Stormwater Treatment & Water Quality Management Plan_April2014

3.2    Irrigation
Plants and trees in raingardens and tree pits will probably require irrigation during the
establishment phase. Irrigation should be applied directly to the surface of the filter media. The
use of Ag pipes for irrigating young trees is not recommended as it creates a short circuit
pathway, or preferential flow path, for stormwater. As illustrated in Figure 3, the stormwater flows
straight down the Ag pipes and into the drainage layer at the base where it is conveyed
downstream to the conventional stormwater system, effectively bypassing any pollutant removal
processes that occur as the stormwater filters through the soil media. Figure 3. Concept
illustration showing how Ag pipes installed for tree watering can result in short circuiting and
reduced stormwater treatment.
3.3    Tree stake removal
Tree stakes are often used to support young trees planted into the filter media of raingardens or
tree pits. Once the trees are adequately established the stakes should be removed and the holes
filled in with filter media. Failure to fill in the holes will result in the creation of a short circuit
pathway, or preferential flow path, for stormwater. Instead of ponding on the surface of the
raingarden, the holes left behind after the stakes are removed allow water to bypass the filter
media and drain directly into the drainage layer at the base of the cell, effectively bypassing any
pollutant removal processes.