Willerby Landscapes, which won the bridge landscape contract, is building an 18m-high soil bund on an exposed site at Deepdale Nurseries, to be planted according to Dan Pearson's planting plan, to see how the different plants cope with the elements they will face high above the Thames.
The Kent-based company's operations director Rick Davies explained: "The purpose is to see how the plants react when they're buffeted and battered. Then we can gain an idea of things like whether there will be different irrigation requirements on either side of the bridge."
The trial run will also show whether the plants can reach the specified planting sizes in the time allotted by the demanding construction schedule. Hard paving and sample balustrades may also be installed. Every element of the bridge's planting is being planned in detail so that when it comes to the real thing it should be easy, Davies told an audience at the Landscape Show on 23 September.
Willerby is now liaising with construction company Bouygues on the pre-construction phase. Bridge sections complete with inbuilt irrigation will be assembled off site and brought up the Thames on barges, as will the landscape supplies. The hefty barges can only come down the river when tides are high enough so plant and soil deliveries must be carefully scheduled. Bagged soil will be loaded onto the bridge by two cranes working simultaneously from either side to keep the structure balanced.
The subsoil will consist of expanded clay aggregate as a drainage layer topped by washed sand. Pearson has specified five planting zones, which will each require their own manufactured soil with different permeability, alkalinity and fertility specifications.
Rigorous soil compliance and drainage testing will be carried out by Tim O'Hare Associates throughout the process because "once the plants are up there we don't want to be trying to take things down again", said Davies.
To minimise machinery compaction, a single track will be laid down the middle of the bridge, along which will run a low ground pressure machine that can reach either edge. Barges will bring the trees and plants on Dutch trolleys, with trees to be underslung and dropped straight into place by crane as topsoil is added. RHS-trained horticulturists will carry out the softer planting and have helped to write up an extremely detailed maintenance plan for each plant to help Willerby maintain the garden for 12 months after the bridge opens in summer 2018.
Pearson is now scouring Europe for the plants required for his design, with cuttings and seeds to be grown by Palmstead Nurseries. Trees will be sourced from the UK where possible but if imported they are being brought in this season, to be containerised for around 15 months before planting. The process is a huge challenge for Willerby but also "really stimulating", said Davies. "It keeps the whole company fresh because we're not just regurgitating the same landscape time and time again."