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January, 1956

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Most, if not all, of our native broadleaved trees have occurred naturally on old spoil heaps. As one would expect, birch has been one of the most important pioneers of both burned and unburned spoils, being more typical of the former where more demanding species appear slowly. Many unburned heaps in favourable situations appear to be proceeding quite normally to oak forest, via typical seral communities of hawthorn, ash, etc. Naturally what can appear is determined by the local seed supply. But there is usually little to suggest any marked abnormality in the spoil heap environment; small heaps in rural coalfields frequently merge completely into the local surroundings and may be difficult to recognise at first sight. No doubt fumes, population pressure in all its aspects, and the exposure all combine to keep the large modern heap in active districts free from vegetation.

Tree planting on spoil heaps in this country has quite a long history, though the activity has been rather sporadic and few plantations exist which have had constant care since planting. Examples will be found in most coalfields, particularly successful ones in the Somerset and Dean districts. The records and surviving work of the Midland Reafforesting Association (most of their plantations have been destroyed) are of value as indicating a number of species which will tolerate extreme atmospheric pollution. Many of the sites planted by this body were probably worse than average, they were mainly old low mounds long settled to heavy clays. No example of planting on the taller modern heaps has been recorded, otherwise heaps of many different shapes and sizes have been attempted. There are perhaps some 100 examples of tree planting on deep-mined spoil in the country. They do not certainly sample all the possible conditions, and all possible species have not been tried, but taken together they present quite an encouraging picture. Few failures have been noted which are not ascribable to :—

(1) Vandalism, or damage by animals.

(2) Incorrect choice of species, usually with regard to atmospheric pollution.

These are remediable causes of failure. Trouble has also been noted from exposure, which is not, of course, remediable. The general appearance is not of any very critical restriction of species due to the special character of the heaps.

Of the conifers, those preferring or best tolerating near-neutral conditions seem to have been most successful; European and Japanese larch, Corsican and Austrian pines have all done well. Scots pine, an adaptable species, has been widely used with some success; it has however compared unfavourably with Corsican and Austrian pines where occurring on the same heap.

The near-neutral reaction of fresh spoils certainly favours broadleaved species, and a number which are considered to have high nutrient demands have grown very satisfactorily, notably common alder, sycamore, ash and poplar. There is not enough evidence to show whether age of spoil or burning can produce wholly unfavourable conditions for these species; it does not seem to be the case. Oak has not been widely planted but has frequently appeared naturally and grown well enough. Elm and lime have also performed well. Alder seems to be specially suited to these sites, as it is to other "raw" soils produced by deep or opencast mining. The usual explanation given is that its nitrogen-fixing capabilities favour it on soils otherwise suitable but deficient in organic matter. It has been widely used for this purpose on the continent, usually in mixture. Common alder has succeeded well on spoil heaps here, and grey alder (Alnus incana) has done well also. It seems very likely that Alnus cordata, which is a very promising exotic on the chalk and also on certain clays, would do well on spoil heaps.

Many other species have been recorded, such as willows, false acacia, rowan, hawthorn, etc. The position is obviously quite open, though the abovementioned species may be regarded as tried subjects. It would seem to be a sound policy to use alders in considerable proportion on all heaps which are reasonably moist or which are weathering to clayey material; likewise burned heaps and lighter materials call for a high proportion of birch. Alder and birch may be regarded as the safest pioneers. With them sycamore may be used freely on all sites. Poplars can be planted where it is certain that there is adequate moisture; they should probably not be carried at all high on the slopes, and may best be confined to screens round the base. Care should be taken to choose canker-resistant varieties; reliable clones of Populus serotina, gelrica, robusta and eugenii are now available.

There is no reason why other broadleaved species should not be used on a limited scale. It seems likely that nothing will attain respectable tree stature high up on tall conical heaps. and if cover is desired it may be necessary to look for something of shrub habit. Hawthorn might fill the role as well as anything; it will at least be inexpensive to try.

What kind of growth can be expected on spoil heaps? It is necessary to view any quoted figures with considerable caution, and the following selected records are not to be regarded as average for any type of heap. They are however examples of height growth on colliery spoil, and serve to indicate the respectable rates of growth obtainable on this material. As has already been mentioned, there is little or no experience of planting on the modern tall conical heaps. Many of the plantations have achieved very respectable rates of growth, and it seems reasonably clear that exposure and smoke pollution are the principal adverse factors operating where growth has been poor.

Preparation of site and planting

It has already been mentioned that there is likely to be no advantage in waiting long periods before planting. There is some generality in this; it has been the experience in afforesting other types of spoil. Of course a deep mine heap is a gradual accretion, unlike for instance an opencast bank, and parts of the heap may be very old before tipping is discontinued elsewhere.

Levelling operations will not usually provide better conditions for tree growth. It has frequently been observed that spoil material of various kinds is best left as it falls. Levelling may bring about packed conditions with poor aeration and drainage. This will not apply so much to the removal of peaks and sharp ridges on large heaps — which may well be a desirable operation — but there is no advantage in removing odd convolutions or in spreading a small heap out to form a low plateau.

It is plain enough that major gully formations must be stopped, but this can easily enough be done by classical methods. Many gullies could obviously be treated quite cheaply by forming small dams of fencing stakes driven in a few inches apart. Erosion will in any case be minimised once some woody cover has been established.

Planting methods have varied considerably. The records of certain old plantations show that the planters took a pessimistic view of the spoil material and transported soil to the planting points. Very large ball plants have sometimes been used. Equally successful results on what appear to be similar sites have been obtained using ordinary forest planting methods and stock; for example mattock planting with the minimum excavations to take roots comfortably with one-plus-one or two-plus-one transplant stock. Manures have been used, basic slag for instance, but again the assumption that these sites are difficult does not seem to have been borne out.

In general one should use normal forest stock, certainly not especially large stock which is apt to blow about, to rock to and fro, and to saw itself off at the neck. It is true that heaps with many large shaly pieces will be difficult to plant. Rather than transport soil however, it is worth considering the use of seedlings raised specially in some form of fibre or cardboard pot which can simply be planted, and then left to disintegrate in position.

Large stock should be confined to poplars or other trees used for screens to be planted low down on the slopes.

General conclusions

Since the plantations seen have by no means covered all the possible combinations of conditions to be met with in the coalfields, and since relatively few have been managed to bring out their full possibilities, it is unsafe to generalise about the capabilities of colliery waste sites. The impression is however that many, particularly the rural ones, are capable of producing respectable crops; and many more, in fact most, are capable of bearing tree growth, always with the vital proviso that plantations can be protected from human depredation.

The main conclusions may be summarised as follows :—

(1) Spoil heaps are usually potentially fertile, and can also provide sufficient moisture to support tree growth.

(2) Difficulties due to instability and erosion are not usually excessive, though eroding heaps of very fine materials such as washery waste may be impossible to plant without special measures to check gully formation.

(3) Actual toxicity is probably rare, and when it occurs often quite local in distribution.

(4) The very high modern conical bings (as dirt heaps are called in Scotland) may present special difficulties largely due to exposure on higher slopes.

(5) The choice of planting species is firstly governed by the local conditions, particularly by the smoke pollution factor, which may rule out all conifers. Secondly there may be some further restriction to less demanding species on burned heaps, and older more acid heaps.

(6) Special measures such as levelling and the importation of soil are not usually warranted; nor is the use of extra large stock, which may, in fact, give poorer results than smaller plants.

(7) Where there has been a failure to establish satisfactory plantations, vandalism has been the main cause.

Drawings and Photographs accompanying the article