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Parking island sizes
The size of the open soil area should be considered. Find the island size or buffer strip width and soil drainage in the following sections that most closely match your planting site.
Tiny islands less than 45 square feet
This tiny island is really too small for most trees. If you must plant, small-maturing trees are most appropriate. Trees that mature at any size can be planted, but they will be dwarfed or bonsaied by the small soil space.
If roots can grow freely under the pavement in well-drained soil, the tree may get reasonably large, but pavement is likely to be damaged by roots unless special provisions are made (see creating root space.)
Small island (45 to 100 square feet) with poor drainage
Trees planted in poorly drained or compacted soil will be shallow rooted. Except for some wet-site-tolerant trees, roots of most trees are not likely to escape the island and grow beneath pavement.
Therefore, large trees are not suited for small islands and they are not recommended because of the possibility of windthrow and future curb and pavement damage.
Although small trees are probably best for this site, most medium-maturing trees may become unstable in very windy weather as they grow older, due to their confined root systems. Growth will be slow and trees will become stunted as roots fill the island space.
Small or medium-sized island (45 to 200 square feet) with good drainage
Loose, well-drained soil is not common in parking lot islands or most urban sites. If soil is loose and well-drained, some roots will get down and under pavement to anchor the trees. Although small- and medium-maturing trees are probably best suited for this sized island, most large-maturing trees should grow fine for a period of time.
Large-maturing trees, such as oaks, are not likely to reach their mature size, and they will be under stress in this size island, which may increase maintenance requirements on large trees.
If medium- and large-maturing trees grow well in the island, they will create shade, but surface roots could cause curb and pavement damage as roots grow next to and beneath curbs and under pavement. Property managers may see the need for curb and pavement replacement to make the area safe for pedestrians; this practice usually severely damages tree roots.
The best alternatives are to increase the size of the island or to live with disrupted curbs and pavement.
Medium-sized island (100 to 200 square feet) with poor drainage
Small- and medium-maturing trees are probably best suited for this site, but osme large-maturing trees, such as oaks, may grow fine for a period of time. Again, trees planted in poorly drained, compacted soil will be shallow rooted.
Large-maturing trees, are not well suited to this size island because of the possibility of windthrow and future curb damage from roots growing along the curb. Again, large-maturing trees will not reach their mature size, and they will be under more stress than smaller trees, making increased maintenance requirements on large- maturing trees.
Large island (greater than 200 square feet) with poor drainage
Poor drainage at this site is likely to confine roots to the soil within the island. Many wet-tolerant trees will grow fine here, but large-maturing trees could become top heavy and fall over in very strong winds as they grow older, due to their confined, shallow root systems.
Large island (greater than 200 square feet) with good drainage
If large-maturing trees grow will in the island, they will create shade, but surface roots could eventually cause curb and pavement damage as roots grow next to and beneath curbs and under pavement. Property managers may see the need for curb and pavement replacement to make the area safe for pedestrians; this practice usually severely damages tree roots.
Small- and medium-maturing trees will not usually cause this type of damage, but they do not provide the shade that larger trees do. The best alternatives are to build islands for large trees that are at least 400 square feet or to live with disrupted curbs and pavement.
