Urban horticulture.

Abstract At the beginning of the 19th century Heinrich von Thunen developed a model on agricultural land usage. The model generated four concentric rings of agricultural activity with dairy and intensive farming being closest to the city centre. The use of a piece of land is affected by the physical potentialities of the soil, but mainly depends on the cost of transport to market. Dairy and intensive farming are closest to the city centre since vegetables, fruit, milk and other dairy products must get to the market quickly. This is one reason for the development of peri-urban vegetable farms in the 19th century that produced high value products with high perishability. Due to its intensive character and important role for feeding people, horticulture in and around the city is widespread and its development is growing in relation to increasing city sizes. Cities of the developing world are somewhat more advanced in the development of multifunctional urban horticulture, as described by de Bon et al. (2015), but very diverse new forms are emerging in Northern hemisphere countries. One challenge of urban production is to produce safe food in an urban, often-polluted environment. In this article, following a description of urban horticulture, some main components are discussed in relation to current concerns and evolutions: the diverse cropping systems and the business models associated with that form of horticulture; the provided ecosystem services; the evolution of technologies in relation with soil and product quality; the diversity of crops; economic values and organizational forms.