Designing a Roof Garden - Organic Substrates

Designing a Roof Garden - A Designers Guide

Peat is a broad generic term for a wide range of products of a very dissimilar age and physical properties. In its raw state peat is almost completely devoid of nutrients but has a high cation exchange capacity of up to four times that of clay. Peat is formed by the partial decomposition of plants growing in areas with high rainfall, high humidity and low Summer temperatures. The older the product the finer it usually is.

Young material tends to be coarser and therefore superior. The cheapest product is usually shipped in bulk and fine in texture particularly if it has been handled several times. Due to its price contractors frequently try to use cheap fine peat. This is not good for long term projects. Specifying coarse grade peat may well be pointless unless the process is well monitored as suppliers will often use the cheapest rather than the best material. As a result a sample of all raw materials should be approved and retained.

There are many sources of peat in Northern Europe and North America but unfortunately there is no standardised international method of classification available, although for horticultural purposes the following are accepted:

Sphagnum moss peats come from peat bogs and should consist of at least 75% of partially decomposed sphagnum moss. The leaves of the moss usually consist of a single layer of cells which give them a high rate of water absorption and retention. The general characteristics of sphagnum peat is a spongy fibrous texture, a high porosity with high water retention, a low inorganic content and usually a low pH. Commercially exploited peat deposits occur naturally in Germany, Finland, Canada and Ireland and have similar properties to the hypnaceous moss peats of the USA.

These are formed mainly from sedges and reeds during the silting up of shallow lakes. As a result they typically hold more nutrients than sphagnum peats, have a higher CEC per unit weight, and are more humidified and decomposed. They have a lower porosity than sphagnum moss peats, a less durable structure and are particularly difficult to re-wet although these problems can be overcome by careful management.

Bark has the advantage that its rate of decomposition is about one third of that of peat. However, in certain cases it can suffer from nitrogen deficiency and the generation of toxic compounds, although these problems usually moderate after the first six months. To avoid problems of toxicity from compounds arising from natural resins and turpins, only those barks which are specifically marketed for substrates should be used.

Most of the bark used in Britain comes from larch and pine, whilst in North America douglas, red and white fir are common sources. The best barks have a loose open structure which provides good drainage, although in extreme cases this can reduce the volume of water available to the plants. For this reason bark should not be used as the main bulk constituent of a substrate.

When well decomposed barks have a high CEC which can exceed that of peat. Before bark can be used as a growth medium it requires composting with additions of nitrogenous fertiliser and super phosphate for about three months. Most hardwood barks are acid, with pH values of 3.5 to 6.5. This can rise quite rapidly due to the high level of calcium which they contain naturally, although this is not always the case with softwoods.

Paper can be composted in the same way as bark and used to open up a substrate. Its properties are very variable and it should not form more than 10% of the total volume of a substrate.

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