They’ll thrive in acid soil but, if you garden on chalk, they’ll flourish in pots, too . One thing it likes plenty of is water, and needs well-draining soil. Everything thrives – giving no credence to the “poisoned soil” theory. The time for renovation of old rhizomatous iris clumps is July. The soil mentioned in the post is alkaline and even acidic rain is not making it acidic. 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. Discover rhododendrons. 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. Frontiers in Ecology and the Environment 11: 251-259. These acid-loving plants can be grown in USDA Zones 4-9 and thrive in filtered shade and well-draining soil. We absolutely adore irises: should we dig up the patch and move it? Kew's recent study provides the first scientific evidence that the grayanotoxins are a natural defence mechanism against herbivores of Rhododendron. 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. However, when the nectar was fed to honeybees they died in a matter of hours. Many plants synthesise defence chemicals so it shouldn’t be a surprise that rhododendrons do as well. Curiously, the thrips rarely damage the terminal 5–10 leaves which seem always to be left healthy and which are renewed each year. The reason is that if the rhizomes were smothered by the periwinkle during last summer (and possibly the summer before that, by the sound of things), they will not have received the real sun-baking needed to form flower buds. 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. Available online, Open today Make soil acidic for rhododendrons by adding elemental sulfur or iron sulfate to the soil. Caffeine in floral nectar enhances a pollinator’s memory of reward. 1. 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. 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. Their fine roots can’t penetrate heavy, compacted soils. Surprisingly, these luscious plants do not need nutrient dense soil, but instead prefer a low nutrient, acidic soil. Egan, P.A., Stevenson, P.C., Tiedeken  E.J., Wright, G.A., Boylan, F. & Stout, J.C. (2016). 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. Is this true? There’s also evidence that they stop other plants growing near their roots, possibly by releasing noxious chemicals in the soil. 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. 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. 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. Rhododendrons do not need substantial Nitrogen for healthy growth. In the process of redoing our driveway, we are removing several very large, old rhododendrons. © Board of Trustees of the Royal Botanic Gardens, Kew, 10am–4.30pm* (last entry 4pm) *Closing times vary. In the absence of a soil test and other cultural or pest problems, use a complete fertilizer with a ratio of 3:1:2 or 3:1:3, such as 12-4-8 or 15-5-15. “Rhododendron poisons the soil around it so that other plants cannot grow.” Plantlife. However, after my Shamley Green experience I resolved to do more research on what I have long believed to be a distinctly dodgy dictum – spread like a horticultural Chinese whisper – that “rhododendrons poison the soil”. These are the youngest leaves and are produced in a whorl each spring prior to flowering. 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. At which point you could dig up the entire patch, throw away all but the outermost (i.e. Can you please advise if there is anything I could add to the water that would kill the flies but not harm the seedlings. If your shrub is adjacent to the lawn, cease using high nitrogen fertilizers in the area. Thrips are tiny winged insects which include some species that are pests of agricultural and horticultural crops around the world, damaging plants by sucking the life out of them. 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. If you are one of those gardeners who cannot bear to be brutal (i.e. 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. This combination of no light and minimal moisture makes it impossible for other plants to survive under or around them. Available online, Tiedeken  E.J., Stout J.C., Stevenson P.C. Botanical Journal of the Linnean Society 155, 373–381. ... and add compost to the soil … The net result is that rhododendrons have spread far and wide. To save much head-scratching and possible confusion, when you sow, stick a label in all pots with the variety name and the date. Both Kew Gardens and Wakehurst are open in the daytime (although some of our buildings are closed). Exceptions are the deciduous shrubs and trees. I suspect, sadly, that there is every chance that these irises will not flower this year, despite their valiant attempts to produce leaves. We aim to enrich everyone’s life through plants, and … We are reviewing the impact of Tier 4 restrictions on Glow Wild at Wakehurst from 26 December and will contact those affected by email. & Stevenson, P.C. 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. Before planting dig in lots of humus and compost. Where I live, the soil is naturally acidic so I don’t have to do much to it. ... How do you grow Rhododendrons? 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. One might expect that this would be bad for the pollinators, or would repel them, but this isn’t always the case. So may I add my twopennyworth to John’s advice? They like acid soil in a pH range of 4.5 to 6. Write to Thorny Problems at helen.yemm@telegraph.co.uk or Gardening, The Daily Telegraph, 111 Buckingham Palace Road, London SW1W 0DT. The toxins responsible for the poisonous effects of Rhododendron are grayanotoxins. Soil Acidity. 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. 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. Journal of Experimental Biology 217: 1620-1625. Membership, Open today Available online, Stout, J.C. (2007b). Other chemicals in the nectar which were not toxic to bees changed very little between the invasive and native ranges. Poisonous nectar. Added to this, their long life ensures that their constantly dropped, leathery leaves (that take years to rot) form a thick carpeting layer beneath them that makes seed germination almost impossible. (She intends to sow little pots of salad weekly from now on in line with John’s instructions). Rhododendrons and azaleas are popular garden shrubs with colorful flowers that bloom in spring, and reblooming varieties will bloom again in summer until fall. 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. Nectar chemistry modulates the impact of invasive plant species on native pollinators. Predominantly, rhododendrons are shrubs that are evergreen. (2014). All parts of walnut trees are toxic to rhododendrons and azaleas. Earlier work had reported that the bumblebees such as Bombus terrestris pollinated Rhododendron so we investigated this bee species further. And it does poison the soil around it so nothing else grows. 'Mad honey' is the toxic honey produced by bees feeding on nectar of Rhododendron ponticum in its native northern Turkey. The Roman soldiers were reported to be confused and vomiting and subsequently defeated in battle after eating the honey. In rhododendron ponticum’s original home, the Black Sea area of Turkey, cases of poisoning have been reported since that century as well. 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. As mentioned, the rhododendrons grow to overtake much of Manderley in the Unnamed Narrator’s dream. (2013).Threats to an ecosystem service: pressures on pollinators. Their leaves will start to turn yellow, and the plant will look quite unhealthy if the soil is too alkaline. Answering as always off the cuff, i.e. The Acid Test: Alan Titchmarsh on growing Rhododendrons RHODODENDRONS are in their prime right now. In collaboration with Prof. Jane Stout at Trinity College, Dublin, we set about trying to find out. You can grow Rhododendrons in two ways. Science 339: 1202-1204. We have just cut back an overgrown mass of small-leafed periwinkle in our new garden and discovered an 18in-wide patch of very old iris rhizomes starting to produce leaves. Christmas at Kew is suspended from 20 December up to and including 1 January due to the introduction of Tier 4 restrictions in London. 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). Despite the fact that Rhododendron honey was first reported as being toxic thousands of years ago, this was the first time the toxin had been identified and quantified in the nectar of the flowers from which the honey was made. By coincidence I was asked a similar question recently at a talk I was giving at Shamley Green in the Surrey rhododendron belt. Dr Alison Scott-Brown. Firstly, we had to isolate the toxin from the flowers using the analytical instrumentation in our laboratories at Kew. 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. So what does this mean for the defence of the plant? Get exclusive updates, offers and more in our monthly newsletter. They do little damage to the roots of well-established plants (most houseplants, for example). Like many woodland plants, Rhododendrons and Azaleas require acidic soil to grow well. 2 “It produces toxins, and suppresses other plants by poisoning the soil as well as year-round Get involved. So the effect was for the plants to reduce or stop entirely the production of nectar toxins to ensure sufficient pollination success. Functional Plant Biology(in review). The Roman garrison marching along the road sometime later stumbled upon this apparent gift from the gods and not recognising the danger, enthusiastically scoffed it, unwittingly poisoning themselves in the process. This was surprising as we know that in Turkey the local sub-species of Apis mellifera visited Rhododendron and made honey from it. Method No. Tip. Secondly, you should know that the rhododendron prefers quick draining soil. We carried out a landscape scale chemical analysis of populations of Rhododendron ponticum in Ireland and compared this to the nectar toxin levels in its range in Spain and Portugal where the species is also native. 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. 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. Bumblebees are not deterred by ecologically relevant concentrations of nectar toxins. “Rhododendron poisons the soil around it so that other plants cannot grow.” Plantlife. Helen Yemm answers readers' garden queries. Many plants synthesise defence chemicals so it shouldn’t be a surprise that rhododendrons do as well. Despite my extensive enquiries I still half expect some science/conservation flak on this one. Leaf trichomes and grayanotoxin 1 mediate plant defence against glasshouse thrips in Rhododendron simsii. Outbreaks among humans in the 1980s were confirmed in Turkey and Austria, as well as in goats in Britain. The toxins occur in nectar too where they ensure that only the best pollinators (that can tolerate the toxins) visit. Chemical analysis of these terminal leaves at Kew revealed they contained very high concentrations of grayanotoxins – the same compounds that poisoned Pompey’s army. 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. 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. Pine needles have a pH between 3.2 and 3.8, which means that they can turn the soil slightly more acidic if you work them into the soil as compost. One of the earliest accounts of mass poisoning dates back to the first century BCE when Roman troops were allegedly poisoned with honey by the Heptakometes of Turkey. Pacific rhododendron exhibiting tall growth pattern Jeff Foott / Getty Images How to Grow Pacific Rhododendron . Rhododendron - planting and cutting. It is only available from April (when the soil generally reaches the correct temperature for the nematodes to become active). Arguably these are the most critical leaf tissues so more important to protect. 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. My thanks to the woodland ecologist Dr James Merryweather, whose information on the subject I found riveting. For the gardener – collecting pine needles is not going to change the soil pH. 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. It could cause issues such as fungus gnats or worse, root rot, and you will notice the flowers drooping as well. ‘Rhododendron ponticum (the Pontica rhododendron) is the invasive type,’ says Stephen. In this case, however, we found that the caffeine was present in nectar at concentrations too low for honeybees to taste but it still had a remarkable pharmacological effect on the bees. 10am–4.30pm* (last entry 4pm) *Closing times vary, Ardingly, Haywards Heath, Sussex, RH17 6TN. 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). 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. It can be harmful to your pet animals such as dogs and cats and especially horses. 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. As a very effective invasive, Rhododendron could be out-muscling native plant species and natural forage sources of other native bee species and honeybees. the best and youngest) rhizomes, trim their leaves back and replant them somewhere else in the garden that is reliably sunny. But rather like the Roman armies of yore that were poisoned by its toxins, Rhododendron is invasive in the British Isles. Professor Phil Stevenson We now believe that they were given honey made from the nectar of the flowering plant Rhododendron luteum. Book a time slot to Kew Gardens or Wakehurst before you visit. Soil testing information is available in HGIC 1652, Soil Testing. 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. Which function takes precedence for the plant: chemicals to optimise pollination or defence? So not only does Rhododendron block out life-giving light, but it poisons the soil as well.” Since they are not tap-rooted they are the first plants to suffer in drought or flood. 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. 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. ‘It was brought over by the Victorians and planted on big estates as cover for pheasant and grouse shooting. So what might the effect of insect toxins in Rhododendron nectar be when encountered by bees? Scott-Brown, A., Gregory, T., Farrell, I. Reproductive biology of the invasive exotic shrub, Rhododendron ponticum L. (Ericaceae). In 2008, relief … Sometimes the responsibility of being a “gardeners’ gardener” makes life complicated. While in their toxin induced stupor they were helplessly slaughtered by the army of Mithridates VI which was lying in wait. Introduction to Britain. Even after 30 days feeding on a nectar containing the toxin at natural levels, bumblebees were fine. Functional Ecology (online). For instance: plant it within the root system of a black walnut tree. 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. Everything you need to know about choosing the right rhododendron for you. Rhododendrons are responsible for producing the world’s first recorded chemical weapons. Adding a large amount of organic matter will raise the bed, which will improve the drainage and aeration of the soil. The Royal Horticultural Society is the UK’s leading gardening charity. Stout, J.C. (2007a). By We have been told by several people that the bushes will have poisoned the soil and that any replacement shrubs we plant will die. Our understanding of plant invasion must consider plant defence not just in relation to protecting against herbivores but the consequences for beneficial interactions too. 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). The buff-tailed bumblebee (Bombus terrestris) was unable to detect the toxin at natural levels so it was certainly not repellent. 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. 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. 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. Lime raises the pH of soil, bringing it into a range that is inhospitable to the roots of Rhododendrons. Plant toxin levels in nectar vary spatially across native and introduced populations. I think a lot has to do with the soil type. Rhododendrons … 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. Journal of Ecology Available online. In your study the soil was a sandy loam, which is much easier to change pH than clay. 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. Pine bark is particularly good because substances in the pine bark are thought to inhibit fungi that cause root rot. The Content Of The Article: Rhododendrons are a genus of the heather family. 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. Available online, Vanbergen A.J. Be sure to test the soil first to determine how much to add to reach the desired pH. This week: rhododendrons' toxic legacy and a plague of fruit flies. Six years ago I dug up and gave away a few large rhododendrons (vibrantly puce-flowered – couldn’t stand them). It does not have deep roots--so it does not compete with other plants for soil nutrients--and tends to thrive in moist, acidic (low pH) soils. Spatially explicit studies of nectar secondary compounds are needed to understand how chemical variation affects floral ecology. 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 … In my garden only the odd tough laurel sapling gets beyond the seedling stage in the gloom under my big rhodos. I start seedlings on my windowsill and I am often plagued by very small flies which appear to hatch from the compost. The common term for rhododendron is also Rosenbaum. In other words, in the invasive range pollination was limited by the toxin so too few species were available for pollination. Bumblebees will spend more time feeding from Rhododendron flowers and therefore more likely transfer pollen from one flower to another of the same species. (2013). Rhododendrons produce toxins to protect against herbivores feeding on their leaves. Milder symptoms of Rhododendron poisoning can range from stomach irritation all the way to abdominal pain. & the Insect Pollinators Initiative. The toxin levels in nectar of invasive plants are lower than that of native plants. These are highly oxygentated diterpenoids that have been presumed to be produced elsewhere in the plant as a natural chemical defence against insects. They do little damage to the roots of well-established plants (most houseplants, for example). One particular species, the glasshouse thrip (Heliothrips haemorrhoidalis), frequently causes damage to plants in the glasshouse collections at Kew. This lovely tree exudes a toxin in the soil which will do in your rhodo. 10am – 3pm (last entry 2pm), Tickets (Rhododendrons and azaleas are, with blueberries, among the few plants that grow best in very acid soils, but mulching is not going to acidify their soil.) Cases of rhododendron poisoning among humans who have eaten toxic honey date as far back as 400 B.C. 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). To do this, it has a nasty trick up its sleeve – the roots are actually toxic to other plants! Apidologie 38 ,1–9. If the soil starts to become waterlogged, this could lead to a bunch of problems with the plant. Having your soil tested is one way to determine if applying fertilizer will benefit your shrubs. Rhododendron ponticum is native to countries in the western and eastern Mediterranean such as Spain, Portugal and Turkey and also occurs eastwards through Asia into China.It is not native to Britain, but was first introduced in the late 18th Century. This is the first written account of “honey intoxication,” in which 10,000 Greek soldiers were affected by eating honey made from rhododendron nectar. Many shrubs and flowers prefer neutral and even alkaline (high pH) conditions. 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. (2016). 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). Introduction 2.1. Pollination of invasive Rhododendron ponticum (Ericaceae) in Ireland. Applied every five weeks, Nemasys GYO will control this and all the big-name soil and compost-based pests through the growing season. Although it didn’t increase mortality in the mining bee, it severely affected their normal behaviours important for foraging such as time spent flying and increased time spent grooming, as well as showing other symptomatic behaviours associated with toxicity. Discover rhododendrons. 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. "For the finer water-holding … 2. So it's win win for Rhododendron and bumblebees. 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. Testing the compounds against a third species, the mining bee, Andrena scotica (previousy A. carantonica) showed it was harmful to this species too. I know this sounds silly, but it means that the leaves will not shade their own rhizomes. If there is only room in the landscape for Rhododendron when they take over, then there is only room for bumblebees. Failure to do this can result in your plant drying out prematurely. 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. Could our lovely bumblebees have unwittingly helped Rhododendron take over huge swathes of our natural landscapes? Giving Your Plant Space. and Then it got out of control and spread everywhere. All rhododendrons need well-drained, loose, friable soil. & Wright G.A. Soil Preparation. Even though it likes acidic soils, it can grow in all kinds of soil … Plant In Acidic Soil. Pacific rhododendrons are full and rich with color. Garden only the best pollinators ( that can tolerate the toxins responsible for producing the world s... 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