Swinsty Reservoir

Saturday 18^th August 2018

Leader: Brian Byrne

Our focus today was on examining the Willows (Salix) along a stretch of Swinsty reservoir. We last visited this site almost exactly a year ago, where a full species list was made. Brian gave an introductory talk in the car park emphasising the difficulties of identifying Willows in the field due to extensive hybridisation. Fertile hybrids can backcross with one of the parent species or with other Willow species to produce plants with a combination of characteristics of the various parents. Brian thought that in many cases it is only possible to say you have a specimen with characteristics of, for example, Eared Willow, Dark-leaved Willow and Tea-leaved Willow (rather than saying that you definitely have a triple hybrid involving those species). An additional complication is that sometimes it is necessary to see characteristics of catkins, young leaves and mature leaves, requiring visits to see the plant in both spring to see the catkins and late summer/early autumn to see the mature leaves. Brian also said it was important to take care with which leaves are examined, for example, looking at branches with at least two years growth and avoiding sucker growth or shoots growing in deep shade, as these can show abnormal growth. The unusually dry summer this year may also have affected growth. One of Brian’s closing remarks was ‘Don’t imagine you can identify every willow you find’.

We then proceeded to examine the Willows along the reservoir edge. Brian was of the opinion that many of the willows here are hybrids, some showing characteristics of two parents, others showing characteristics of three or possibly even four different parents. These parents included Eared Willow (Salix aurita), Dark-leaved Willow (S. myrsinifolia), Tea-leaved Willow (S. phylicifolia), Grey Willow (S. cinerea) and Osier (S. viminalis). With the exception of one or two species such as Osier, few of the pure parent species were conclusively found here today.

If the specimen has shiny red twigs, Brian informed us this indicated it was a hybrid involving one or both of Tea-leaved Willow and Dark-leaved Willow. For example, a specimen we found with shiny red twigs showing characteristics of Tea-leaved Willow (leaves shiny on the upper surface and thick to the touch) but also showing characteristics of Grey Willow (underside of the leaf with some rusty red hairs) could be the hybrid between these two species.

At times there were passionate debates as to the identification of a willow we found here. Some members of the group were of the opinion that if it shows the characteristics of say 90% Eared Willow, they would record it as Eared Willow, however others, such as Brian, felt it important to list the potential other parent or parents. Do any specimens of these pure Willow species exist or are they all complex hybrids? How much variation is there within each species? These questions are clearly not easy to answer. However, we got a feel for the characteristics to look for and an insight into the complexities of identifying Willows.

There was agreement on the identification of Cricket-bat Willow (Salix alba var. caerulea) which had the underside of the leaves clearly blue-green in colour and were only sparsely hairy rather than silky hairy as in White Willow (S. alba). We also found a variety of Creeping Willow which had silky-hairy leaves (S. repens var. argentea), which may have escaped from cultivation.

Although willows were the main focus, we did occasionally look at other species which caught our eye. The water level in the reservoir was unusually low. Abundant on the sandy beach was Trifid Bur-marigold (Bidens tripartita), Sneezewort (Achillea ptarmica), Silverweed (Potentilla anserina) and the invasive New Zealand Pigmyweed (Crassula helmsii).

A particularly interesting find was Tasteless Water-pepper (Persicaria mitis), which is only found in a number of scattered sites throughout the British Isles. We also saw Mudwort (Limosella aquatica) which is also scattered but decreasing throughout the British Isles.

Other finds included Sand Spurrey (Spergularia rubra), Lesser Hawkbit (Leontodon saxatilis), Skullcap (Scutellaria galericulata), Corn Mint (Mentha arvensis) and Whorled Mint (hybrid between Mentha arvensis x M. aquatica).

In damper sections of the beach were Common Spike-rush (Eleocharis palustris), Bottle Sedge (Carex rostrata), Bladder-sedge (Carex vesicaria) and the hybrid between Slender Tufted-Sedge and Common Sedge (Carex acuta x Carex nigra).

Thanks were given to Brian for a thoroughly interesting and enjoyable day. The weather had been decidedly chilly today, in contrast to the fine hot weather we’ve enjoyed recently, however at least there had been no rain.

Text and photographs by Susan and Tom