Wildflower of the Month: July: Sulphur Clover

Sulphur Clover at Brampton
Meadow SSSI, Hunts.
Image: K. J. Walker

This year, BSBI Head of Science Kevin Walker has been focusing each month on a different wildflower. From March's purple saxifrage to June's bee orchid, we've all found out more about some of our best loved and most iconic wildflowers. But July’s Wildflower of the Month is a plant that many people are unlikely to have heard of, never mind seen. 

Over to Kevin to tell us more:

"Sulphur Clover Trifolium ochroleucon, a lovely pale-yellow clover, is a speciality of grasslands on the boulder clays of East Anglia stretching in a wide arc around the fenland basin. Here you are most likely to stumble across it on one of the ‘protected road verges’ that are scattered across the six East Anglian counties. Due to the catastrophic loss of old meadows and pastures, these narrow strips of grassland are fast becoming wildflower arks for some of our most threatened species. If we needed a flagship species to encapsulate their plight then Sulphur Clover would be a good candidate; its recent history provides us with insights into why these road verges are so important and also the challenges their wildflowers face due to mismanagement, pollution and increased disturbance from human activities.

Sulphur Clover, Honeydon road verge
Image: P. Stroh

Post-glacial relic

"Sulphur Clover isn’t particularly rare by British standards. In conservation jargon it is Nationally Scarce, that is it occurs in less than 5% of the hectads (10 x 10 km grid squares) that make up Great Britain. Nor is it highly threatened – it’s title ‘Near Threatened’ means that it’s not quite ‘Vulnerable’ but then again certainly not of ‘Least Concern’, although I’d wage money on it being classified as ‘Vulnerable’ or even ‘Endangered’ on the next iteration of the GB Red List. But categorisations aside it is indicative of good quality habitat, the sort that is worthy of conservation at the local if not the national scale. It seems to be a pioneer species of raw mineral soils and as such is a poor competitor that was probably more suited to the harsh conditions that existed in East Anglia during the early post-glacial period before the development of woodland. So today it hangs on where summer droughts prevent the spread of more competitive species. Thankfully, like other perennial clovers it is probably quite long-lived and produces lots of seeds that are readily dispersed, most likely by animals and machinery (Walker, 2019).

Crested cow-wheat
Image: P. Stroh

Surveys

"I first became aware of Sulphur Clover when I worked at Monks Wood experimental station near to Huntingdon. It was something of a local celebrity with populations easily accessible during lunchtime walks. It even grew on the edge of the ride in Monks Wood although it had gone, possibly due to shading, before I arrived. In some local sites it grew in excellent company, most notably with Crested Cow-wheat Melampyrum cristatum and Bath Asparagus Ornithogalum pyrenaicum on a spectacular verge near to Honeydon in Bedfordshire, which I first visited with colleagues in 1997. [Ed.: if you'd like to see what  Sulphur Clover, Crested cow-wheat and other wild flowers look like growing on a protected road verge in Hunts., check out this short video by BSBI England Officer Pete Stroh].  

"The plant interested me so much that I devoted a chapter of my PhD to it. Using historical records I managed to trace 35 localities in ‘Old Huntingdonshire’ where it had been recorded in the past. Over two hectic summers I revisited 30 of these sites collecting information on populations sizes, habitats, threats and management. The results were worrying (Walker & Pinches, 2009). At only 13 sites could we find any plants, eight of which were on road verges. Even more alarming was the decline in old meadows and pastures; only four of 11 populations had survived. Thankfully one of the best sites was a SSSI in the middle of Huntingdon Racecourse where 1000s of plants occurred in a large area of species-rich grassland. We were not alone in charting the fortunes of this lovely plant. Around the same time similar surveys were carried out in Cambridgeshire, Essex and Norfolk. The results in some counties were even worse in Cambridgeshire, for instance, three-quarters of its populations had perished and it had virtually disappeared from meadows and pastures (Cadbury, 2012).

Sulphur Clover on a road verge
Image: P. Stroh

Causes

"So why had Sulphur Clover declined by so much? The simple answer was that many of its former grassland haunts had been ploughed-up or improved as farmers either shifted to arable or intensified grassland management. The few surviving meadow populations were often protected as SSSIs or County Wildlife sites. The declines on roadsides, though less marked, were more difficult to explain. A paper by Ken Adams in Essex provided some much-needed answers (Adams, 2007). The management of road verges had clearly changed dramatically over the last 100 years. Before the Second World War verges were usually cut by local farmers relatively infrequently and usually during the late summer. The arisings were often removed as hay and is some cases broad verges might have been grazed. Hedgerows would have been managed less intensively and inputs of nutrients from human activity, in particular from farming and cars, would have been negligible. 

Crested Cow-wheat on the road verge, Honeydon
Image: P. Stroh

"By the end of the C20th, however, the management had changed dramatically. Many roads had been widened to cope with increased traffic and cutting was carried out by contractors much more frequently and earlier in the year. Contractors also left the cuttings in place thereby increasing soil fertility. Further eutrophication was caused by deposition of nutrients from adjacent farmland and from traffic and other airborne sources. As a consequence, our roadsides have become less diverse and dominated by a small number of tall grasses and herbs such as Cow Parsley Anthriscus sylvestris.

Sulphur Clover
Image: P. Stroh

A last refuge?

"A few years ago Plantlife published an excellent report on the importance of road verges for our threatened wildflowers (Plantlife, 2017). This included a league table in which Sulphur Clover came in fourth with 68% of its populations confined to road verges, although those above it were confined to just a single or a handful of sites. So road verges really are its final refuge. If we are to save it and many other roadside wildflowers for future generations, then we need to improve their protection and management.

"One of the key problems we face is making sure that highway managers know where the best road verge sites are. This might sound simple but it has so far proved problematic. We can erect signs, posts and big flashing beacons but this still doesn’t stop contractors trashing them with depressing regularity. Wouldn’t it be great if we could set-up a national inventory of the best sites that could be easily accessed by all those involved in their conservation and management? This can’t be beyond our wit! Most county councils have a list of protected verges and many have undertaken surveys to establish which wildflowers are present. Combining this information with the BSBI’s records on a webpage could be a game-changer for our roadside wildflowers if sufficiently well-publicised and supported.

Crested Cow-wheat and Sulphur Clover
on the verge: Stocking Lane
Image: P. Stroh

"Once they are ‘on the map’ then we need to ensure that road verges are managed appropriately. Thanks again to the efforts of Plantlife, we know what good management looks like. Their best practice guidance provide highway managers with all they need to know – in a nutshell, cut at least once a year but only after plants have had a chance to seed, remove the cuttings and, if you can, maintain a bit of structure to benefit other wildlife (Plantlife, 2016; Bromley et al., 2019). And this campaigning appears to be paying dividends; highway managers in different parts of the country are starting to manage road verges with wildflowers in mind. There is still a long way to go but at least there is now more hope for the road verge specialities such as Sulphur Clover, Crested Cow-wheat and Bath Asparagus. By increasing awareness and improving management, their life on the edge of our highways and bye-ways may not be as precarious as it once was".

References

Adams, K.J. 2007. Notes on Essex specialities. 12: the status and distribution of Sulphur clover, Trifolium ochroleucon Hudson, in Essex and Eastern England. Essex Naturalist 24: 115-118.

Bromley, J., McCarthy, B. & Shellswell, C. 2019. Managing grassland road verges. A best practice guide. Plantlife, Salisbury. 

Cadbury, C.J. 2012. Sulphur Clover Trifolium ochroleucon: its decline in Cambridgeshire (v.c.29). Nature in Cambridgeshire 54: 44-53.

Plantlife, 2016. The good verge guide. A different approach to managing our waysides and verges. Plantlife, Salisbury. 

Plantlife, 2017. Road verges. Last refuge for some of our rarest wildflowers and plants. Plantlife, Salisbury. 

Walker, K.J. 2019. Trifolium ochroleucon Sulphur Clover, in P. Stroh, K.J. Walker, S. Smith, R. Jefferson, C. Pinches & T. Blackstock (comp. & eds.). Grassland plants of the British and Irish lowlands. BSBI, Durham, pp. 307-308.

Walker, K.J. & Pinches, C.E. 2009. The status of Trifolium ochroleucon in Huntingdonshire. Nature in Cambridgeshire 51:3-12.