Designing a Roof Garden - Mulches

Designing a Roof Garden - A Designers Guide

Half the irrigation water which is applied to landscape is lost by evaporation. Probably the best way of retaining water is by the use of a mulch. Reference is often made to the use of a dust mulch. A layer of dry soil does reduce water loss but the benefit is not great. Bark is the most commonly used material.

In the search for lower costs, a large number of extremely unattractive products are now sold under this heading. To be effective the mulch should be shredded to give pieces 2 to 3cm across and left to compost for 6 months with an application of nitrogen based fertiliser to encourage it to break down. This ensures that it does not absorb nitrogen from its surrounding environment.

To control the growth of weeds, the layer of mulch needs to be thick. The trouble is that the industry refers to mulch in its fluffed up form. This means that a 50mm thick mulch is only 25mm thick when it has settled. It is important to anticipate this when specifying the thickness of the layer, and probably to specify a 75 or even 100mm thick layer. It is also necessary to insist that the thickness be restored at the end of each year.

One problem with bark is that it will float in a water feature. Where planted areas abut water, it is better to use a gravel mulch. The horticultural industry commonly uses woven plastic sheeting, such as Mypex to suppress weeds. This solution can be incorporated into a roof garden. The sheet not only controls the growth of weeds but provides a good barrier to evaporation. In an ideal situation a drip irrigation system can be laid under the sheet. The sheet can then be camouflaged with a gravel mulch.

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