Hydroplanting: the way forward for extensive green roofs

Citation

(1998), "Hydroplanting: the way forward for extensive green roofs", Facilities, Vol. 16 No. 3/4. https://doi.org/10.1108/f.1998.06916cab.004

Hydroplanting: the way forward for extensive green roofs

Hydroplanting: the way forward for extensive green roofs

Despite the influence of environmental pressure groups and a steadying of the population growth, changes in lifestyle aspirations and working practices mean that there is no end in sight to the urbanization of the UK landscape. It is essential, therefore, that some compensatory measures are taken to counter the negative effects of this urban sprawl. This is one of the reasons why green roofs ­ landscaped flat roofs ­ are increasingly viewed in the UK with the same enthusiasm they have enjoyed on the Continent over the past three decades.

The concept of green roofs is moving from the fringes of the planning and design debate to centre stage. One of the reasons is that ecological demands and economic interests complement each other in this case; even though the relationship is not always without friction.

One of the main reasons for this sea change is the emergence of hydroplanting ­ a technology developed from transport landscaping which brings the environmental benefits of traditional green roofs while offering construction times and cost savings more in keeping with modern fast-track projects.

Essentially, hydroplanting mechanizes the planting process. While the equipment now used for hydroplanting (or spray greening) of roofs was derived from the machines used on motorway cuttings and civil engineering revetments, it faces many new demands in roofing applications. It must, for instance, have a pressure output adequate for high or distant roofs. At the same time, the process must be gentle enough not to damage the plants themselves. To ensure a consistent finish, efficient mixing is essential, while the output nozzle must have stepless control in order to be able to deal with small surfaces and connecting areas. Finally, the equipment must be easily cleaned, so that residues of special plant mixtures can be removed.

Equipment with these properties, together with suitable additive materials, produces a nutrient and growth mixture which is excellently suited to sowing vegetation on extensive roofs. Many such roofs now feature hydroplanted vegetation sustained by purpose-designed mulches. Consisting of cellulose, compost, substitute peat or other suitable materials, mulches provide structure to the sprayed mass and an effective covering for seedlings and seeds. Applications of between 5 litres and 10 litres of solid material per sq.m are ideal.

Slow-release fertilizers can also be used, although care must be taken to ensure that these types of fertilizer are not washed out by rainfall. A nutrient supply which is slow and as balanced as possible is particularly important. Nutrients that are released too quickly and in an uncontrolled manner can lead to environmental pollution due to washout. Modern spray equipment applies the nutrient and growth layer at high pressure using a shallow spray. Ideally, the pure fertilizers should be applied with a balanced nutrient composition at a rate of between 3g and 6g per sq.m.

When selecting a suitable media for sealing and compacting the ground, the priority should be building up a balanced micro-organism structure while also consolidating the substrates and increasing the water-carrying capacity and absorption rate. Only those binding agents which are absolutely harmless to the vegetation should be used. A complete biological breakdown within a period of approximately six months and approval for use in water conservation areas are both desirable.

A very wide range of seeds and seedlings can be used when hydroplanting extensive green roofs. Suitable seedlings include Sedum acre, Sedum album (Coral Carpet), Sedum cauticolum, Sedum floriferum (Father Christmas Gold), Sedum hybridum (Evergreen), Sedum reflexum, Sedum sexangulare (White Tatra), Sedum spurium, and Sedum spurium. The usual application is 50g per sq.m.

Suitable seeds include Achillea millefolium, Agrimonia eupatoria, Agrosthemma githago, Allium schoenoprasum, Anthemis tinctora, Antyllis vulneraria, Campanula rotundifolia, Centaurea cyanus, Daucus carota, Leucanthemum vulgare, Dianthus carthusianorum, Dianthus deltoides, Galium verum, Hypericum perforatum, Inula conyca, Linum perenne, Lychnis viscaria, Lotus corniculatus, Melampyrum pratense, Origanum vulgare, Papaver rhoeas, Prunella grandiflora, Prunella vulgaris, Sanguissorba minor, Salvia pratense, Salvia verticilata, Silene vulgaris, Thymus pulegioides and Thymus serphyllum.

The current trend is to make widespread use of indigenous grasses. These can include: Festuca ovina duriuscula (Bornito), Festuca ovina tenuifolia (Fertal), Festuca rubra commutata (Lirouge), Bromus erectus, Anthoxanthum odoratum, Poa pratensis (Compact), Poa compressa (Rubens), Deschampsia flexuosa, Briza media and Koeleria machrantha.

This list is by no means extensive as the ability of many individual plants to adapt to life on a green roof is still unknown. Ongoing trials at various research institutes involving long-term observation will throw some light on this. It could be just as important to see the results of practical experience in greening extensive roofs using hydroplanting. Many new types will be discovered which will be suitable for extensive roof greening while some types which we consider good today may in time prove to be less durable.

The weather conditions in the area and the exposure of the roof surface, the amount of sunshine and the properties of the substrate (depth of layer, water retention and absorption capacity) and the slope of the roof are the main criteria when selecting species. The philosophy of extensive roof greening also assumes that plants brought in from outside should have the opportunity to develop in the course of succession.

When sprouting sedum seedlings, herbs and grasses, a mixture of seedlings of eight to ten different types of sedum spread as widely as possible, in conjunction with at least 20 different seeds, may be used, with the grass proportion remaining as small as possible in order to reduce subsequent maintenance. Great care must be taken in choosing the types of herb and grass. The ability to regenerate, ecological value, endurance, competitive behaviour and ability to withstand drought should be considered.

The composition of the growing plant communities is naturally affected by the substrate site. Hydroplanting can be used for all substrates which are suitable for extensive roof greening, including single and multi-layer systems. The type and quantity of the described additives must be matched to the substrate. The positional stability of the bulk material on the surface, the water storage capacity, the amount of organic substances and the concentration of nutrients in the greening construction are factors requiring particular consideration. When correctly applied, the spray method minimizes wind erosion of the nutrients and growth layer. It also creates a germination layer for rapid and certain vegetation colonization. At the same time it provides internal linkage of the upper substrate layer with the additive materials.

Other benefits of hydroplanting include the acceleration of the germination process by steeping the seed and seedlings in the sprayed mass which is exacerbated by the high pressures in the pipes. Hydroplanting also tends to improve the substrate's properties, particularly with regard to water capacity, substrate invigoration and capillary action.

The high capacity of the equipment means that hydroplanting is best used on cohesive roof greening areas above approximately 150m² to 200m². The larger the greening area, the more positive the effect of the spray method, particularly regarding economies of scale. This method of greening extensive areas is also particularly suitable where different skills are involved. The technical construction of the roof greening layers can be accomplished by different tradesmen who can then leave the landscaping to a specialist.

The hitherto normal annual gardening cycle for seeding can be substantially extended using the spray method. The system offers seeds and seedlings extensive protection against drying out and frost. Experience over several years has shown that spraying can be used throughout the year except during periods of persistent drought and frost or where the substrate is frozen. This offers an advantage with regard to erosion protection of the overall structure and the economic use of specialized equipment and highly-qualified personnel. Projects carried out during mild winter conditions are particularly successful as there is a rapid colonization of the vegetation in the following spring.

In conclusion, hydroplanting enables extensive roof greening to be carried out at reasonable cost. Ecologically-effective, durable vegetation colonies are created where there is an optimum interrelationship between the roofing and horticultural elements of the roof. The spray method is excellent in this respect, and is helping to further the penetration of green roofs to the benefit of everyone in society.

The author acknowledges the pioneering work of Fritz Hämmerle of Paul Bauder GmbH & Co and, in particular, his article "Extensive Dachbegrünung im Anspritzverfahren", published in Das Gartenamt 44, February 1995.

For more information contact: Graham Spall, Green Roof Division, Erisco-Bauder Ltd, Broughton House, Broughton Road, lpswich, Suffolk IP1 3QS. Tel: 01473 257671; Fax: 01473 230761.