Magnificent May

How did it get to be May already? It seems a very short time ago that we were looking at the low sun and listening to the lone robins sing, bare earth and branches waiting for a temperature hike. Well, the Garden has plumped up with leaves and life, almost fluorescent in its vibrancy. It’s a wonderful time of year, even when it rains you can almost see the plants growing. 

While the rain is soaking into the May soil, it also threatens the flowers of one group of plants in our Chinese Herb Garden. This year we have completed our peony garden, a unique display here in the west country, and on Sunday 12th May we’re holding a day dedicated to peonies in celebration. One thing we’d like for people to see in this area is of course the flowers of peonies, and the weather was doing its best to rain on our peony parade. So we decided to pamper these plants with an umbrella each. It might seem over the top, but it’s a treatment that some of them are accustomed to. In days gone by the gansu mudan peony has led a life of privilege; ancient China knew it as the Emperor’s flower and law decreed that it was only grown in his gardens. Specialist growers were tasked with cultivating it for use in the imperial borders, but if anyone got ideas above their station and sneaked some in their own garden, well, they were executed! So some of these peony flowers have the air of ‘an umbrella is no more than I deserve’.

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Bumblebees who brave the winter

By Nicola Temple

This past weekend, my family and I met with friends in the village of Shipham, in Somerset, for a walk. It was torrential rain, yet we were determined. We dressed ourselves and three children under the age of 10 in waterproofs and set out. We arrived at a local country pub, not more than 3 km away, resembling drowned rats. And as a Canadian living here in the UK, I still marvel at the fact that nobody took one bit of notice at the state of us. It’s what you do. You get wet. You find a pub. You hunker down for a hot Sunday lunch. And you hope it tapers off before you have to head out again. (It didn’t.)

Pollinators, at least of the flying insect variety, aren’t terribly keen on this kind of weather either. Most hunker down for the winter months as there is generally not a lot of nectar to forage this time of year anyway. How they do this depends on the species. Honeybees reduce the colony to a minimal size and rely on their honey stores to see them through, while they dance in order to regulate the temperature of the hive. Most bumblebee colonies die out completely and the queens that mated at the end of the season find a place to hibernate. Solitary bees may hibernate as adults or as larvae, emerging only when the weather conditions are suitable. To each their own.

Martin Cooper spotted this buff-tailed bumblebee queen
foraging on his Mahonia flowers in Ipswich on a sunny
January day in 2015.
Photo credit: Martin Cooper [via Flickr CC]

However, there is one flying pollinator that can be spotted this time of year here in Bristol, and indeed, other warmer regions of the UK. It is the common buff-tailed bumblebee (Bombus terrestris). This species was first spotted during the winter of 1990, in Exeter. Sightings have been increasing ever since and include nest-founding queens, workers and males, suggesting this is a winter generation of the species.

The mated queen will emerge from her subterranean dormant state (diapause) during warm winter weather and set about establishing a new colony. The potential cost of waking up early is that the warm weather could be short-lived and temperatures could plummet. The benefit, of course, is that there’s nobody to compete with for food. If successful, the queen can establish a colony before the other pollinators even wake up from their winter nap.

Introduced plants provide winter forage

Of course, there is potentially another cost to emerging early – there could be nothing to eat. Bees are able to forage at temperatures around 0oC, but if there aren’t enough plants in flower, they won’t find the pollen and nectar needed to sustain the colony. Few native UK species flower in winter, but species introduced by avid gardeners to bring some winter colour to the garden, also bring some much-needed food to the buff-tailed bumblebee.

Researchers at Queen Mary University of London and The London Natural History Society, conducted a study of buff-tailed bumblebees foraging in London parks and gardens during winter about ten years ago. They wanted to see just how much food the bees were finding as food is directly related to the success of the colony.

The researchers found that there was plenty of forage to sustain the colonies and, in fact, the foraging rates they recorded near the end of winter were equivalent to peak foraging rates found in the height of summer. This doesn’t mean that the winter-flowering plants, such as the evergreen shrubs of the Mahonia spp., are providing more pollen and nectar than all the plants in the height of summer. But it does mean that each flower might have more pollen and nectar available because there aren’t other pollinators out and about also using the resource. The bumblebees, therefore, don’t need to go as far to find an equivalent amount of food and so they can collect it at a faster rate.  

Strategies for tolerating cold

Buff-tailed bumblebees aren’t as tolerant to cold as some other bee species; workers will freeze solid at about -7.1oC while queens freeze at -7.4oC. The bumblebees can obviously find warmth in the colony, but they need to forage and therefore be able to tolerate short spells of cold during the winter months. They may even need to tolerate cold temperatures for up to 24 hours as bumblebees often overnight away from the colony when they are unable to return from foraging.

Researchers from the University of Birmingham looked at the different cold tolerances of this bumblebee species a few years ago. They found that 50% of workers died after being exposed to 0oC for 7.2 days while queens could last over 25 days at this temperature – likely due to their fat reserves. However, as the forage study showed, the bees seem capable of finding food sources closer to the colony during winter months, which may reduce the likelihood of them having to endure cold temperatures for a lethal period of time.

These bumblebees may also have adopted some strategies to help reduce their possibilities of freezing. Pollen is an ice-nucleating agent in that it promotes the development of ice at higher temperatures. Other insects have been observed to expel any ice-nucleating agents from their gut when they experience low temperatures to avoid freezing. While this wasn’t observed in the bumblebees, it is a strategy that individuals might employ when caught out in the cold.

The more frequent observation of buff-tailed bumblebees in winter is thought to be a result of warmer autumn temperatures brought about by climate change. In a study from 1969, researchers reported a 6-9 month dormancy of all bumblebees in southern UK, so in a relatively short period of time there has been a considerable change in their seasonal pattern. There seems to be some flexibility in these patterns among bumblebees and for now, establishing winter colonies seems to be working for the buff-tails. However, with so many of our pollinators under threat, there is obviously also concern among the scientific community that more frequent extreme weather events could also spell disaster for these colonies that have selected to brave the winter months. As gardeners, we can perhaps do our bit by planting some winter forage species.

This year, the University of Bristol Botanic Garden will embrace a pollinator theme, with the aim of highlighting some of the lesser-known pollinators that are so important here in the UK. We love our pollinators, but research is still revealing so much about their unique and complex relationships with plants. So watch this space as we share some of these wonderful stories through our blog. We will also be posting pictures of pollinators we see in the Botanic Garden on our Twitter feed and Facebook page. But to see these polli
nators in action, take some time to visit the Botanic Garden. Make space in your busy schedule to watch nature at its best – it’s worth it.

Sources:

Alford DV (1969) A study of the hibernation of bumblebees (Hymenoptera: Bombidae) in Southern England. Journal of 
     Animal Ecology 38: 149-170.
Owen EL, Bale JS, Hayward SAL (2013) Can winter-active bumblebees survive the cold? Assessing the cold tolerance of 
     Bombus terrestris audax and the effects of pollen feeding. PLoS ONE 8(11): e80061.          
     doi:10.1371/journal.pone.0080061
Stelzer RJ, Chitka L, Carlton M, Ings TC (2010) Winter active bumblebees (Bombus terrestris) achieve high foraging 
     rates in urban Britain. PLoS ONE 5(3): e9559. doi: 10.1371/journal.pone.0009559 

‘Tis the season…or is it?

By Helen Roberts

As I sit at my desk this morning, staring out the window, the weather is dire. There is slanting torrential rain and high winds, a typical December day perhaps.
Here in the UK, the seasons are changing and we are experiencing extremes of weather. For example, we have had wetter, milder winters in the southwest over the last couple of years along with increased flooding, particularly on the Somerset Levels. And then there was the very slow start to spring this year, with temperatures well below average in April. This was followed by a very hot end to the summer and warmer-than-average temperatures throughout autumn.
These changes to the seasons are linked to global climate change and are throwing the UK’s wildlife into disorder and affecting the fine balance of habitats and ecosystems. This is not a good scenario for biodiversity in the UK. Seasonal timing is off. When seasons start and end is shifting, and the length of the season itself is changing, making ‘growing seasons’ a more fluid concept. There is also increased risk for most gardeners of a ‘false spring’. Many plants and animals are changing their geographical ranges in order to adapt to these changes.
One of the most significant effects has been the disruption of lifecycle events and these are manifesting themselves in different ways. Bird migration, insect emergence, incidence of pests and diseases and flowering times are being thrown out of kilter.  
Researchers from the University of East Anglia recently analysed 37 years worth of data from the UK Butterfly MonitoringScheme (UKBMS) and found that extreme weather events were causing population crashes of butterflies. Uncommonly high rainfall events during the cocoon life stage affected 25% of UK butterfly species. And more than half of species were affected by extreme-heat during the overwintering life stage, possibly due to the increased incidence of disease or the effect of a ‘false spring’, causing butterflies to emerge too early only to be decimated by a return to cooler temperatures.
Warm temperatures are not all bad for butterflies though, as they will benefit from hot temperatures over the summer months when they are in their adult form and resources are plentiful. However, if populations crash more frequently than they expand, these extreme weather events could threaten UK butterflies.
The spider orchid (Ophrys sphegodes).
Photo: Jacinta Iluch Valero via Flickr [Creative Commons]

Changes in seasonal timing are also knocking the relationships between plants and animals out of sync, including the delicate balance between plants and pollinators. Thiscan be detrimental to the balance of entire ecosystems. An elegant study carried out by scientists from Kew and the University of East Anglia found that earlier springs brought about by rising temperatures are affecting the relationship between a rare spider orchid (Ophrys sphegodesand its sole pollinator, the solitary miner bee (Andrena nigroaenea).   

This particular orchid has a flower that resembles and smells like a female miner bee and it uses this deceit in order to lure the male miner bee in. The male attempts to mate with the flower and by doing so, pollinates the flower. The plant has evolved to flower at the same time as the male bees emerge, but before the females do.
What the researchers discovered, by looking at the data set going back to 1848, was that rising temperatures are causing the relationship between orchid and bee to break down. Although rising temperatures cause both the bee to emerge and the orchid to flower earlier, the effect on the bees is much more pronounced. The male bees emerge much earlier and the orchids now flower as the female bees emerge. This means the males are not “pseudocopulating” with the flower because the real thing is already available and so the rare spider orchid is having fewer pollinations.
However bleak this picture may seem, plants and animals do have the ability to adjust to seasonal changes caused by climate change, it is just whether they can adapt quickly enough for these intricate ecological relationships to remain intact.
Helen Roberts is a trained landscape architect with a background in plant sciences. She is a probationary member of the Garden Media Guild and a regular contributor to the University of Bristol Botanic Garden blog.


References

Latecomers to the summer flowering party

By Helen Roberts

It’s the time of year when most people think that gardens are nearing the end of the full flush of summer blooms. Mid summer flowers may be dwindling but there are numerous late flowering species that still provide a riot of colour. I have always been interested in gardens at this time of year because we are often rewarded with a spell of bright sunny weather in autumn. I want to be outside enjoying the garden, hanging onto the summer for as long as I can before the cold deepens and the nights draw in. So planning for some autumn colour in the garden can be very rewarding.
  
With thoughts of designing my own garden for a prolonged season of flowering, an excursion to the University of Bristol Botanic Garden was due. I met up with Froggie who showed me the bounty of colour at this time of year in the gardens.
First stop was the hot borders which can be found in front of The Holmes, which were evidently at their most scorching in terms of vivid colours, with swathes of Hemerocallis, Penstemon, Helenium, Rudbeckia, Tithonia, Fuchsia and Dahlia. Froggie pointed out the lovely Verbena bonariensis, which is such a great plant for pollinators and one that self seeds profusely. Verbena adds some soft architectural form to borders and provides flowers for months on end. A plant that I was less familiar with was the rather cute and tender Cuphea cyanea or Cigar flower with red and yellow tipped flowers. There is also another variety of Cuphea called the Pink Mouse – each flower does look like a miniscule mouse!
Providing structure to the hot borders were the awesome sub tropical Abyssinian Banana (Ensete ventricosum), which were still looking amazing but will soon be lifted and taken into the greenhouses. The staff keep a close eye on the weather at this time of year, any sign of frost forecast and they must move quickly to take in the tender species.
Froggie explained, “We had some hard winters a few years back and we lost quite a lot of plants so lifting plants into the greenhouses ensures they are protected. They are our insurance against a very cold winter.”
Salvia uliginosa can be found flowering this time of year
by the Botanic Garden’s main pond.
Photo credit: Helen Roberts.
Many of the shrubby salvias are in this tender category. Froggie showed me Salvia confertiflora, an exotic late flowering species with beautiful fuzzy crimson inflorescences about 0.5m tall. This will be moved inside soon when the weather cools. Another that caught my eye in the pollinator beds located on one side of the main pond was Salvia uliginosa, a very tall plant with vibrant sky blue inflorescences that were buzzing with bumblebees.
I have to admit to an obsession with shrubby salvias, which started after many visits to the garden of plant guru Derry Watkins over the course of this summer. Her passion for these beautiful plants is contagious. They are an extraordinary group of plants that flower continuously from June until October and the flower colours are exquisite. The colours really pack a punch in terms of vividness. I purchased Salvia microphylla ‘Cerro Potosi’, which started producing vibrant magenta flowers back in June and is still putting on a show of pink in October. I plan to take cuttings of this to provide a back up plan in case I lose my original plant (I am going to risk leaving mine out over winter).
Toad Lilies (Tricyrtis macropoda). Photo credit: Helen Roberts.
In amongst the buzzing pollinator borders were the very pretty and delicate Toad Lilies (Tricyrtis macropoda). I watched bees visiting these inflorescences and collecting nectar by robbing it through the back of the flowers. The pink flowered society garlic (Tulbaghia violacea) and Japanese Anemone (Anemone hupehensis) were very subtle in hue combined with dazzling yellow Rudbeckia and deep purple drooping flowers of Agapanthus inapertus ‘Midnight Cascade’. Many of the hummingbird-pollinated plants were in flower including the pineapple relative Ochagavia litoralis and the terrestrial bromeliad Fascicularia pitcairnifolia. The latter, at present, is visually screaming, “Come pollinate me!” with the centre of the rosette turning an intense scarlet with a dense cluster of blue flowers tipped with bright yellow pollen.
Throughout the gardens, as I toured around with Froggie, there were interesting flowering species and the colours varied tremendously from vivid red and pink to deepest indigo. The flower forms were diverse too; delicate dangling umbels, ‘in your face’ discs of blooms, hooked and lipped nectar-rich inflorescences and some which were just plain weird looking. The gardens simply still looked stunning and I left knowing that it’s not yet time to put gardens to bed, there’s plenty more flowers to come.
More species that are flowering now in the garden include:
  • Abutilon sp. (Chinese lantern)
  • Agastache sp. (Giant hyssop)
  • Campsis sp. (Trumpet vine)
  • Caryopteris x clandonensis (Bluebeard)
  • Colchicum agrippinum
  • Commelina tuberosa Coelestis Group (Day flower or Sleeping Beauty)
  • Crinodendron hookerianum (Chilean lantern tree)
  • Erica tetralix (Cross-leaved heath)
  • Impatiens tinctoria
  • Lantana camara (Yellow sage)
  • Liriope muscari (Big blue lily turf)
  • Tropaeolum peregrinum (Canary creeper)
  • Verbena peruviana (Peruvian verbena)