Caithness Field Club

Caithness Field Club Bulletin
1978 - October

THE WILD PLANTS OF THURSO RIVERSIDE
J. K. Butler

The first mile of the Thurso riverside, stretching from the sea to the southern end of the Salmon Pool, is a fine recreation area for the town, and at the same time has the remnants of a very good northern river valley flora, which could be completely lost if it is not better appreciated. This detailed description will be useful to people who walk in the area, and may help others to realise its value. It is a valley cut into clay till which is some 30 metres deep, so that the river runs at bedrock level and most of the vegetation is on the alluvial valley floor or on the steeply sloped rich calcareous clay banks.

The river arises deep in the Caithness peat-moors which drain to Loch More. At Westerdale the ground becomes fertile clay till which is intensively farmed, so that fertiliser products wash into the water over the rest of the river's length, enriching it with nitrogen, phosphorus, potassium and calcium. This water pervades the alluvial floodplain in times of spate helping to maintain a fertile valley floor. The clay slopes also provide nutriment by way of underground springs which wash mineral salts out of the rich clay.

An important feature is the shelter from winds and salt spray provided by the valley, allowing a shrub and tree community to develop when it might not do so on the open plain.

I have divided the valley into sections bounded by the bridges across the river, and given the bridges obvious names: starting at the sea the "seaward footbridge" is followed by the "main bridge", then the "mill footbridge" and finally the "cemetery footbridge". Upstream of the cemetery footbridge is a low weir at the inlet to the mill stream and upstream from the weir the "salmon pool" extends for 500 metres to the bend in the river which marks the end of the area under consideration.

The English names of plants correspond with those found in popular books and those used in the Field Club booklet "Wild Flowers of Caithness".

The Banks of the Lower Reaches
By the sea the banks have been recently modified including the spreading of foreign grass seed, and the immature vegetation is changing rapidly. The harbour has the uncommon grass Puccinellia capillaris which occurs in similar situations all along the north coast. By the bus-park there is frequent dumping of rubbish and waste soil, so the poisonous hemlock, the unusual grass Agropyron repens var. aristatum, the slender speedwell and the green alkanet are all established introductions to be seen here. Upstream of the seaward footbridge the grassy west bank has bindweed as a prominent introduction while the east bank has planted trees and chicory. Beyond the main bridge the west bank vegetation is long-established and is a typical mixture of the natural and the introduced plants of a place that man has interfered with over many years. The trees and bushes - elm, elder, snowberry, horse-chestnut, willow, aspen. and Japanese knotweed are introduced, as are the white butterburr, bindweed, opium poppy, fireweed, Russian comfrey, chicory, slender speedwell and alkanet. No doubt the flag iris, nettle, meadow foxtail, rough meadow-grass, hedge parsley, sorrel, timothy-grass and bramble were there originally.

The River, the Mill Stream and the Ponds
The main river bed has only a small plant population because it is stony and the river is fast flowing. It can sometimes get covered with blanket weed in years of low rainfall. But more often the unbranched bur-reed is to be seen isolated in mid stream and a search below the water level will reveal alternate water-milfoil and pondweeds Potamogeton alpinus and P. berchtoldii. The margins of the river have more plants in the mud: the common ones are the common spike-rush, jointed rush, sharp-flowered rush, common sedge and the water sedge (Carex aquatilis). There are unconfirmed reports of the Wick sedge growing among this water sedge community.

The mill stream is slower flowing than the river and has a muddy bottom supporting a thick tangle of vegetation. There is branched and unbranched bur-reed, the pondweeds Potamogeton berchtoldii, P. natans and P. x nitens, with watercress, water forget-me-not and occasional flag iris.

The mill stream provides the water supply for the boating pond and then the small pond by the main bridge. The boating pond is scraped clean quite frequently but the least bur-weed and the pondweed Potamogeton x nitens have nevertheless become established there. The small pond usually has a community of bulbous rush and toad rush of variable extent.

The Floodplain
The level ground of the valley floor is made up of an alluvial deposit of sand, clay and gravel, and the degree of drainage is the most important factor in determining what grows on it. There are areas of floodplain between the mill footbridge and the cemetery footbridge, on the east bank covering a large area and on the west bank a lesser area; on the west bank by the salmon pool the natural vegetation has been so much altered by grazing that it is of no botanical interest.

The wettest areas are often flooded, especially where the mill stream spills over its banks. The dominant species are reed canary-grass, meadow-sweet, flag iris, common marsh-bedstraw, and great willowherb forming a tall-herb community. Wet areas without tall herbs have marsh marigold, water-mint, marsh pennywort, water forget-me-not, jointed rush, marsh horsetail, brooklime, bottle sedge, lesser spearwort, bog stitchwort and common valerian.

The drier parts of the floodplain form a herb and grassland community of considerable diversity. There are areas dominated by few species such as the yorkshire fog and tufted hair-grass patches, but mostly mixed groups occur with silverweed, sheep's fescue, common sedge, clover, crested dog's-tail, knapweed, northern marsh-orchid, common mouse-ear, goosegrass, creeping bent, hedge parsley, hogweed, lesser stichwort, cocksfoot, ribwort plantain, bush vetch, tufted vetch, daisy, false oat-grass, creeping thistle, sorrel, yellow rattle, birdsfoot trefoil, greater plantain, pignut, creeping buttercup and milfoil. One special plant of this area is the shady horsetail (Equisetum pratense) on the east bank in its only known locality in the county.

At the south-east corner of the salmon pool there is a fragment of floodplain which is rather different from the rest in having mare's-tail, amphibious bistort, water horsetail, unbranched bur- reed and marsh ragwort as well as the commoner species. It also has a hybrid foxtail (Alopecurus pretensis x geniculatus) which is rare in Britain, and which was first noticed by the riverside more than 100 years ago. It is surviving quite robustly in spite of severe grazing and trampling.

The river banks in the floodplain areas are similar to the wet or dry floodplains (depending on the height of the bank), but there are some extra plants which favour the banks, particularly globeflower, water avens, and the monkey flower (Mimulus). One might wonder about the identity of the monkey flower, for it produces no seed and has to propagate long the river by pieces breaking off and taking root downstream. It seems to be the hybrid Mimulus guttatus x luteus which has been cast out from gardens. The marsh ragwort and the rampant hybrid woundwort Stachys x ambigua (= Stachys palustris x sylvatica) both prefer the riverbank.

The Railside Wood
From the mill footbridge to the cemetery footbridge, on the west bank, extends the railside wood covering the steep clay bank of the valley. It is all planted woodland of uncertain age, consisting mainly of sycamore and ash with some rowan, hazel, whitebeam, alder, hawthorn and horse-chestnut. The overhead canopy is so dense in summer that little sunlight penetrates and the ground beneath is bare or sparsely covered. In a natural wood there would be bulbous spring flowers, but here there are none. Part of the area has been colonised by the white butterburr which is a garden escape which has colonised the cinder-strewn upper slopes where it can creep without hindrance. throwing up white flowers in February and broad coarse leaves in summer. The margin of the wood is more interesting, with hedge parsley, nettle, hogweed, sorrel, bush vetch, false brome-grass, wild angelica, honeysuckle, goosegrass, bramble, hedge woundwort, meadow-grasses, wild roses, willow, red campion, bracken, meadowsweet, meadow vetchling and wood avens. The wood avens cross breeds with the nearby water avens to produce a range of peculiar offspring.

The Clay Slopes
The steep clay slopes are quite different from the valley bottom. The calcareous mineral content of the clay varies and the amount of flushing of the clay from underground springs changes the soil wetness. These flushes turn the wet clay to a pasty grey colour due to the high calcium content, and the characteristic plants associated with then are Grass of Parnassus, glaucous sedge, marsh horsetail, heath spotted-orchid, butterwort, quaking grass and, just occasionally, bog pimpernel. The flushes spill down to the valley floor to enrich the floodplain, and the characteristic vegetation may follow.

The drier slopes have more complex vegetation, for they are by nature scrub woodland but restrained by grazing, burning and trampling. On the west bank, just upstream of the cemetery bridge, the first slope lacks a distinct spring, and has a rich coating of vegetation. Shrubs are dominant, especially hazel, eared willow, gorse and creeping willow. Herbs include hawkweed, lady's bedstraw, fragrant orchid, false brome, water avens, milkwort and slender St. John's Wort. The second slope upstream has a very thick canopy of shrubs and trees, mainly aspen, hazel, gorse, sycamore, burnet rose and honeysuckle. There are flushes in the centre and short dryer vegetation on the side slopes. The herb community includes harebell, yellow rattle, clover, ribwort plaintain, birdsfoot trefoil and meadow vetchling. The third slope upstream has some small shrubs in the lower part, while the upper part is thickly covered with cinders from the railway. The cinders are invaded by common toadflax, which is common on railway cinders further south, but rare in the county.

The other large extent of clay slopes is on the east bank and spans the distance from the mill to the south end of the salmon pool. From the mill it extends along the side of the mill stream and this slope is either cultivated or dominated by gorse, so lacks interest. Beyond the mill stream entrance the slopes have a short turf with low shrubs similar to that of the first and second slopes of the west bank, with dominant gorse, yellow rattle, cocksfoot grass, knapweed, lesser stitchwort, harebell, tufted hair-grass, lady's bedstraw, bracken, devil's bit scabious with occasional agrimony and columbine. One part has dense hazel scrub with hawthorn, aspen, burnet rose, false brome and Smith's pepperwort.

Saltmarsh
There is very little saltmarsh in the county, and the small patches in the river estuary are the more important because of it. The influence of salt can be seen on the vegetation for some distance past the main bridge, with sea milkwort and scurvy grass occurring in the bank vegetation. There are small patches of saltmarsh below the bus-park and a series of saltmarsh islands in the river between the seaward footbridge and the main bridge. The expected binding grass (Puccinellia maritime) is replaced by red fescue, and the other plants are orache (Atriplex glabriuscula), scurvy grass, sea plantain, sea milkwort, lesser sea-spurrey and spike-rush.