A deep planting zone is an area that is dedicated to landscape and is able to sustain large subtropical shade trees.
The aim is to retain or provide significant vegetation to urban areas for shade, amenity, offset of carbon emissions and to mitigate the heat-island effect in urban areas caused by dense built-form and hard surfaces that fill cities.
Brisbane City Council has some of the most specific requirements that include:
- Area is 100% open to the sky
- No driveways, maneuvering or hardstand areas and pedestrian paths
- No underground development (basements, tanks) or infrastructure (services)
- In natural ground (not in cut/fill area or a planter)
- Can incorporate existing trees
- Sometimes must meet a minimum dimension
Clearly, not all of the landscape area typically indicated on a site plan meets these requirements.
Multi-storey apartment buildings often include garden areas at entry areas that are located beneath upper levels. Multiple services and structural footings are often located beneath landscape zones.
4 Common Intrusions to Deep Planting Zones
(transformers, fire boosters, water meters, switchboards)
A common and significant (size) intrusion to a deep planting zone is the introduction of a pad-mount transformer. Ironically, the requirements for locating transformers are quite similar to deep planting zones so of course this is exactly where they end up.
Understandably, the requirement for these is often not certain until after a DA is achieved. Because of this, it is now common for Brisbane City Council to require transformers to be indicated on site plans for development approval.
(overland flow zones, stormwater pipes, bio-basins)
Overland flow zones are often accompanied by restrictions to the density and type of vegetation that can be planted within it.
Sometimes it means no planting at all. Where a stormwater pipe is shallow, any large tree planting will pose a risk to the integrity of the pipe. Many tight development sites make use of underground stormwater treatment devices.
But when the need to an open bio-retention area is identified late in the design process, often the only location left on the site to put these are landscape areas.
At times a bio-retention area may be designed to accommodate trees, but this falls short again, as the very design of a bio-zone involves replacing the filter media at a future time, thereby removing any vegetation.
Footings to walls and structures can significantly impact landscape areas in terms of the amount of growing media that is available to plants. Boundary walls that require footings to be contained within the site can result in significant footing extents that commonly impact landscape areas.
For instance, a typical landscape buffer zone might be 2 metres wide, which normally allows small columnar trees and screen shrubs to be planted.
If such a buffer happens to be located adjacent to a wall that has a footing intruding 800mm into the landscape area, it could mean that no trees can be planted in that zone unless footings are greatly lowered, adding significant cost to the whole exercise.
(footpaths, hardstand, parking spaces, driveways)
Fire exits and access paths to service areas are items that are at times resolved quite late in a project. Where landscape areas are minimal, these can have a big visual impact on the resulting amount of planting area, as well as affecting what areas can be counted as deep planting.
The challenge here is that many of these components remain in a conceptual form throughout the DA phase. Often their resolution occurs after key structural components of buildings and structure and it is then simply too late.
While each may be small individually, their combined impact on the amount of deep planting can be significant to the point where landscape conditions cannot be fulfilled.
The key is to factor in realistic area estimates required for all components and services at the early stages. Getting a Landscape Architect involved early in the site planning phase allows an opportunity for feedback that can lead to a more robust proposal that will withstand inevitable change.
What does the client need to know?
An area designated as deep planting should be treated as a planted garden bed area that includes trees. Turf is not considered an acceptable solution.
It is important that a client/developer understands this as there are reasonable design and cost implications.