An important element of maintenance is ensuring that plants receive adequate nutrition. This section provides information on nutrition for roofs and facades. Walls are not addressed here because green wall installers will provide specific instructions for meeting the nutrition requirements of the particular plants in the particular wall. Fertilisers for green walls are delivered in liquid form, via the irrigation system (see here).
In consultation with the designer and client, establish the lowest appropriate application rate for controlled release fertilisers (CRF). The aim is to provide sufficient nutrition for strong plant performance while minimising nutrient loss into irrigation/stormwater run-off. As such, fertiliser rates are usually significantly lower than those recommended for garden or container plants. Where the plantings are limited to succulents there may not be any ongoing addition of fertiliser.
CRF is the most suitable choice for plant nutrition in green roof settings or green facades in containers. The fertiliser comes in the form of water permeable resin granules or beads and is applied on the substrate surface but should be raked or mixed through, ensuring an even distribution. Each rain or irrigation event dissolves a small amount of the inorganic nutrients stored in the bead. If the roof or container is being irrigated from below (a sub-irrigation system) then it is important to mix the fertiliser well into the substrate.
The elevated temperatures on a roof or facade can lead to excessive fertiliser loss and damage plant growth. For Australian natives on green roofs, low phosphorus CRF can be used at half to quarter of the rate recommended for garden or container plants.
Liquid fertilisers are not suitable for routine use on green roofs, as nutrients are more likely to leach out of the mix and leave via stormwater run-off.
Facade greening planted in the ground in a good quality, sandy loam topsoil should not require additional fertiliser. The addition of composted organic matter once or twice a year to garden beds where facade greening is planted will supply nutrients and improve water retention. CRF suitable for the species planted may be incorporated into the garden beds, if desired, according to the rates recommended by the manufacturer.
Visual inspection plays an important part in determining whether plants are experiencing nutrient deficiencies (see Growing Media for Ornamental Plants and Turf for further information).
Monitoring of fertiliser levels
It is useful to undertake soil testing of pH and electrical conductivity to establish the conditions under which plants were started. Ongoing, strong plant growth and coverage is the best sign that growing conditions are suitable. As long as there are no problems with the performance of the plants, the manufacturer’s recommendations for rate and frequency of fertiliser application may be followed. If plants do not perform well, or there are any signs of nutrient deficiency, a sample of growing substrate should be collected for testing of pH and electrical conductivity (EC) or total dissolved solids (TDS), following the same test procedure as was used at planting. Additional fertiliser should be added if EC or TDS results suggest nutrient levels are simply running low. If possible this should be incorporated through the growing substrate to ensure its even distribution, though this may be difficult to achieve for green facade containers installed at elevation.