22.5: Urban Green Infrastructure

We have discussed concerns related to urban soils in the context of crops and food production. But natural areas, parks and ornamental gardens are also highly treasured by residents and visitors. Similarly, yards and gardens are small areas of relief from the urban bustle and are cherished by city dwellers. Urban areas also have a lot of food waste, tree leaves and tree trimmings that can be turned into compost or mulch and used to improve the soil—done at the municipal scale or in home backyards. Under ideal conditions even pet waste can be safely composted. (While cities also generate a lot of sewage sludge at wastewater treatment plants, there are often concerns with contamination by industrial and household products that keep it from being used to grow food.)

With the de-industrialization of many cities, urban renewal projects frequently involve the redevelopment of former manufacturing and transportation sites into housing and office developments, or urban parks. Care needs to be taken to study the nature of the previous land uses and the associated possible contamination as we discussed earlier in this chapter.

Remediation

Similar to establishing urban farms, the development of green spaces needs to consider different options. Most green spaces involve perennial plants, and much of the soil health considerations need to be addressed up front. Generally you want the soil to support attractive vegetation at a low maintenance cost. This requires good drainage, high waterholding capacity, good rooting and low weed and disease pressure. This is usually accomplished through the same practices we discussed earlier: loosening compact soil, adding compost and fertilizer, balancing soil pH and mulching.

Except in extreme cases of contamination when the soil may need to be removed, landscaped areas can generally have poor soil buried by trucking in good top soil. Or the soil that is there can simply be improved with amendments. Burying soil in place is often sufficient for the remediation of industrial or built sites that contain various debris materials. Building raised beds or berms is a common approach in urban gardens, both to address poor soil quality and to improve drainage. Placing a layer of landscaping fabric on the soil surface before adding the imported soil for the raised beds helps to limit roots reaching the original soil and lessens mixing of the imported material with the surface layer.

When the soil is compacted or has low organic matter, and when there is little chemical contamination or other waste materials in the soil, the best option is probably to improve what is there through mixing and adding organic materials. The physical, chemical and biological quality of the soil can then be improved by applying and incorporating compost using excavators or bucket loaders. The so-called “scoop-and-dump method” works well when there is no existing vegetation on the site (Figure 22.5). If there are trees and other large plants that need to be saved, an air spader (a device that blows soil away with high air pressure) may be used to gradually remove the compacted soil around existing roots and then replace it with healthy soil.

Figure 22.5.Left: scoop-and-dump method for de-compaction of soil and mixing in compost. Right: Ameliorated soil with plantings and mulch. Photos by Cornell University, Urban Horticulture Institute.

Special Soil Mixes and Street Trees

Plants in pots, planter boxes and green roofs require clean soil mixes that allow for excellent drainage (because of the low gravitational drainage potential due to shallow depth), high water- and nutrient-holding capacity, and low weight. Soil for rooftop gardens needs to be light enough so that it doesn’t overburden the roof, and heavy enough that it anchors the plants and won’t be dislocated by wind or water. Soil mixes are typically combinations of special minerals like vermiculite clay (treated by heating), perlite (expanded volcanic rock particles) and organic materials like peat moss, compost or biochar. These manufactured soil materials have favorable physical, biological and chemical characteristics with low density, but they generally cost more than traditional soils. Containers for growing plants need to have holes in the bottom to allow for water drainage to avoid saturated conditions when watering.

Street trees are valuable assets to a neighborhood because they moderate the microclimate and improve the aesthetics. Special challenges exist with trees in sidewalks and parking lots. Unlike those in parks, cemeteries or green strips along boulevards, street trees are growing in a paved environment. The pavement substrate (the soil material immediately underneath) is often highly compacted in order to meet the bearing capacity standards to support the sidewalk pavement plus the additional loads from possible emergency vehicles. Oftentimes the tree roots grow big and break or tilt the sidewalks, thereby creating a health hazard and liability for the municipality. They also frequently die prematurely due to the highly constricted rooting environment combined with salt, and heat and moisture stresses.

Street trees therefore create a dilemma between the engineering requirements for a strong pavement that supports high loads (requiring a compacted substrate), and the need for a healthy rooting environment for the trees. One solution is the so-called gap-gradedsoils that can meet both objectives (Figure 22.6). Such materials contain only particles of certain sizes, with some sizes deliberately left out in order to ensure that there will be good amounts of pore space. This soil material commonly uses large, uniform stones as a skeleton matrix that can support high loads from the pavement, while allowing large pores for tree root protrusion under the pavement. These pores are partially filled with high quality soil material to support the tree functions. On golf courses and other greens, similar gap-graded soil materials are applied (typically sand, with certain sizes omitted) to better support foot traffic while still maintaining healthy turf growth.

Figure 22.6.Left: Planting hole in gap-graded soil material that supports high loads from the pavement while allowing large pores for tree root protrusion. Pores are partially filled with fine soil particles and organic matter to provide plant growth functions. Right: Healthy street trees in a sidewalk with gap-graded soil. Photos by Cornell University, Urban Horticulture Institute.