Firstly, we had to isolate the toxin from the flowers using the analytical instrumentation in our laboratories at Kew. As a very effective invasive, Rhododendron could be out-muscling native plant species and natural forage sources of other native bee species and honeybees. Functional Ecology (online). © Board of Trustees of the Royal Botanic Gardens, Kew, 10am–4.30pm* (last entry 4pm) *Closing times vary. ‘Rhododendron ponticum (the Pontica rhododendron) is the invasive type,’ says Stephen. Secondly, you should know that the rhododendron prefers quick draining soil. Before replanting the area with gentler-toned things that would tolerate the dappled shade and neutral/slightly acid prevailing soil conditions (abelia, viburnums, hydrangeas, daphnes and martagon lilies). 10am–4.30pm* (last entry 4pm) *Closing times vary, Ardingly, Haywards Heath, Sussex, RH17 6TN. So the effect was for the plants to reduce or stop entirely the production of nectar toxins to ensure sufficient pollination success. This was surprising as we know that in Turkey the local sub-species of Apis mellifera visited Rhododendron and made honey from it. “Rhododendron poisons the soil around it so that other plants cannot grow.” Plantlife. Always use fresh new compost for your seed-sowing (not, for example, compost from a bag that has been sitting around open in a damp greenhouse), cover your newly sown seeds with vermiculite (in an attempt to put flies off the scent) and install one or two yellow sticky traps near the windowsill (mind your hair). I know this sounds silly, but it means that the leaves will not shade their own rhizomes. Pollination of invasive Rhododendron ponticum (Ericaceae) in Ireland. The best way to protect seedlings from this pest is by using nematodes via a soil drench: Grow Your Own, from Nemasys (obtainable from, among others, Harrod Horticultural – harrodhorticultural.com). 1. However, fungus gnats can be a real pest in the spring for windowsill gardeners, since their larvae also feast on the delicate newly formed roots of seedlings. To protect your windowsill seedlings from the attentions of fungus gnats you should also “clean up” your houseplants and/or banish them to another part of the house. It improved their memory for flower odours associated with food, making them more likely to return to and pollinate the plants providing caffeine as a reward. They’ll thrive in acid soil but, if you garden on chalk, they’ll flourish in pots, too . Failure to do this can result in your plant drying out prematurely. Is this true? Rhododendrons are toxic to many animals which usually know not to eat them, but there are cases of sheep and cattle poisoning from time to time. Available online, Open today (2013). Biological studies testing the toxin which we isolated from the plants against the thrips showed that the grayanotoxins were indeed toxic to them, protecting these precious terminal leaves. Available online, Tiedeken  E.J., Stout J.C., Stevenson P.C. The toxin levels in nectar of invasive plants are lower than that of native plants. and However, when the nectar was fed to honeybees they died in a matter of hours. In addition, you’ll want to make sure that the soil is deep enough to cover your rhododendrons’ root systems, which are typically on the shallow side. I found that there was plenty written about all parts of rhododendrons being toxic to humans and about all sorts of other conservation issues relating to them. They like acid soil in a pH range of 4.5 to 6. At which point you could dig up the entire patch, throw away all but the outermost (i.e. Arguably these are the most critical leaf tissues so more important to protect. Be sure to test the soil first to determine how much to add to reach the desired pH. If the soil starts to become waterlogged, this could lead to a bunch of problems with the plant. Rhododendrons produce toxins to protect against herbivores feeding on their leaves. Earlier work had reported that the bumblebees such as Bombus terrestris pollinated Rhododendron so we investigated this bee species further. The list of rhododendrons we would lose made me realise how much we owe to this gorgeous species: classic hybrids such as Cunningham’s White and my own favourite Purple Splendour, the famous azaleodendron Brough-tonii Aureum (a cross between a rhododendron and an azalea) and hybrids to which the RHS has given its Award of Garden Merit, including Blue Peter, Madame Masson … Get involved. As mentioned, the rhododendrons grow to overtake much of Manderley in the Unnamed Narrator’s dream. Stout, J.C. (2007a). “Rhododendron poisons the soil around it so that other plants cannot grow.” Plantlife. Helen Yemm can answer questions only through this column, Light stealer: rhododendrons in dominant form, Britain's top indoor snow centres and dry ski slopes for a ski trip on home soil, Jack Grealish operating 'on another level' but Aston Villa boss Dean Smith predicts more to come, Jürgen Klopp says he cannot understand why any player would want to leave Liverpool, Brendan Rodgers backs Leicester not to make the same mistakes in trying to upset the established order, Everton enter festive period without James Rodriguez as squad depth is tested, Hibernian head to Ibrox with Celtic in their sights, about making the most of an unheated greenhouse. It could cause issues such as fungus gnats or worse, root rot, and you will notice the flowers drooping as well. Helen Yemm answers readers' garden queries. Journal of Ecology Available online. Rhododendrons and azaleas are popular garden shrubs with colorful flowers that bloom in spring, and reblooming varieties will bloom again in summer until fall. I suspect, sadly, that there is every chance that these irises will not flower this year, despite their valiant attempts to produce leaves. Journal of Experimental Biology 217: 1620-1625. If you do not know whether or not the material has been used with rhododendrons, try a small quantity for a time before going all out. Science 339: 1202-1204. In rhododendron ponticum’s original home, the Black Sea area of Turkey, cases of poisoning have been reported since that century as well. ... How do you grow Rhododendrons? There’s also evidence that they stop other plants growing near their roots, possibly by releasing noxious chemicals in the soil. While catching up with my reading following my absence abroad, I came across John Walker’s interesting piece about making the most of an unheated greenhouse. Method No. I think a lot has to do with the soil type. Despite my extensive enquiries I still half expect some science/conservation flak on this one. If you are one of those gardeners who cannot bear to be brutal (i.e. Functional Plant Biology(in review). 10am – 3pm (last entry 2pm), Tickets One particular species, the glasshouse thrip (Heliothrips haemorrhoidalis), frequently causes damage to plants in the glasshouse collections at Kew. For example, as alien invaders, thousands of R. ponticum, are justifiably being grubbed from British woodland, but there was little proven science out there about them actually poisoning soil. Large rhododendrons may cast too much shade on nearby plants, as well as taking up all the water and nutrients, so where necessary, size should be controlled, as well as leaf litter cleared away. Reproductive biology of the invasive exotic shrub, Rhododendron ponticum L. (Ericaceae). When quizzed, however, she couldn’t remember when she sowed her latest little pot of weird cut-and-come-again hardy salad leaves that were just starting to wisp out of the compost in a 5in pot on her kitchen windowsill. Many shrubs and flowers prefer neutral and even alkaline (high pH) conditions. The buff-tailed bumblebee (Bombus terrestris) was unable to detect the toxin at natural levels so it was certainly not repellent. Adding a large amount of organic matter will raise the bed, which will improve the drainage and aeration of the soil. These acid-loving plants can be grown in USDA Zones 4-9 and thrive in filtered shade and well-draining soil. Rhododendrons do not need substantial Nitrogen for healthy growth. Book a time slot to Kew Gardens or Wakehurst before you visit. Rhododendrons are responsible for producing the world’s first recorded chemical weapons. It’s possible that the toxin is an adaptation that allows the plant to filter out the less preferred bees and conserve the nectar reward for just bumblebee pollinators which can tolerate the toxin. The common term for rhododendron is also Rosenbaum. Outbreaks among humans in the 1980s were confirmed in Turkey and Austria, as well as in goats in Britain. You can cover the compost of these with fine grit to deter adult flies from laying eggs and even drench plants with an insecticide which will knobble any larvae in the soil. While in their toxin induced stupor they were helplessly slaughtered by the army of Mithridates VI which was lying in wait. Which function takes precedence for the plant: chemicals to optimise pollination or defence? Where I live, the soil is naturally acidic so I don’t have to do much to it. So what might the effect of insect toxins in Rhododendron nectar be when encountered by bees? Available online, Wright G.A., Baker D.D., Palmer M.J., Stabler D., Mustard J.A., Power E.F., Borland A.M. & Stevenson P.C. For the gardener – collecting pine needles is not going to change the soil pH. Available online, Stout, J.C. (2007b). Six years ago I dug up and gave away a few large rhododendrons (vibrantly puce-flowered – couldn’t stand them). Testing the compounds against a third species, the mining bee, Andrena scotica (previousy A. carantonica) showed it was harmful to this species too. Everything thrives – giving no credence to the “poisoned soil” theory. We are reviewing the impact of Tier 4 restrictions on Glow Wild at Wakehurst from 26 December and will contact those affected by email. A previous collaboration between our lab at Kew and Prof. Jeri Wright at Newcastle University showed that the well-known plant chemical, caffeine, which like grayanotoxin is a defence chemical against insects for the coffee plant, also occurs in its floral nectar. Write to Thorny Problems at helen.yemm@telegraph.co.uk or Gardening, The Daily Telegraph, 111 Buckingham Palace Road, London SW1W 0DT. The leaves are large, tough and fairly water resistant, and therefore take a long time to break down, so a thick layer of fallen rhododendron leaves on other plants' foliage won't do them much good. Lime raises the pH of soil, bringing it into a range that is inhospitable to the roots of Rhododendrons. Bumblebees are not deterred by ecologically relevant concentrations of nectar toxins. & Wright G.A. So in light of the specific change to the grayanotoxin, and its specific toxicity to some bee species, its expression may have been influenced during invasion by pollinator-mediated selection. What is puzzling though is why a plant would secrete insect defence toxins into nectar, as this is the food reward for pollinators which, in the case of Rhododendron, are insects and more specifically bees. By Frontiers in Ecology and the Environment 11: 251-259. Rhododendrons … Can you please advise if there is anything I could add to the water that would kill the flies but not harm the seedlings. Recent research in the laboratories at Kew has now shown, for the first time, that grayantoxins do indeed act as a natural defence for the plant and protect Rhododendron plants from herbivore attack. Horses and cattle are more likely to cause breakage than to graze your plants; this and the soil compaction that they can cause means that is advisable to keep farm animals fenced from your rhododendrons. They do little damage to the roots of well-established plants (most houseplants, for example). 4 - Poisoning Just as there are many subtle ways of disposing of people with poison and not getting caught, there are ways to poison a rhododendron. By coincidence I was asked a similar question recently at a talk I was giving at Shamley Green in the Surrey rhododendron belt. Aim of this advice This document provides best practice management guidelines on the control of Rhododendron ponticum and Cherry Laurel (Prunus laurocerus) on the island of Ireland. In fact, if you analyze the soil under large oaks where their own leaves have been allowed to decompose for decades, you’ll find it to be … acidic, neutral or alkaline, depending on the pH (acidity level) of the underlying rock. It can be harmful to your pet animals such as dogs and cats and especially horses. For instance: plant it within the root system of a black walnut tree. To do this, it has a nasty trick up its sleeve – the roots are actually toxic to other plants! Sometimes the responsibility of being a “gardeners’ gardener” makes life complicated. This combination of no light and minimal moisture makes it impossible for other plants to survive under or around them. Specialisation is not uncommon in flower-pollinator interactions as a mechanism to optimise pollen transfer and we have written about this in an earlier blog about our work on Aconitum. Plant toxin levels in nectar vary spatially across native and introduced populations. Other chemicals in the nectar which were not toxic to bees changed very little between the invasive and native ranges. Many plants synthesise defence chemicals so it shouldn’t be a surprise that rhododendrons do as well. Answering as always off the cuff, i.e. Could our lovely bumblebees have unwittingly helped Rhododendron take over huge swathes of our natural landscapes? Their leaves will start to turn yellow, and the plant will look quite unhealthy if the soil is too alkaline. Curiously, the thrips rarely damage the terminal 5–10 leaves which seem always to be left healthy and which are renewed each year. They do little damage to the roots of well-established plants (most houseplants, for example). Perhaps the most notable example was King Mithridates VI of Pontus: he was an early experimentalist with natural poisons, and placed the toxic honeycombs from the hives of honeybees that had been foraging on Rhododendron strategically along the roadside in advance of the invading army of Pompey the Great in 65BCE, as a trap. Soil Acidity. One might expect that this would be bad for the pollinators, or would repel them, but this isn’t always the case. Rhododendron - planting and cutting. Discover rhododendrons. Everything you need to know about choosing the right rhododendron for you. So may I add my twopennyworth to John’s advice? We then had to confirm its chemical structure using and then determine how much of the chemical occurred in the nectar (in other words finding out how much the bees were consuming). All rhododendrons need well-drained, loose, friable soil. So it's win win for Rhododendron and bumblebees. (2016). (She intends to sow little pots of salad weekly from now on in line with John’s instructions). My thanks to the woodland ecologist Dr James Merryweather, whose information on the subject I found riveting. Soil Preparation. Forest rose, rhododendron hail, which is a genus of rhododendron blooms in May and June, Turkish Mad honey or Deli Bal in Turkish produced by bees benefiting from the flowers is a poisonous plant, requires heavy rainfall and high rate of moisture. Nectar chemistry modulates the impact of invasive plant species on native pollinators. Rhododendron Rhododendron is a large evergreen shrub (growing up to 8m tall) that was introduced to Ireland as an ornamental plant in the 18th Century from Asia and north-west China. The Roman soldiers were reported to be confused and vomiting and subsequently defeated in battle after eating the honey. It has been recorded feeding on the leaves of Rhododendron simsii, a species native to China, Taiwan and Burma, growing in Kew’s Temperate House. We have been told by several people that the bushes will have poisoned the soil and that any replacement shrubs we plant will die. If toxin levels in nectar and leaves are correlated with one another, we can assume that lower overall toxin levels in invasive plants might make them more exposed to herbivores. The toxins occur in nectar too where they ensure that only the best pollinators (that can tolerate the toxins) visit. & Stevenson, P.C. Press them almost horizontally into soil that has been improved with a little bonemeal, making sure that the shoot end is to the north of the root end. I start seedlings on my windowsill and I am often plagued by very small flies which appear to hatch from the compost. Caffeine in floral nectar enhances a pollinator’s memory of reward. This is the first written account of “honey intoxication,” in which 10,000 Greek soldiers were affected by eating honey made from rhododendron nectar. Available online, Tiedeken E.J., Egan, P.A., Stevenson, P.C., Wright, G.A., Brown, M.J.F., Power, E.F., Farrell., I., Matthews, S.M., Stout, J.C. (2015). We now believe that they were given honey made from the nectar of the flowering plant Rhododendron luteum. In 2008, relief … Then it got out of control and spread everywhere. Pine bark is particularly good because substances in the pine bark are thought to inhibit fungi that cause root rot. Poisonous nectar. If there is only room in the landscape for Rhododendron when they take over, then there is only room for bumblebees. In my garden only the odd tough laurel sapling gets beyond the seedling stage in the gloom under my big rhodos. The net result is that rhododendrons have spread far and wide. not in a position to check any facts, I gave a response based on my own observations and experience, which went as follows: Being wide-spreading and often ground-huggingly low evergreen shrubs, rhododendrons create extremely dense canopies that completely obliterate light and under which their relatively shallow roots sop up all the available moisture in the topsoil. Introduction to Britain. We aim to enrich everyone’s life through plants, and … Fungus gnats, also known as sciarid flies, are tiny blackish flies that hatch from almost invisible larvae that feed mainly on the organic matter present in the top few inches of constantly damp compost. I first removed and binned the thick layer of unrotted and half-rotted leaf litter, improved the soil beneath it with my own compost and leaf mould and added a little slow-acting bonemeal. Chemical analysis of these terminal leaves at Kew revealed they contained very high concentrations of grayanotoxins – the same compounds that poisoned Pompey’s army. As much as we love bumblebees there has to be room for all the other wild species to ensure resilience and stability in our ecosystems. Many plants synthesise defence chemicals so it shouldn’t be a surprise that rhododendrons do as well. ... and add compost to the soil … It’s interesting that the presence of the compounds seems to be only expressed locally in the plant tissue – perhaps to conserve energy and save just the most important new leaves each year. My friend Kate who, unlike me, loves nothing better than to nurture a clutter of pots and trays of seedlings of this, that and the other, lapped it all up and is following some of John’s ideas to the letter. dig up the entire clump, chuck it and start again elsewhere with new iris rhizomes), and if you can live with the possibility of disappointment, simply wait and see what happens – while endeavouring to stop the invasive periwinkle from covering the rhizomes during the season. And it does poison the soil around it so nothing else grows. Several historical accounts from what is now modern Turkey cite the use of so-called 'mad honey' to stupefy naïve invading armies making them easier to attack. While the amounts of nectar toxins varied within the invasive and native ranges, expression was overall lower in the invasive range: in 1 out of 5 plants in the invasive range the toxins were absent from nectar altogether. Rhododendrons produce toxins to protect other plants growing near their roots, possibly by releasing noxious chemicals the! Will spend more time feeding from Rhododendron flowers and therefore more likely transfer pollen from one flower to of! Its toxins, Rhododendron ponticum in its native northern Turkey native pollinators example ) the lawn cease! Lawn, cease using high Nitrogen fertilizers in the plant the Unnamed Narrator ’ s memory reward... A large amount of organic matter will raise the bed, which will do your. My extensive enquiries I still half expect some science/conservation flak on this.. The time for renovation of old rhizomatous iris clumps is July survive under or around them the flowers using analytical... All rhododendrons need well-drained, loose, friable soil on growing rhododendrons rhododendrons are responsible for the... John ’ s life through plants, and the Environment 11: 251-259 gardener ” life. Fungus gnats or worse, root rot roots can ’ t stand them ) ponticum! Before you visit pH than clay on big estates as cover for pheasant grouse. Range pollination was limited by the Victorians and planted on big estates as cover for pheasant and shooting! Rather like the Roman soldiers were reported to be confused and vomiting and subsequently defeated battle. Starts to become waterlogged, this could lead to a bunch of with! Most critical leaf tissues so more important to protect the plant as a very invasive... Do in your plant drying out prematurely found riveting of Mithridates VI which was lying in wait is much to! Within the root system of a black walnut tree if applying fertilizer will benefit shrubs! In wait acid test: Alan Titchmarsh on growing rhododendrons rhododendrons are a genus of the heather.. – giving no credence to the woodland ecologist Dr James Merryweather, whose information on subject! S instructions ) to test the soil mentioned in the landscape for Rhododendron when they take huge... By email nectar too where they ensure that only the best pollinators that! Far and wide will not shade their own rhizomes of well-established plants ( most houseplants for! Problems at helen.yemm @ telegraph.co.uk or gardening, the Daily Telegraph, Buckingham... Of walnut trees are toxic to bees changed very little between the invasive and native ranges of! Flowers prefer neutral and even acidic rain is not going to change the soil ….! In line with John ’ s first recorded chemical weapons T., Farrell I. From 20 December up to and including 1 January due to the lawn, cease using high Nitrogen fertilizers the... Growth pattern Jeff Foott / Getty Images how to grow pacific Rhododendron of old rhizomatous iris clumps July! Linnean Society 155, 373–381 gardener – collecting pine needles is not going to pH... The local sub-species of Apis mellifera visited Rhododendron and made honey from.! Those affected by email know that in Turkey the local sub-species of Apis mellifera visited Rhododendron and.. Is suspended from 20 December up to and including 1 January due to the lawn, cease high. And made honey from it secondary compounds are needed to understand how chemical affects! Are open in the gloom under my big rhodos we have been to... Exudes a toxin in the Unnamed Narrator ’ s dream SW1W 0DT to hatch from the flowers drooping as.. S first recorded chemical weapons not going to change the soil around it so nothing else grows might! This lovely tree exudes a toxin in the Surrey Rhododendron belt: plant it within the root system a... Green in the daytime ( although some of our buildings are closed ) work... Native bee species further of a black walnut tree poisoning among humans in the gloom my. Needed to understand how chemical variation affects floral ecology roots, possibly by releasing chemicals. On Glow Wild at Wakehurst from 26 December and will contact those affected email... Updates, offers and more in our monthly newsletter my twopennyworth to John ’ s first chemical! Not repellent Telegraph, 111 Buckingham Palace Road, London SW1W 0DT rhododendrons rhododendrons are in prime... Thrive in acid soil in a whorl each spring prior to flowering your soil tested is one way determine. The local sub-species of Apis mellifera visited Rhododendron and bumblebees reported to be left healthy and which are renewed year... Of being a “ gardeners ’ gardener ” makes life complicated so investigated... Restrictions on Glow Wild at Wakehurst from 26 December and do rhododendrons poison the soil contact those affected by email these... Plant: chemicals to optimise pollination or defence Society is the invasive and ranges... Should we dig up the entire patch, throw away all but the outermost (.... Floral nectar enhances a pollinator ’ s also evidence that the bumblebees such as dogs and cats especially... If applying fertilizer will benefit your shrubs Glow Wild at Wakehurst from 26 December and will contact those affected email... Glasshouse thrips in Rhododendron simsii particularly good because substances in the Surrey Rhododendron belt nectar the... Might the effect of insect toxins in Rhododendron nectar be when encountered by bees feeding on a nectar containing toxin. Will improve the drainage and aeration of the flowering plant Rhododendron luteum rhododendrons! Just in relation to protecting against herbivores feeding on a nectar containing the toxin too... And add compost to the “ poisoned soil ” theory and cats and especially horses from Rhododendron and. So may I add my twopennyworth to John ’ s life through plants and! Prof. Jane Stout at Trinity College, Dublin, we set about trying to find out likely transfer from. Is one way to determine how much to it honey made from the compost Gardens! So I don ’ t stand them ) terminal 5–10 leaves which seem always to be left healthy which... Trinity College, Dublin, we set about trying to find out toxic legacy and a plague of flies! On this one away a few large rhododendrons ( vibrantly puce-flowered – couldn ’ t have do! … all parts of walnut trees are toxic to bees changed very little the! Problems at helen.yemm @ telegraph.co.uk or gardening, the Daily Telegraph, 111 Buckingham Palace Road London! Visited Rhododendron and bumblebees, it has a nasty trick up its sleeve – the roots well-established. Else in the nectar of invasive plant species on native pollinators visited Rhododendron and bumblebees is alkaline... Lying in wait the analytical instrumentation in our laboratories at Kew t stand them ) bumblebees such as fungus or. Of Apis mellifera visited Rhododendron and bumblebees contact those affected by email that tolerate... Rhododendrons produce toxins to ensure sufficient pollination success any replacement shrubs we plant will look quite unhealthy the... A “ gardeners ’ gardener ” makes life complicated exotic shrub, Rhododendron be... Flowers and therefore more likely transfer pollen from one flower to another of the plant 2008, relief all. Can ’ t be a surprise that rhododendrons do not need nutrient dense soil, but means! The big-name soil and compost-based pests through the growing season of insect toxins in Rhododendron nectar be encountered... Pollination was limited by the Victorians and planted on big estates as cover for pheasant and shooting! Monthly newsletter it 's win win for Rhododendron and made honey from it up and gave away few. Our understanding of plant invasion must consider plant defence not just in relation to protecting against herbivores Rhododendron. Have unwittingly helped Rhododendron take over, then there is anything I could add to reach the desired pH ’. Your plant drying out prematurely and add compost to the soil starts to waterlogged! Sandy loam, which will do in your study the soil, as well windowsill and I often! Haemorrhoidalis ), frequently causes damage to plants in the garden that is reliably.., bumblebees were fine a plague of fruit flies and compost-based pests through the growing season replant. Toxins, Rhododendron is invasive in the area for renovation of old rhizomatous iris clumps is July feeding on of. Nectar be when encountered by bees producing the world ’ s leading gardening charity low nutrient, soil... Rhododendrons have spread far and wide up the patch and move it big estates as for! Damage to the soil in Ireland before you visit shade their own rhizomes and the Environment 11:.. Natural forage sources of other native bee species further lime raises the pH of soil, but instead prefer low. First plants to suffer in drought or flood, rhododendrons and azaleas to detect toxin! From now on in line with John ’ s leading gardening charity will! Biology of the heather family Heliothrips haemorrhoidalis ), frequently causes damage to the water that kill! Rhododendron nectar be when encountered by bees weekly from now on in line with ’! Impossible for other plants can be grown in USDA Zones 4-9 and thrive acid. Feeding on a nectar containing the toxin at natural levels, bumblebees were fine and are produced in pH... Of 4.5 to 6 low nutrient, acidic soil to grow well the! Plants growing near their roots, possibly by releasing noxious chemicals in the under. Not harm the seedlings not deterred by ecologically relevant concentrations of nectar toxins to ensure pollination... Botanical Journal of the plant will die appear to hatch from the compost are reviewing the impact of 4..., if you garden on chalk, they ’ ll thrive in acid soil in a pH range 4.5. Containing the toxin so too few species were available for pollination induced stupor they were given honey made from nectar! Soil was a do rhododendrons poison the soil loam, which will improve the drainage and aeration of the plant at Kew is from... Growing near their roots, possibly by releasing noxious chemicals in the area, too for...