For facility managers and building managers whose buildings have a green roof on their roof or balcony, it is important to understand what these assets are, how they create value, and what type of maintenance is required.

Green roofs are long term investments. These are investments made for a purpose, and the maintenance crew or green roof gardener plays a key role in cultivating the garden to ensure it fulfils its intended purpose.

The key attributes in a green roof maintenance crew are:

• Horticultural knowledge and experience
• Problem-solving skills
• Understanding of the green roof’s supporting built layers and their implications for maintenance

Time frames on a green roof

Most green roof owners want their plants and vegetation to cover a significant amount of the soil or substrate. Yet individual plants take time to grow. They take time to achieve the desired level of coverage. This can lead building owners or communities to become impatient.

One way to avoid this potential issue is to plant mature plants. However, mature plants are far more costly as you are paying for years of care and space supplied in nurseries. In addition, they often suffer greater difficulty than young plants in adjusting to their new site and the change in soil or substrate they are growing in. Plants grown from a young age in the same soil or substrate as that on the roof, and better still, grown from a young age in the roof garden site, tend to be more resilient than plants transplanted in their maturity. This means fewer demands, and lower costs, in maintenance.

The decision whether or not to specify young plants is normally made toward the end of the design phase or early in the construction phase. The choice is often determined by the capital budget. Note, however, that some notion of the maintenance budget should also guide the choice. There is no point reducing capital costs to meet the capital works budget only to increase maintenance costs beyond what is viable.

From a maintenance perspective, the important point is to make sure the building owner or investor understands the consequences of this choice. This is one reason why having a facility manager or building manager who has a good understanding of green roofs is invaluable during the design process.

Note that these choices are in essence no different from choices made about ground-level landscapes, except that green roof gardens are often held to a higher standard of performance without this being stated explicitly. Unless he or she is fully conscious of the choice being made, the building owner may have a lower tolerance for the pace of growth of young plants than would be the case for a ground level garden.

Choices made about the age of plants specified directly affect the green roof maintenance required.

Establishment maintenance

When plants are shifted to a new location, they get a shock. Plants vary according to their age in how long they take to recover from this shock. Young plants may become established in six weeks, plants a year old may take three months, and older plants may take six months. During this establishment period, they need more intensive care than they will need later on. For this reason, several green roof companies provide maintenance at least during the establishment period as part of their construction contract.

Be aware of the intensive care required by plants during the establishment period. Either this care must be arranged and scheduled, or at the very least, access to the roof must be provided to the green roof construction company at relatively frequent intervals, probably starting at once a fortnight.

The intensity of care and maintenance required in the establishment phase, like the frequency required, varies according to the age of the plants. Very young plants will require the greatest intensity of care, though they become established more quickly. Mature plants generally require less intense care, but go on requiring particular care for a much longer period.

Note that these remarks are rules of thumb. Each type of plant has its own particular needs which vary from specimen to specimen and from garden site to garden site. You will need a horticulturalist to advise on the particular needs of your garden in terms of frequency and intensity of maintenance both during the establishment period and later.

The good news is that advising on this point, and monitoring plant specimens to assess their specific needs, is all part and parcel of a day’s work for a horticulturalist. The only difference on a green roof is that the frequency and intensity of its maintenance tends to have an impact on a greater number of people than in a ground level landscape, starting from the need for access to the roof or balcony.

The fact that conducting maintenance affects many people in the building, often in ways that are not immediately evident (especially to horticulturalists unaccustomed to working within buildings), means that communicating maintenance requirements is particularly important where green roofs, as opposed to other landscapes, are concerned. You need a maintenance crew leader who will communicate regularly to you about changing maintenance requirements, and you need a good means of communicating this information to the community in your building.

Seasons on a green roof

Like any garden, the frequency and intensity of cultivation and maintenance a green roof requires varies from season to season according to the climate zone.

In Australia’s temperate climate zones, expect to need a greater intensity and frequency of maintenance and higher levels of irrigation in the warmer months of the year.

Time frame for a green roof asset to mature

Providing a certain level of vegetated (and flowering) coverage is just one way in which green roof plants and gardens deliver value to their owners and building communities. Indeed, this is one of the simplest, most immediate ways in which green roofs create value. But the reason why green roofs are created tends to be far more complex and substantial than simply providing coverage (although this may be an important part of the purpose).

In serving complex, substantial purposes, the living systems in green roofs – their ecological processes – play a vital role. Ecological processes are the natural functions performed by the plants and their growing media. Examples include flowering, evapotranspiration, nutrient cycling, pollination and water filtration. The capacity of a green roof to serve its purpose derives mainly from these ecological processes. They generate the vast majority of the complex value that green roofs deliver.

But the living systems that are vital in green roof gardens – ecological systems – take time to evolve. A garden which has just been planted out does not have highly functioning ecological systems. Even once the roots of individual plants have become established, a garden’s ecological systems will take time to evolve. What’s more, it is only as these ecological systems evolve that they will reach their full potential in terms of delivering the value for which they have been created.

Less often discussed than establishment maintenance is the time required for a brand new garden to develop highly functioning ecological systems. A number of variables affect the time required, but as a rough guide, you might expect it to take two years to emerge from juvenility, and a further three years to reach maturity. Depending on your plants, you may need 15 to 20 years (consider the time it takes a large tree to grow to maturity.)

Once a garden is well-grown and its role in the culture of a building is well established, a green roof becomes a mature asset, by which we mean an asset that realises its potential value and fulfils the specific purpose for which the building owner (or sometimes the tenants or facility manager) has invested and continues to invest money in it.

The maintenance crew or green roof gardener is, quite literally, responsible for growing the value of the green roof asset.