As usual, I’m writing this late in the day when we are already finished May and spring has accelerated away from me, as it always seems to do at this time of year!
One of the loveliest things in the gardens in April is the unfurling of leaves everywhere dispelling the brown hues of winter. Ferns are a particular personal favourite, unfurling their fronds in a distinctive pattern known as circinate vernation.
The Gunnnera manicata or Giant rhubarb coming into leaf looks pretty much like super-sized garden rhubarb, with huge leaves up to 2.5 meters across and cone-like flower spikes weighing up to 13 kg. Gunnera is a flowering plant, more recently arrived on land than ferns and their allies. A native of SE Brazil, it loves damp, boggy ground and thrives in the woodland part of the Botanical gardens. I won’t be planting it round my new pond though as it would soon shade out everything else and has definite invasive tendencies!
The other part of the garden undoubtedly springing into life is the arctic-alpine garden – things are flowering earlier here than they would in their harsher natural habitat, with both higher temperatures and longer days.
Little did I know when I saw Mountain avens and Creeping willow in the gardens in April that I would be seeing masses of both growing wild at the end of May in the Burren in Ireland! More on this anon…
The Rhododendrons and Azaleas in flower in April and May are also mostly mountain plants, many originally from the Himalayas. I first saw Rhododendron ferrugineum, the type species for the genus Rhododendron, growing wild in the Ötztal region of the Austrian Alps on an undergraduate field trip led by Alan Pearson and Brian Huntley.
Rhododendron is the largest genus in the plant family Ericaceae (heaths and heathers) which are a plant family adapted to acidic, nutrient poor growing conditions – hence the need for special ‘Ericaceous’ compost when growing them in a pot. There are over 1000 species of Rhododendron, including all the Azaleas in a sub-group whose flowers contain just five anthers rather than the usual ten.
Though Rhododendrons undoubtedly produce valuable early pollen and nectar for pollinating insects, Rhododendron pollen also contains neurotoxins known as grayanotoxins. Grayanotoxins bind to sodium-ion channels in cell membranes and keep them open, so that neurons keep firing indefinitely, over-stimulating the nervous system and producing heart arrhythmias and altered states of consciousness in humans. Most Rhododendron species contain relatively small amounts but some, including the purple-flowered R. ponticum often regarded as invasive in UK woodlands, contain sufficient that bees which feast on their nectar can produce so-called ‘mad honey’. Mad honey has long been treated in Nepal as a recreational drug but toxic honey and mead have also been used as early chemical weapons in and around Turkey for over 2000 years (Pain, 2015). In 69 BC Mithridates, ruler of Pontus in northern Anatolia, tricked an invading Roman army into eating toxic honey, then waited until they became nauseous and delirious before attacking them.
The toxic effects of grayanotoxins on humans normally wear off in a few hours but they are deadly to some animals and there are suggestions that invasive plants bearing such toxins may be partly responsible for the well-documented decline in pollinators. A study on Irish bees found that R. ponticum grayanotoxins killed honeybees within hours, whilst causing only short-term symptoms in mining bees and not affecting worker bumblebees at all (Tiedeken et al., 2016). Species of bee which cannot tolerate the grayanotoxins seem much more likely than others to be negatively affected by R. ponticum coming to dominate an area, both because they can be poisoned directly by the nectar and because the flowers on which they normally depend for nectar may have been out-competed.
But I digress… Why would it be to a plant’s advantage to kill potential pollinators? Maybe it’s about ensuring that the ‘best’ pollinators for a particular species are attracted and valuable nectar is not wasted on insects which are less effective. Large-flowered Rhododendrons are better served by big, hairy bumblebees than by smaller bees, so nectar which sustains them and not other pollinators makes good sense from the plant’s point of view (if it had one!). However that raises as many questions as it answers about how only bumblebees have evolved to be able to tolerate grayanotoxins…. Especially as, in places where mad honey is common, honeybees must also have developed tolerance to the toxins!
This month I also enjoyed watching tiny seed cones starting to grow on the Lawson cypress (Chamaecyparis lawsoniana) trees from last month’s tiny blue ovule bearing cones….
….as well as discovering that the garden has a Pinetum I knew nothing about, with specimens of gymnosperms collected from all over the world including Monkey Puzzle trees which will, I guess, eventually replaced the huge one which, sadly, had to be felled last year.
Don’t forget to take a look at The Friends of Durham University Botanic Garden (friendsofdurhambotanicgarden.org.uk) to find out more about what’s going on there this summer.
Tiedeken, E.J et al., (2016) Nectar chemistry modulates the impact of an invasive plant on native pollinators. Functional Ecology, 30, 885–893