processes 200 to 400 kg/year of organic waste and produces 60 to 120 kg/year of compost with a nitrogen content of 1.5% to 2.5%, phosphorus of 0.5% to 1.5% and potassium of 0.5% to 1.0% (Haug, 1993). Vermicomposting with Eisenia fetida accelerates the process to 60-90 days and generates worm castings with a water retention capacity of 200% to 300% of their dry weight, ideal for container substrates on rooftops where weight is a limiting factor.

Urban gardens contribute to city sustainability through reduced emissions linked to food transport, improved urban biodiversity and stormwater management. The conventional food supply chain generates 0.5 to 2.5 kg of CO₂ per kg of food transported from field to urban consumer, considering an average distance of 1,500 to 5,000 km (Weber and Matthews, 2008). Food grown in urban gardens reduces these emissions by 50% to 95%, eliminating refrigerated transport, cold storage and post-harvest losses that reach 30% in the conventional chain. A 500 m² community garden in a Mediterranean city produces approximately 2,500 kg/year of vegetables, avoiding the emission of 1.2 to 3.5 tonnes of CO₂ equivalent annually by substituting imported produce. The urban garden network in Havana (Cuba), with more than 44,000 plots producing 70,000 tonnes/year of vegetables, demonstrates on a large scale the capacity of this revolution to feed significant urban populations.

In terms of biodiversity, urban gardens function as ecological corridors and pollinator refuges. A study by the University of Bristol (Baldock et al., 2019) documented a pollinator density in UK urban gardens of 50 to 250 individuals per transect of 100 meters, 30% higher than in conventional public parks and 60% higher than in ornamental residential gardens. Urban gardens harbor an average of 45 plant species, 25 bird species and 12 butterfly species per 200 m² plot in temperate climates. Stormwater management is another quantifiable benefit: a 100 m² garden with permeable soil and a 1,000-liter collection tank retains and infiltrates between 60% and 85% of annual precipitation, reducing pressure on urban sewer systems. In cities facing flood problems such as Copenhagen (which suffered 800 million EUR in damage from the July 2011 storm), urban gardens are integrated into water resilience plans as green retention infrastructure at an implementation cost of 15 to 40 EUR/m², compared to 200 to 500 EUR/m² for expanded conventional drainage systems.

Urban garden management models range from individual efforts on balconies and terraces to large-scale municipal infrastructure and commercial urban agriculture ventures. Community gardens managed by neighborhood associations are the most widespread model in Europe: plots of 25 to 75 m² allocated by lottery or waiting list, with annual fees of 30 to 150 EUR and regulations prohibiting synthetic pesticides and requiring organic cultivation. The cooperative model, such as Agrópolis in Barcelona (2012), manages 4 hectares of peri-urban garden with 120 members paying 80 EUR/month who receive a weekly basket of 6-8 kg of seasonal organic vegetables. Vertical indoor farming companies represent the most capital-intensive model: AeroFarms (Newark, USA) operates a 6,500 m² vertical farm with an investment of 100 million USD that produces 900 tonnes/year of leafy greens, equivalent to the output of 160 hectares of conventional farming, using 95% less water and no pesticides.

The social benefits of growing in the city are documented by more than 200 scientific studies published between 2010 and 2023. Community garden participants consume 40% more fruits and vegetables than the average for their neighborhood (Alaimo et al., 2008), exhibit cortisol (stress hormone) levels 20% to 30% lower after a 30-minute gardening session (Van den Berg and Custers, 2011), and report a 15% to 25% increase in social interactions with neighbors. In terms of inclusion, therapeutic urban gardens for people with mental health disorders have demonstrated reductions of 25% to 40% in anxiety and depression symptoms measured using the DASS-21 scale after 12-week programs (Soga et al., 2017). School gardens, with more than 7,000 operating in Spain according to the School Garden Network, increase vegetable consumption among participating children by 20% to 50% and improve academic results in natural sciences by 15 percentage points compared to control groups without gardens.