PLANT GROWTH TRIALS IN RAPID FILTRATION MEDIA

Rapid bio filtration treatment systems have gained Washington State Department of Ecology General and Conditional Use Level Designations as basic level and enhanced level stormwater treatment devices in the USA. The media used in rapid bio filtration devices have hydraulic conductivity rates exceeding 2500 mm/hr. Stormwater360 NZ has partnered with Contech USA to develop a local version of this rapid filtration media. One of the reasons current bio retention guidance limit the hydraulic conductivities of media to less than 350 mm/hr is to support plant growth. This research examined the capability of four NZ groundcover plant species to establish in the rapid filtration media made using locally available materials. A typical ‘TP10’ rain garden mix was used as a control. Both treatments were covered with 7 cm of shredded bark mulch.

The four NZ plant species that were used for the plant growth trials were:-

- Poa cita / Silver tussock
- Hebe speciosa / NZ hebe
- Dianella latissima / Flax lily
- Carex testacea / Orange sedge

Over a period of 12 weeks the plants were watered with tap water and stormwater taken from a local carpark sump according to a fixed schedule. Plant root and shoot growth and biomass showed 3 of the 4 species successfully established in both the rapid filtration and TP10 media, developing extensive root systems with markedly different structures. Carex testacea did not establish in either media. Hebe foliar growth was greater and visibly greener in the TP10 mix, likely due to significantly higher levels of available nitrogen and phosphorus. Hebe and Dianella had high root mass, with Hebe having a root:shoot ratio between 1.7 and 2.6.