BOSK, Leeuwarden, Netherlands

A walking forest in the heart of Leeuwarden consisting of 1,200 trees planted in 800 containers this is, at its core, the BOSK art project. Over a period of 100 days, hundreds of volunteers will move the trees daily through the city along a 3.5-kilometre route. Both passers-by and participants are thereby made aware of the added value of greenery in a city. BOSK is essentially a temporary oasis and shows well the added value of greenery and the spatial choices you can make as a city. It balances on the cutting edge of mobility, liveability, culture and nature.

Leeuwarden's city centre has become very petrified over the years. By literally making trees move, visitors are made aware of the need for a sustainable living environment and the impact of greening in a playful and original way. By adding temporary greenery, a sharp contrast is set and this immediately makes clear what nature has to offer a city.

Smart containers with mud paint
The containers used are basically made of wood, this to match the trees in the most natural way possible. For optimal movement during the event, sturdiness and weight are also decisive factors. To prevent the trees from being blown over easily, an underground anchorage in the crate was chosen. Soil leaching is prevented by finishing the inside of the crates with capillary cloth. This has the positive feature of optimally distributing moisture and providing the soil with an even supply. Finally, it was decided to ‘paint’ the boxes with mud, made from Frisian clay. This creates a beautiful natural effect. For moving the boxes, carts were made which can be pushed manually and have a short turn to get the trees in the right position.

Climate-adaptive and biodiverse assortment
The choice of species for the trees used in the project was made in close consultation with the municipality of Leeuwarden, the Province of Friesland and tree nursery Ebben. A range was chosen that is heat-resistant (in view of the urban location) and easy to apply on the clay soils of Friesland, where the trees will be planted afterwards. Part native and part non-native were deliberately chosen. For optimal connection to local biodiversity, indigenous material, to be planted later in the outskirts of the municipality and province, such as elm (Ulmus laevis), alder (Alnus glutinosa) and hornbeam (Carpinus betulus), was used. For application in and around cities, a diverse selection of non-native planting material was chosen, as these trees can better cope with climatic extremes such as heat stress and peak showers, such as Gleditsia tricanthos, Ginkgo biloba and Zelkova serrata. In addition to trees, lower shrubs were used in the containers and the remaining space was sown with a wildflower mixture. The main criteria in choosing shrubs were rapid growth to provide privacy at eye level and, where possible, edible for animals. For example, plants chosen bear berries in early autumn, as a food source for birds.

The lasting effect of BOSK
It was estimated in advance that there might be complaints from business owners and residents in the city centre. After all, the containers take up quite a lot of space and obstruct the view of shops and catering establishments. Space for bicycles and parking is also limited during the project. Support among everyone turned out to be greater than previously estimated. Partly because BOSK is part of a much broader cultural project. In addition, hundreds of volunteers are working daily to move the containers and water the plants. Reactions to the greenery are heartwarming, the greenery softens the city and the most frequently heard question is whether the trees can't stay. Terraces are considerably fuller and the city centre radiates more tranquillity. BOSK has shown that nature should be an integral part of the living environment to create an attractive, healthy city. That urban planning is not static but dynamic has been exemplified by the municipality of Leeuwarden through its full cooperation. With good species choice and thoughtful approach, relocatable greenery can also have impact.