Courtyards are a common architectural feature of healthcare sites in the UK. Whether designed principally as internal gardens, lightwells or to facilitate ventilation, enclosed spaces close to wards and clinics are likely to be the principal view of the outdoors from surrounding windows, and access to sheltered open-air areas for rest and recuperation may be greatly valued by patients and staff. Many will think trees in courtyards aren’t possible, but this isn’t the case.
In this blog, we will consider some of the practicalities for incorporating trees in courtyards and enclosed gardens. We will be using ‘courtyard’ to mean outdoor spaces at ground level that are open to the sky, but wholly or partly enclosed by buildings or walls. And our focus will also be trees planted directly into the ground, rather than in pots, raised beds or other form of container that restrict root spread.
Is the space suitable for a tree?
An obvious question to ask at the outset will be whether your site is appropriate for trees in courtyards. There are several factors that may limit or preclude planting in such spaces:
Sunlight is essential for photosynthesis, and many of the UK’s native tree species are highly light dependent, requiring full sun to thrive. In a courtyard setting it is likely that bounding structures will cast shade for at least part of the day, so you will need to consider carefully how much direct sunlight your courtyard receives throughout the year, and which areas within this space may be more or less shaded. Assuming the height of the perimeter walls is equal on all sides, south facing areas and the core of the courtyard are likely to receive proportionally more sunlight over the course of each day.
Growing space will be limited by the bounds of the courtyard. Given the potential for trees to cause structural damage it’s important to consider both potential height and canopy spread. As a general rule it is a bad idea to plant trees any closer than a distance equivalent to the mature height of the tree from buildings or walls, and we suggest a minimum of 5 metres regardless of species. It would also be advisable to check for any underground services such as pipes or cables that may be present.
Access will also be an important consideration, as mature trees – many times larger than when initially planted – may become an obstruction. Low level foliage may extend well beyond the trunk, so leaving sufficient clearance adjacent to pathways and paved areas within the courtyard will help to avoid additional maintenance as trees establish, and disturbance by tree roots.
Shading by mature trees may impinge on light reaching the ground and how that space can be used. Large tree canopies will also restrict light levels to windows overlooking the courtyard – albeit this may be an asset if direct sunlight creates uncomfortable temperatures during the summer months.
Subsurface soil is necessary for a tree to establish its root system. Just how much soil will depend on the size of the tree at maturity, but a minimum depth of 60 cm for both the top and subsoil combined is recommended, ideally with no stones larger than 2.5 cm (and these not exceeding 5% of overall soil volume). The projected mature canopy diameter is a good indication of root spread.
Watering and maintenance routines will differ for trees in an enclosed space opposed to trees in the wider environment. Courtyard trees may have restricted access to groundwater and routine pruning will need to be undertaken proactively to maintain the size and shape of the tree over time; establishing responsibility for the tree’s upkeep at the outset will avoid it being neglected or left to grow in a way that is unsuitable for the available space.
Trees in courtyards – which species are the most suitable?
Here are some of NHS Forest’s suggestions for species suited to an enclosed space. We have focused our recommendations on native trees, but you could also consider more exotic ornamental species.
 | Prunus species such as bird cherry (Prunus padus) and wild cherry (Prunus avium) will tolerate poor soils and produce colourful blossoms. |
 | Crab apple (Malus sylvestris) is a compact and seasonally interesting species, with sweetly scented blossom. A good choice for wildlife. |
 | Elder (Sambucus nigra) is a smaller tree with fragrant flowers, cultivated varieties may be selected for their colourful foliage and flowers. |
 | Hornbeam (Carpinus betulus) is a larger species that responds well to regular pruning and can be trained into interesting shapes. |
 | Rowan (Sorbus aucuparia) can grow well in thin soils but requires plenty of light. A good choice for wildlife, especially birds. Miniature cultivars may work better in small spaces. |
 | Dwarf fruit varieties including apples, cherries and figs can be a lovely – and edible – addition to a courtyard garden and can be selected to tolerate confined spaces. |
What are the alternatives to tree planting?
Not every courtyard space will be suited to tree planting. One possible alternative might be to incorporate smaller shrubbier plants or develop a “woodland carpet” of understory plants such as ferns, grasses, herbaceous perennials and wild flowers. The Royal Horticultural Society has a blog on creating a woodland feel even in small spaces.
Further reading
NHS Forest has a couple of other blogs focused on trees for small spaces that may be insightful:
Planting Trees in Small Spaces Part 1 considers tree size / spacing and has recommendations for smaller UK native species.
Planting Trees in Small Spaces Part 2 looks at care and maintenance with a view to restricting the ultimate size of planted trees.