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Somerset Archaeology and Natural History: the proceedings of the Somerset Archaeological and Natural History Society for 1989, Vol. 133, pages 231-242. It is quite difficult to find authentic fragments of this work, but we managed not only to process this historical find, but also to create a whole project dedicated to the study of the origin of plants in the Chu Valley. This project brought significant results to the field of natural science. If you intend to work in our research team, you can pay for letter of recommendation here https://elitewritings.com/buy-a-letter-of-recommendation.html.


During the two years 1988 and 1989, 305 walls in the Chew Valley were kept under observation, with the particular aim of assessing the relative frequency of different plant species on the various kinds of walls in the area. Only flowering plants and pteridophytes were studied. Stone bridges and some buildings were included in the survey but not quarries, cliff faces or any other semi-natural habitats. It seems that no comparable study has previously been made in the west of Britain, though Thompson (1925) recorded 104 species growing on pollarded willows and alders along our River Chew - a somewhat similar habitat - and West (1911) listed 78 species growing on a single wall near the sea in the extreme south west of Ireland.

The Chew Valley, in North Somerset and Avon, being the area drained by the small River Chew and its tributary streams, extends from above Chewton Mendip in the south to Keynsham in the north (where the Chew flows into the Avon), and from Marksbury and Emborough in the east to Compton Martin and Felton in the west, a total area of some 140km2. The altitudinal range is from 285m to 15m. The area is almost entirely rural with a few large villages but only one town, Keynsham. For this reason pollution is not an important factor in determining the composition of the flora: in any case the higher plants are much less affected by pollution than lichens and bryophytes. Large parts of the area have a high rainfall, the average on the Mendip Hills (where the Chew rises) being some 45 ins per annum, Chew Magna (in the centre of the valley) 38 ins, while down at Keynsham the total is probably about 32 ins.

Almost all the walls are of local stone and predominantly calcareous in nature - White Lias, Carboniferous Limestone, Dolomitic Conglomerate and Grey and Red Sandstone. Even where the stone is not calcareous, eg. Pennant Sandstone at Pensford, the mortared joints, where most plants grow, provide a localised calcareous environment. There are virtually no brick walls except for a very few railway bridges and in suburban roads in the southern districts of Keynsham. There are many miles of drystone walls, particularly at the higher altitudes.

The 305 walls studied were chosen with three criteria in mind: firstly, to cover the whole area (the initial aim being to select at least one wall in each 1km square): secondly, to include reasonable numbers of walls of particular kinds, eg dividing and retaining walls, river walls, churches, drystone walls etc - some of these categories may, of course, overlap; thirdly, to select, as far as was consistent with these two more important criteria, walls with a relatively rich flora, so as to obtain as large a sample as possible of plants colonising walls in the area. Needless to say, every wall selected had at least one plant species growing on it during some part of the survey period: as Kent (1961) noted in his paper on Middlesex walls, it was not uncommon to find on a later visit that a wall had been repointed or rebuilt to the detriment of the flora! Retaining walls with a substantial depth of soil on top were generally excluded from the survey, since plants growing on their tops could not reasonably be distinguished from the flora of the adjoining field or wood. For the same reason plants at the extreme base of a wall which appeared to be rooted in the ground were not recorded.

Except for churches (see below), the unit for the survey was either a single wall or, occasionally, a series of very short adjacent walls of identical construction and aspect (for example, a farmyard wall with a break for a gate in the centre). In the case of long drystone and other field walls a sample length of up to 50 metres was chosen.

As Darlington (1981) points out, the angle of inclination of a wall has an important effect on colonisation, in that the nearer to the horizontal the greater the range of plants. Partly for this reason the survey excluded the shelving river embankments along the lower Chew in Keynsham and also the sloping concrete and stone walls of the Chew Valley Reservoir. With the single exception of the aqueduct revetment in Harptree Combe, which is marginally out of the vertical, all the walls surveyed had vertical sides.

The nomenclature of plants used in this paper is that of Clapham, Tutin and Moore (1987), and to save space the names of authors are omitted.


A total of 291 species of flowering plants and ferns were seen on the Chew Valley walls, but of these, 72 (25%) occurred on only one wall, with another 42 (14%) or only two walls. The species are listed below, with the number of walls on which they were seen.

Taraxacum officinale agg  174    Arenaria serpyllifolia  9    Hordeum murinum  2
Urtica dioica  161    Bromus ramosus  9    Hyacinthus non-scriptus  2
Hedera helix  159    Potentilla sterilis  9    Hypericum hirsutum  2
Cymbalaria muralis  140    Adoxa moschatellina  8    Hypericum tetrandrum  2
Geranium robertianum  138    Agrostis capillaris  8    Lycopus europaeus  2
Galium aparine  136    Alopecurus pratensis  8    Malva sylvestris  2
Poa pratensis  135    Erophila verna agg  8    Melica uniflora  2
Asplenium ruta-muraria  123    Rumex obtusifolius  8    Moehringia trinervia  2
Dactylis glomerata  117    Sedum reflexum  8    Oxalis acetosella  2
Sambucus nigra  112    Sorbus aucuparia  8    Papaver somniferum  2
Rubus fruticosus agg  108    Anthoxanthum odoratum  7    Phleum pratense  2
Arrhenatherum elatius  94    Arum maculatum  7    Polygonum aviculare  2
Festuca rubra  92    Aubrieta deltoidea  7    Primula vulgaris  2
Anthriscus sylvestris  91    Cerastium tomentosum  7    Rorippa palustris  2
Asplenium ceterach  90    Chamaenerion angustifolium  7    Rumex crispus  2
Asplenium trichomanes  84    Corylus avellana  7    Sonchus arvensis  2
Bromus sterilis  81    Euphorbia peplis  7    Slellaria holostea  2
Sonchus oleraceus  74    Ribes uva-crispa  7    Torilis japonica  2
Asplenium scolopendrium  69    Rumex acetosa  7    Trifolium dubium  2
Epilobium montanum  65    Sedum album  7    Tripleurospermum inodorum  2
Lapsana communis  65    Symphoricarpos albus  7    Valerianella locusta  2
Holcus lanatus  64    Syringa vulgaris  7    Veronica filiformis  2
Lamium album  58    Veronica polita  7    Viburnum lantana  2
Cardamine hirsuta  57    Buddleia davidii  6    Acer campestre  1
Brachypodium sylvaticum  53    Chelidonium majus  6    Agrostis gigantea  1
Plantago lanceolata  50    Clematis vitalba  6    Arctium minus  1
Glechoma hederacea  44    Crepis vesicaria  6    Artemisia vulgaris  1
Veronica hederifolia  44    Hypochoeris radicata  6    Atriplex patula  1
Senecio vulgaris  42    Ilex aquifolium  6    Atriplex prostrata  1
Poa annua  40    Lamiastrum galeobdolon  6    A vena sativa  1
Polypodium interjectum  40    Leucanthemum vulgare  6    A venula pubescens  1
Lolium perenne  38    Parietaria judaica  6    Betula pendula  1
Alliaria petiolata  37    Plantago major  6    Bidens tripartita  1
Acer pseudo-platanus  35    Trisetum flavescens  6    Campanula cochlearifolia  1
Poa trivialis  35    Bromus erectus  5    Campanula poscharskyana  1
Sedum acre  34    Elymus caninus  5    Campanula trachelium  1
Veronica arvensis  34    Ligustrum vulgare  5    Cardamine impatiens  1
Geranium lucidum  32    Lonicera periclymenum  5    Carex divulsa  1
Crataegus monogyna  31    Mercurialis annua  5    Centaurea nigra  1
Fraxinus excelsior  31    Nasturtium officinale  5    Cerastium glomeratum  1
Galium mollugo  30    Ranunculus bulbosus  5    Chenopodium album  1
Ranunculus repens  30    Stellaria neglecta  5    Circaea lutetiana  1
Rosa canina agg  30    Allium ursinum  4    Cirsium palustre  1
Alnus glutinosa  28    Aphanes arvensis agg  4    Cotoneaster integrifolius  1
Myosotis arvensis  28    Aquilegia vulgaris  4    Cotoneaster buxifolius  1
Sonchus asper  28    Arabis caucasica  4    Cotoneaster franchetii  1
Angelica sylvestris  27    Asplenium adiantum-nigrum  4    Crataegus crus-galli  1
Saxifraga tridactylites  26    Bellis perennis  4    Epilobium obscurum  1
Epilobium ciliatum  24    Chaerophyllum temulentum  4    Euphorbia helioscopia  1
Leontodon autumnalis  24    Cheiranthus cheiri  4    Euphrasia nemorosa  1
Agrostis stolonifera  23    Fagus sylvatica  4    Fumaria officinalis  1
Bromus hordeaceus  23    Hieracium pilosella  4    Geranium rotundifolium  1
Mycelis muralis  23    Leontodon hispidus  4    Hieracium acuminatum  1
Cirsium vulgare  22    Ribes sanguineum  4    Hieracium trichocaulon  1
Lamium purpureum  21    Sempervivum tectorum  4    Hypericum androsaemum  1
Rumex sanguineus  21    Ulmus glabra  4    Hypericum calycinum  1
Silene dioica  21    Veronica persica  4    Hypericum perforatum  1
Veronica chamaedrvs  21    Ajuga reptans  3    Juniperus communis  1
Achillea millefolium  20    Apium nodiflorum  3    Lactuca serriola  1
Cotoneaster horizontalis  20    Arabidopsis thaliana  3    Linaria purpurea  1
Elymus repens  20    Carex pendula  3    Lolium multiflorum  1
Scrophularia auriculata  20    Chamaecyparis lawsoniana  3    Lonicera nitida  1
Cirsium arvense  19    Chrysosplenium oppositifolia  3    Lunaria annua  1
Heracleum sphondylium  19    Deschampsia caespitosa  3    Luzula campestris  1
Senecio jacobaea  19    Epilobium tetrandrum  3    Lycium barbarum  1
Dryopteris filix-mas  18    Euonymus europaeus  3    Malva moschata  1
Umbilicus rupestris  18    Hieracium maculatum  3    Melissa officinalis  1
Aegopodium podagraria  17    Kerria japonica  3    Milium effusum  1
Centranthus ruber  17    Lotus corniculatus  3    Myosotis sylvatica  1
Cerastium fontanum  17    Meconopsis cambrica  3    Nepeta x faassenii  1
Festuca ovina  17    Medicago lupulina  3    Nigella damascena  1
Oenanthe crocata  17    Prunella vulgaris  3    Phalaris canariensis  1
Poa nemoralis  17    Quercus robur  3    Phygelius capensis  1
Stachys sylvatica  17    Ribes rubrum  3    Polys tichum aculeatum  1
Epilobium parviflorum  16    Taxus baccata  3    Polystichum setiferum  1
Eupatorium cannabinum  16    Trifolium repens  3    Prunus domestica  1
Festuca gigantea  15    Verbascum thapsus  3    Ranunculus sceleratus  1
Mercurialis perennis  15    Viola reichenbachiana  3    Rubus idaeus  1
Convolvulus arvensis  14    Aesculus hippocastanum  2    Sagina procumbens  1
Calystegia sepium  13    Antirrhinum majus  2    Scrophuluria nodosa  1
Ranunculus ficaria  13    Aster lanceolatus  2    Sedum spurium  1
Corydalis lutea  12    Ballota nigra  2    Senecio squalidus  1
Epilobium hirsutum  12    Campanula portenschlagiana  2    Silene vulgaris ssp maritima  1
Geum urbanum  12    Capsella bursa-pastoris  2    Sison amomum  1
Tanacetum parthenium  12    Cardamine pratensis  2    Soleirolia soleirolii  1
Cardamine flexuosa  11    Carduus acanthoides  2    Solidago gigantea  1
Crepis capillaris  11    Carpinus betulus  2    Symphytum x uplandicium  1
Poa compressa  11    Cotoneaster x watereri  2    Thymus drucei  1
Rubus caesius  11    Cotoneaster sternianus  2    Tragopogon pratense  1
Solanum dulcamara  11    Cotoneaster dielsianus  2    Trifolium pratense  1
Cystopteris fragilis  10    Dipsacus fullonum  2    Tussilago farfara  1
Filipendula ulmaria  10    Draba muralis  2    Veronica agrestis  1
Fragaria vesca  10    Equisetum arvense  2    Veronica montana  1
Potentilla reptans  10    Euphorbia amygdaloides  2    Vicia sativa  1
Stellaria media  10    Geranium dissectum  2    Viola riviniana  1
Ulmus procera  10    Geranium molle  2    Vulpia myuros  1
Vicia sepium  10    Hordeum distichon  2    


The table below, which is based on a similar table in Kent (1961), compares the relative frequency of the 21 commonest wall plants in this survey with the frequency of some of them in earlier wall surveys in Wiltshire (Grose 1957), Middlesex (Kent 1961), London (Kent 1961), Essex (Payne 1978), Warwickshire (Cadbury et al 1971), Cambridge (Rishbeth 1948) and Durham (Woodell and Rossiter 1959).

Percentage occurrences (where known) in various surveys of the 21 commonest species in the Chew Valley survey

Essex Walls
Middx Walls
London Walls
Durham Walls
Warks Walls*
Camb Walls*
1  Taraxacum officinale  57  83  16  13  11  64    (2)
2  Urtica dioica  53  67  16  4        
3  Hedera helix  52  78  27  2      (2)  
4  Cymbalaria muralis  46  56  17  24  7    (3)  (5)
5  Geranium robertianum  45  50  <5  <!      (19)  
6  Galium aparine  45  56  12  <!        
7  Poa pratensis  44  61  7  5    23    
8  Asplenium ruta-muraria  40  50  <5  2      (1)  
9  Dactylis glomerata  38  44  11  1    40    
10  Sambucus nigra  37    19  6  1  41    (7)
11  Rubus fruticosus  35    15  2    23    
12  Arrhenatherum elatius  31    12  2        
13  Festuca rubra  30    <5  <!        (2)
14  Anthriscus sylvestris  30    7  <!        
15  Asplenium ceterach  30  63  <5  <!      (19)  
16  Asplenium trichomanes  28    <5  1        
17  Bromus sterilis  27  61  14  2      (19)  
18  Sonchus oleraceus  24  44  35  19  13      
19  Asplenium scolopendrium  23    10  4      (6)  
20  Epilobium montanum  21    <5  8  17  38    
21  Lapsana communis  21    5  <!        
   Total No of spp  291    286  204  83  168    186

* Order of frequency

It is hardly surprising that the Wiltshire figures, though based on a very small sample of walls, come closer to those in the present survey than any from more distant parts of England.

The most striking absentee from a high place in the Chew Valley list is Poa annua, which occurred on only 13% of our walls, compared with 56% in Wilts, 54% in Essex, 48% in Middlesex, 40% in Durham and 38% in London, while it topped the Cambridge list. Other surprising species are Dryopteris filix-mas on only 6% (35% in Middlesex, 28% in London, 22% in Essex), Chamaenerion angustifolium on 2% (53% in Durham. 24% in Middlesex and 12% in Cambridge), Senecio squalidus on only 1 wall (29% in Essex, 25% in Middlesex, 22% in London) and Sagina procumbens also on only 1 wall (18% in Essex, 17% in Middlesex, 10% in London).

Poa annua, Chamaenerion angustifolium, Senecio squalidus and Sagina procumbens are all plants primarily of artificial, even urban, habitats, which are relatively infrequent in the pastoral Chew Valley, so there may be a comparatively small reservoir of populations of these species. Bolton (1985) found the Sagina abundant on walls in Exeter (an urban area), though Darlington (1981) refers to it as a common mural species generally.

The scarcity of Male Fern is harder to explain.

There may be two other factors to consider. The Chew Valley has a relatively heavy rainfall, which perhaps leads to annual and small species like P. annua and Sagina being overwhelmed by perennials, such as nos. 11, 12, 13, 14 and 21 on our list. And since many of the limestone walls in the lower parts of the Valley are surrounded by soils with low lime content, local races of plants may not readily adapt to the lime of the walls.


Drystone walls, which of course contain no mortar, initially offer little or no nutriment to the higher plants, but when they have been colonised by lichens and then mosses, some flowering plants and ferns soon appear and the drystone walls of the upper parts of the Chew basin support a flora which is to some extent distinctive. A factor in this distinctiveness may be the higher rainfall on Mendip.

The commonest species on the 40 drystone walls surveyed were as follows, with the percentage of such walls on which they were noted:-

Geranium robertianum  88%
Galium aparine  85
Urtica dioica  78
Arrhenatherum elatius  73
Taraxacum officinale agg  70
Poa pratensis  68
Dactylis glomerata  63
Hedera helix  63
Festuca rubra  58
Polypodium interjectum  58
Cardamine hirsuta  53
Asplenium trichomanes  50
A. ceterach  48
Anthriscus sylvestris  48
Geranium lucidum  48
Saxifraga tridactylites  45
Sedum acre  45
Holcus lanatus  35
Lapsana communis  33
Bromus sterilis  30
Cymbalaria muralis  30

Species which showed a statistically significant preference for drystone walls (on the basis of the test) were:-
chi² value

   chi² value
Polypodium interjectum  75.17
Saxifraga tridactylites  73.26
Geranium lucidum  62.68
Sedum acre  49.40
Arrhenatherum elatius  35.30
Cardamine hirsuta  32.13
Geranium robertianum  31.79
Galium aparine  28.57
Festuca rubra  14.87
Crataegus monogyna  13.05
Urtica dioica  10.45
Asplenium trichomanes  10.38
Dactylis glomerata  10.20
Poa pratensis  9.02
Anthriscus sylvestris  5.92
Lolium perenne  5.38

On the other hand, the following two species showed a marked relative aversion to
drystone walls :-

Asplenium ruta-muraria  8.91
Asplenium ceterach  6.20

[The statistical tests were performed by constructing 2 x 2 contingency tables (species present or absent vs. selected wall category or other) and calculating chi² with one degree of freedom. Yates’ correction for continuity was applied, but in view of the large number of observations it made virtually no difference.]


In this category are included vertical walls embanking the River Chew and its tributary streams, as well as revetments of stone bridges over streams. Only plants growing on the sides of these walls were recorded, ie those most likely to be influenced by the permanent proximity of water.

The commonest species on the 69 river walls were:-

Urtica dioica  64%
Asplenium ruta-muraria  55
Cymbalaria muralis  49
Taraxacum officinale agg  46
Asplenium scolopendrium  45
Rubus fruticosus agg  45
AInus glutinosa  41
Asplenium trichomanes  38
Brachypodium sylvaticum  38
Geranium robertianum  36
Hedera helix  36
Anthriscus sylvestris  33
Angelica sylvestris  29
Scrophularia auriculata  29
Oenanthe crocata  25
Dactylis glomerata  23
Holcus lanatus  22
Sambucus nigra  22
Galium aparine  19
Poa pratensis  19

Species which showed a significant preference for river walls were:-

   chi² value
AInus glutinosa  85.04
Scrophularia auriculata  61.00
A ngelica sylvestris  52.14
Oenanthe crocata  50.64
Epilobium hirsutum  33.89
Filipendula ulmaria  24.16
Brachypodium sylvaticum  23.62
Eupatorium cannabinum  22.84
Cardamine flexuosa  16.89
Rumex sanguineus  13.24
Nasturtium officinale  11.59
Parietaria judaica  8.28
Tanacetum parthenium  6.56

Species with a relative aversion to river walls were:-

Asplenium ceterach  17.81
Hedera helix  8.91
Sonchus oleraceus  7.69
Sedum acre  7.67
Geranium lucidum  7.28
Cardamine hirsuta  7.08
Festuca rubra  5.20
Senecio vulgaris  5.01


In an attempt to show the influence of a shaded situation on the composition of a wall flora. 32 walls overhung by trees were selected. The selection was often difficult, in that a wall might well be under dense shade in summer but more or less exposed in winter when the overhanging trees were leafless. To that extent any conclusions drawn from this part of the survey are probably of less value than those relating to other, more clearly defined categories of walls.

The commonest species on the 32 shaded walls were:-

Urtica dioica  88%
Galium aparine  78
Geranium robertianum  75
Hedera helix  66
Taraxacum officinale agg  63
Aspleniiim scolopendrium  61
Rubus fruticosus agg  50
Brachypodium sylvaticum  47
Alliaria petiolata  44
Anthriscus sylvestris  44
Asplenium trichomanes  44
Poa pratensis  44
Sambucus nigra  41
Lapsana communis  35
Dactylis glomerata  33
Crataegus monogyna  31
Cymbalaria muralis  31
Epilobium rnontanum  31
Polypodium interjectum  31
Rosa canina agg  31

Species showing a significant preference for shaded walls were:-

   chi² value
Adoxa moschatellina  51.76
Mercurialis perennis  44.00
Alliaria peliolata  30.30
Asplenium scolopendrium  27.94
Festuca gigantea  23.80
Brachypodium sylvaticum  21.48
Urtica dioica  16.56
Rosa canina agg  15.89
Bromus ramosus  15.41
Crataegus monogyna  14.97
Galium aparine  14.79
Geranium robertianum  11.47
Fraxinus excelsior  10.53
Polypodium interjectum  8.62

Perhaps surprisingly, the species showing the most marked aversion to shaded walls was a fern:-

Asplenium ruta-muraria  12.83


All the old parish churches in the Chew Valley had plants growing on their outside walls: for the purpose of this survey each church was treated as a single unit. Church-yard walls were not included in this category. With the addition of five 19th-century nonconformist chapels, 27 churches were surveyed.

Of the 63 species noted on church walls (including their towers) the commonest was Sambucus nigra, which occurred on no less than 20 of the 27 churches. The dispersal of Elder by birds no doubt accounts for the frequent seedlings seen high up on church towers. Other common species were:-

Asplenium ruta-muraria  59%
Hedera helix  59
Rubus fruticosus agg  37
Taraxacum officinale agg  33
Poa annua  30
Epilobium montanum  26
Festuca rubra  26
Cardamine hirsuta  22
Cymbalaria muralis  22
Arrhenatherum elatius  19
Geranium robertianum  19
Senecio vulgaris  19

Only Sambucus nigra showed a significant preference for church walls (chi² value 16.07). On the other hand, three species were significantly less frequent on

Geranium robertianum  7.48
Taraxacum officinale agg  5.78
Cymbalaria muralis  5.68


Several farm buildings, mainly old barns, were included in the survey, together with two tall disused chimneys: 17 stone structures in all.

Of the 50 species noted from these buildings, the most frequent were:-

Urtica dioica  47%
Bromus sterilis  29
Dactylis glomerata  29
Hedera helix  24
Asplenium scolopendrium  24
Taraxacum officinale agg  24

The data are insufficient to warrant any further comments.


In a similar survey of the wall flora in Essex some years ago (Payne 1978) it was found that the walls of railway bridges and stations had a distinctive flora. In the Chew Valley, however, apart from the main Bristol to London line that runs through Keynsham at the extreme northern tip of our area, the one railway that formerly extended for some 4 miles through the Valley, the Bristol and North Somerset, was finally closed in 1968. Although every accessible bridge and other structure was examined - with the exception of Pensford Viaduct, access to which proved to be impracticable - there is little evidence that any distinctive flora has survived the 20 years of disuse. It may just be significant that the commonest plant overall was a fern, Asplenium ruta-muraria, which occurred on no less than 90% of those railway walls which supported any of the higher plants, and that one old bridge near Pensford still harbours one of the very few colonies of Asplenium adiantum-nigrum in the valley. (Hedera and Taraxacum came a poor joint second to Wall Rue). Railway walls, particularly those at stations, have long been known to botanists as a favourable habitat for ferns, presumably because of the damp micro-climate brought about by the frequent discharges of steam from locomotives in earlier times: once established, ferns tend to persist in such sites long after the demise of steam trains.


Some 66 stone garden walls were included in the survey. Only the outside faces of such walls were recorded, since it is unlikely that plants would have been deliberately introduced there. (In any case, as Clement (1984) has pointed out, it is far from easy to establish a plant on a wall). Even so, a high proportion of garden plants occurred on these walls, presumably self-seeded or dispersed from the adjacent gardens by birds. It is hardly surprising that all the occurrences of the following species were on walls backing on to gardens: -

Antirrhinum majus
Aquilegia vulgaris
Arabis caucasica
Aubrieta deltoidea
Campanula poscharskyana
Crataegus crus-galli
Euphorbia helioscopia
[a common garden weed]
Hypericum calycinum
Juniperus communis
Kerria japonica
Linaria purpurea
Lonicera nitida
Lunaria annua
Lycium barbarum
Melissa officinalis
Nigella damascena
Papaver somniferum
Phygelius capensis
Sedum spurium
Sempervivum tectorum
Soleirolia soleirolii
Solidago gigantea
Syringa vulgaris

On the other hand, only half the records of Cheiranthus cheiri and Centranthus ruber were from garden walls, while in the genus Cotoneaster 11 out of the total of 29 records were on walls well away from gardens, doubtless dispersed through the agency of birds feeding on their berries. Other species originally introduced into England as garden plants but which are increasingly becoming established outside gardens, and which occurred on non-garden walls in this survey, are Buddleia davidii. Corydalis lutea. Leycesteria formosa and Meconopsis cambrica.


A comparison was made between the floras of dividing and retaining walls. Retaining walls, being backed up by a deep mass of earth, tend to retain moisture and nutrients to a much greater extent than isolated dividing walls, and might therefore be expected to harbour a different association of plants.

The following species showed a significant preference one way or the other:-

Dividing Walls

   chi² value
Poa pratensis  25.34
Dactylis glomerata  20.90
Saxifraga tridactylites  15.17
Cymbalaria muralis  12.62
Sedum acre  12.06
Asplenium ceterach  10.65
Galium aparine  9.21
Bromus hordeaceus  8.96
Cotoneaster horizontalis  8.73
Sonchus asper  8.28
Geranium lucidum  8.15
Bromus sterilis  7.91
Myosotis arvensis  6.26
Senecio vulgaris  6.05
Poa compressa  5.68
Veronica arvensis  5.46
Poa annua  5.46

Retaining Walls

   chi² value
Alnus glutinosa  25.16
Scrophularia auriculala  24.12
Brachypodium sylvaticium  22.82
Oenanthe crocata  21.72
Angelica sylvestris  20.44
Eupatorium cannabinum  9.53
Epilobium ciliatum  8.06
Rubus fruticosus agg  7.12

However, it should be noted that some of the species showing a preference for retaining walls are markedly associated with river walls, which of course were all of this type.


The more or less flat top of a wall clearly receives more light and rain than the vertical sides, and accumulates more debris which leads to the formation of rudimentary soils. Fruits and seeds dropped or evacuated by birds are much more likely to arrive on the top of a wall than on its sides. It seemed probable, therefore, that certain species would tend to be found on the exposed tops of walls, while shade-loving plants and those not primarily dispersed by birds would be more at home on their sides. To this end, separate records were kept of occurrences of species on tops and sides, and the following species were found to show a significant preference for one or the other habitat:-


   chi² value
W Poa annua  18.15
B W Dactylis glomerata  18.09
B Cotoneaster horizontalis  15.26
B Plantago lanceolata  14.93
W Bromus hordeaceus  12.86
W Arrhenatherum elatius  12.34
B Crataegus monogyna  12.33
B Sambucus nigra  12.29
B Galium aparine  11.67
B W Taraxacum officinale agg  9.72
W Fraxinus excelsior  8.68
W Lolium perenne  8.15
B W Senecio vulgaris  7.44
W Poa pratensis  6.85
W Alopecurus pratensis  6.26
Veronica arvensis  6.17
W Cirsium arvense  5.29


   chi² value
Asplenium ceterach  75.76
A. ruta-muraria  68.64
Hedera helix  51.39
Asplenium trichomanes  50.38
Cymbalaria muralis  29.27
Asplenium scolopendrium  20.74
Rubus fruticosus agg  15.85
Umbilicus rupestris  8.87
Urtica dioica  8.64

B Known to be dispersed by birds
W Known to be dispersed by wind
(Cannon 1957, Ridley 1930, Salisbury 1961)

The high proportion of grasses preferring wall tops is notable, while ferns dominate the other side of the table (with very high chi² values).


Darlington (1981) draws attention to ways in which the aspect of a wall may affect plant life growing on it. Thus a south-facing wall is exposed to desiccation by direct insolation, while on a north wall water will remain longer because the sun's rays reach it only peripherally and temperatures are less extreme on a north wall, sc plant life may often be more luxuriant. Records were kept of the aspect of each of the walls in the survey, but it is disappointing to have to admit that very few marked preferences appeared to be shown, perhaps because of the relatively high rainfall in the area. However, on the basis of the chi² test Dryopteris filix-mas, Asplenium trichomanes, A. scolopedrium and Angelica sylvestris showed a significant preference for north-facing walls as against all other aspects, while Sonchus oleraceus very markedly disliked the north aspect.


Although the height above sea-level of each wall was noted, the results of the survey suggested that the overall range of only some 270 metres was insufficient to constitute a significant factor in determining the wall flora.


Ivy (Hedera helix), which more or less covers a high proportion of walls in the area, often smothering other plants, was only recorded when it appeared to be established in crevices without making contact with the ground.

Brambles (Rubus fruticosus agg) are always difficult for the non-specialist to identify, and the difficulty is made worse when they are growing on walls, because such plants normally fail to produce flowers or fruit. The same problems arise with wild roses, which I have treated as Rosa canina agg.

Special attention was paid to grasses and ferns. Poa compressa is generally regarded as a typical grass of old walls, and P. nemoralis as a woodland species: indeed Segal (1969) says that nemoralis is hardly ever found on walls. Nevertheless on Chew Valley walls nemoralis was almost twice as frequent as compressa, showing however some preference for the sides of walls (the more shaded position), while compressa evinced a marked preference for the tops. The surprising infrequency of Poa annua has already been commented on. Poa subcoerulea and P. angustifolia have both been recorded as typical wall species, the latter particularly in Eastern England, but neither was seen during the present survey.

Elymus caninus may seem a surprising colonist of walls, but each of the five walls on which it was noted adjoined either a wood or a shady hedgerow in which the grass was also growing. The seeds of Melica spp are dispersed by ants (Ridley 1930, Segal 1969), and this may explain the presence of M. uniflora on two walls.

It is difficult to account for the total absence of Desmazeria rigida from the Chew Valley walls, a grass described as common on limestone walls in Wiltshire (Grose 1957) and a wall-top plant as near to the survey area as Wells.

Asplenium adiantum-nigrum was relatively much scarcer in the Chew Valley than I had found it in Essex (Payne 1978), perhaps because it may be more partial to brick walls (Page 1988).

Polypody, unlike all the other ferns in the survey, seemed equally happy on the tops of walls as on their sides, although showing some preference for shaded sites. Because of the practical impossibility of separating in the field the three species (and three hybrids) into which this genus is now divided, I have treated all the wall Polypodies as P. interjectum, which is by far the most abundant of the segregates in this area.


Special thanks are due to Mr S.M. Taylor, who helped me greatly with statistical
calculations; Mr M.W.J. Paskin and Mr A.G. Smith also discussed statistical matters with me. I am grateful for help in identifying plants to Mrs J. Fryer and Mr J.R. Palmer (Cotoneaster), and to Mr J.G. Keylock and Mr R.D. Randall (mainly Hieracium). Mr I.M. Kinchin introduced me to several relevant papers dealing with wall flora. Dr K. Allen advised me on the geology of the area, and Mr R.A. Janes, Mr Paskin and Mr A.C. Titchen made useful comments on early drafts of the paper.


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