Like any garden, gardens on roofs cannot survive without a gardener. Without a gardener, they cease to be gardens and become wild places.

This may sometimes be desired; for example, the UK’s brown roofs are created not as gardens, but as places where habitat for insects and birds is allowed to evolve organically and with little interference. In Australia, however, at present the vast majority of green roofs are created as gardens. Even when the style of the garden is wild or natural, it is achieved through gardening and cultivation.

What do green roof gardeners need to know?

In commercial environments, the gardening and cultivation of green roofs is commonly referred to as maintenance. This term has its pitfalls. It masks the horticultural knowledge required for good results. But it has developed for at least one important reason: namely, that green roofs are one of a building’s assets. Like any building asset, their engineered components and built layers require maintenance.

On a well-designed green roof, the greater part of the maintenance of the engineered components and built layers simply involves regularly checking for any issues, but it is crucial that the gardener of a garden on a roof understands the nature of these engineered components and built layers. Horticultural practices that are normal on the ground, such as knocking a stake into the ground, must be avoided on green roofs or conducted with great care, for they risk damaging the integrity of the support layers. Penetrating the waterproofing membrane on the roof results in extremely costly damage and must be avoided.

The focus of green roof gardeners

Although maintenance of the built systems and engineered components is one part of the role of the people who look after gardens on roofs, it is really a secondary task. Maintenance of the built systems and engineered components of a green roof is a task which supports the chief purpose and role of green roof gardeners. So what is the chief purpose of green roof gardeners and what roles and responsibilities must they fulfil?

In a commercial or retail building, the gardening and cultivation of a green roof is normally the responsibility of a maintenance crew which is managed by the facility manager. In an apartment block, it may be conducted either by a maintenance crew managed (and perhaps contracted) by the building manager, or by the building manager himself or herself. In a house, normally the owner either conducts the gardening or cultivation of the garden on the roof or engages a gardener to do so.

The maintenance crew responsible for cultivating gardens on commercial, retail and multi-residential buildings must have various responsibilities, and the facility managers and building managers who are responsible for managing the green roof gardening or maintenance crew must be aware of these. If you are thinking of cultivating a garden on a roof yourself, or if you own a green roof or are a property developer or architect creating a green roof and thinking through the design requirements, here is what you need to know:

Why green roofs need cultivation

All green roofs have a purpose, even though over time it may change. Various Australian green roofs are designed to serve any of 40-odd different purposes.

The designer of a green roof is (or should be) well aware of its purpose. Good green roofs are designed to fulfil their purpose with a deep understanding of how this purpose will be fulfilled – of the functions through which the green roof will fulfil its purpose.

In the handover from construction to green roof operations (use and maintenance), the purpose of the green roof should be clearly documented and communicated to the facility manager or building manager. Ideally, the way in which the design features are intended to function in order to fulfil the purpose should also be documented and communicated.

It is the responsibility of the facility manager or building manager to ensure the maintenance crew understands the purpose of the green roof, and the intended role of the design features in fulfilling this purpose. This matters because gardens on roofs, or green roofs, are made of living systems supported by engineered layers. The ecology of these living systems is the chief means or technique by which green roofs serve their purpose. The principal role of a green roof maintenance crew or gardener is to cultivate the ecology of the living systems that is required for the green roof to reach its full potential and fulfil its purpose.

What does this mean in practice?

Note that you are dealing with living systems. Some design features may not perform the functions they were intended to perform. Every garden requires testing over time; it is impossible to guarantee the performance of a living plant in advance. The green roof gardener or maintenance crew plays a crucial role in selecting replacement design features (plants, substrate, mulch and irrigation systems) that will perform the intended function.

This may sound obvious, yet it is easy for a maintenance crew which has not been told the purpose of the green roof and the intended function of its various components to assume mistakenly, for example, that a particular set of plants were selected for their colour, and accordingly to replace them, if they fail, with plants of a similar colour. But if the intended function of the original plants was in fact to filter water, the new replacement plants may not have the requisite qualities.

The core attributes a green roof gardener or maintenance crew needs are:

• Knowledge of horticulture
• Problem-solving abilities
• Understanding of the built layers and engineered components of green roofs

Time frames

One factor you must keep in mind is that the living systems that are vital in gardens – ecological systems – take time to evolve. A garden which has just been planted out does not have highly functioning ecological systems. At the most basic level, the plants need time to grow to achieve the desired level of coverage. Even once the individual plants have become established, the ecological system will take time to evolve and reach its full potential.

For a green roof to function as designed, the plants need to become established. Next, the plants need to grow and the garden needs to evolve into a whole system and to mature. These developments take time, and they also require care and cultivation by the maintenance crew or gardener.