A range of planting stock can be used for green roofs, including seed, cuttings, seedlings, tubestock and larger containerised plants. See here for information on species selection.
Planting is best completed in autumn and winter to assist plant establishment before summer. Irrigation should be provided while the plants establish. Depending on the time of year and rainfall, establishment irrigation could be required for up to six months.
It is helpful to acquire container-grown nursery plants that have been grown in the substrate they are to be planted into; otherwise, reduce the amount of potting mix held around the roots when transplanting into the green roof.
Controlled release fertiliser (CRF) can be added to the growing substrate or applied after planting is completed (top dressing). Careful consideration of rates and application methods is needed to ensure proper distribution and to limit any rapid discharge of nutrients (more information on plant nutrition is provided here).
Trees planted on green roofs will require a deep substrate as well as anchorage to prevent wind-throw. Various anchorage systems are available as illustrated in Figure 19. Tree bracings must be checked regularly to ensure they are functioning as specified, and not causing damage to the tree. Trees will require construction of a tree ‘pit’ in the substrate to house the tree roots: this needs to be deep and wide enough for lateral root growth to ensure tree stability. Consult an arborist for specialist advice.
Plants can be sourced from retail nurseries or grown at home for small projects. Generally, plants for larger projects will be grown to a contract with a wholesale nursery. Contract growing lead times may range from several months to more than a year, depending on the type of vegetation that is required. The growing contract will specify a date for completion and delivery of plants. It should outline the terms and conditions of a further holding period, for instance, if the planting date is postponed because of delays in construction. Ensure plants purchased are weed and pest free – look for nurseries with good hygiene. If in doubt, remove the top centimetre of growing medium to reduce the weed seed bank.