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Rhododendron ponticum L., Rhododendron

Account Summary

Introduction, neophyte, uncommon, widely scattered, but locally dominant.

1946; MCM & D; near Colebrooke Park.

Throughout the year.

Growth form and preferred habitats

Originally planted around B & I in the larger estates and gardens probably around the early to middle period of the 19th century as a decorative plant and for upland game cover tolerant of exposure and peatland soils, R. ponticum soon became firmly established, locally dominant and then spread to adjacent areas of acid terrain. This large, vigorous evergreen shrub is especially frequent on peaty and sandy soils, but really in terms of soil conditions it avoids only limestone.

A study of invasive spread in N Wales by Thomson et al. (1993) found that whenever a seed source of R. ponticum existed, factors favouring colonisation were a northerly aspect, steep slopes and a habitat where soil is occasionally disturbed by some form of management, eg tree planting or thinning, scrub clearance, moor burning or grassland ploughing. Despite the toxins it contains, intensity of sheep grazing appears to help inhibit Rhododendron establishment, possibly by removing or trampling small seedlings. However, ungrazed vegetation may simply provide a more favourable microclimate for Rhododendron seedlings. Spread of R. ponticum is further inhibited by excessive wind exposure on higher ground and it avoids permanently boggy ground at all altitudes (Thomson et al. 1993).

Fossil history and native distribution

The fossil pollen record preserved in peat bogs shows that R. ponticum was widespread in southern C Europe and extended northwards into Ireland in the Gortian interglacial warm period, 250 to 200 thousand years ago (equivalent to the Hoxnian Period in England) (Jessen et al. 1959; Watts 1959; Mitchell & Watts 1970). In the current interglacial the native distribution of the species is much further south in the Mediterranean basin and it is very disjunct − the main region being the Pontic and Anatolian regions of Turkey, Bulgaria and the Russian Republics (W Caucasia) on the S & E shores of the Black Sea and the immediately adjacent areas, plus a small outlier in the Lebanon (Chamberlain & Cullen 1982, p. 315, Map 94). The secondary native area of the species is comprised of three small Tertiary relict populations, surviving, but now vulnerable, in C and S Portugal and the Gibraltar area of S Spain (Cross 1975; Bean 1976; Mejias et al. 2002).

Introduction to B & I and origin

The original introduction of R. ponticum to B & I either took place around 1763 (the date usually quoted eg by Elton (1958) and Harvey (1988)), or more probably (at least for the commercial introduction) between 1775 and 1780 (see D.L. Clarke, in: Bean 1986, p. 742). The original material was almost certainly imported via Gibraltar − thanks to the Rock's strategic importance and its colonial status. Confirmation of the Iberian origin of most B & I R. ponticum, rather than from the Pontic or other eastern region, has been obtained from a recent genetic study of chloroplast DNA and nuclear ribosomal DNA haplotypes (Milne & Abbott 2000). Examining a total of 260 samples from sites around B & I, these workers found that 10% were of a unique Portuguese haplotype and 89% were of a type almost exclusively from S Spain.

Interestingly, the remaining 1% of accessions examined possessed chloroplast and nuclear ribosomal DNA of other introduced Rhododendron species that are cultivated in B & I, but which have never escaped into the wild. Two American species are undoubtedly involved, R. maximum and R. catawbiense, the latter closely allied to R. ponticum despite it being a plant of Eastern USA origin. R. catawbiense was first implicated as being introgressive along "with other influences here and there", by Cox & Hutchinson (1963) when they inspected the wide range of R. ponticum variation in Turkey and compared it with naturalized material growing wild in Britain.

The DNA study showed that introgression involving R. catawbiense was significantly more abundant in E Scotland, Britain's coldest region. Milne & Abbott (2000) suggested that the B & I naturalised R. ponticum may have increased cold tolerance conferred on it as a result of this introgression, a factor which may have allowed the plant to colonise colder parts of these islands to which the Iberian material was not preadapted (Abbott et al. 2003). Further study is required to eliminate the possibility of this particular introgressed material having been introduced to E Scotland by mere chance, rather than the distribution reflecting such an advantageous adaptive feature.

The introgressive genetic material that the evidence suggests must have originated from crosses R. ponticum made with other introduced Rhododendron species, or with hardy hybrids during their joint garden cultivation and these genetic pairings probably date from the early years of the 19th century (D.L. Clarke, in: Bean 1986, p. 743). Nelson (1994) has reported that R. ponticum was in cultivation in Ireland by at least as early as 1800.

Naturalization and spread

The species appears to have been first recorded in the wild in Britain by at least 1894 and it appears to have spread particularly rapidly in the early years of the 20th century, both by seed and by layering itself (Cross 1975). Little detail is known about the dates of naturalization and the rate of invasive spread into the wild in either Ireland or Britain, due to a general lack of documented evidence, or perhaps to its current obscurity. It is however known that the plant was recognised as having "strong regenerative ability" by the middle of the 19th century, for this was reported to J.D. Hooker by his correspondents in Hampshire and S Wales while he was still in India collecting rhododendrons and writing his major work on the genus in the Himalaya (Hooker 1849; Desmond 1999).

Flowering and conditions for seedling establishment

Unlike some Rhododentron shrubs, R. ponticum flowers when it is still very young. It is self-compatible and like other members of the heather family it produces massive quantities of tiny seed. Each Rhododendron inflorescence produces around 5,000 seeds and Cross (1981) estimated that in Muckross, Co Kerry, a small bush 11 m in circumference and just 2 m high in a semi-shaded position could produce over one million seeds.

In producing vast numbers of small, readily dispersible seed, the reproductive strategy of the Rhododendron shrub resembles that of species in the Pyrolaceae and the Orchidaceae. This is very unlike other B & I woodland shrubs, which produce seed on average 100 times heavier than those of R. ponticum (Salisbury 1942; Cross 1981). R. ponticum seeds are short-lived, surviving only a few months. They require light to germinate and in favourable conditions do so about five or six days after release. Seedlings survive best on slopes where there is a thin carpet of moss or liverwort, since when small they are extremely susceptible to drought. Other factors limiting the species involve the facts that young seedlings are unable to survive burial by shifting soil litter and they do not have much competitive ability against taller herbaceous vegetation (Cross 1981).

Once the shrub has become established, grown taller and branched to form a domed shape, the heavy evergreen shade that it casts and the toxins it contains allow the species to dominate the vegetation. Indeed, typically R. ponticum excludes all other plants from beneath it. In suitable growing conditions in woodland and with the physical support of trees, naturalized specimens of R. ponticum can reach a height of 7 to 8 m and form an impenetrable thicket of branches (Cross 1975, 1982).

Fermanagh occurrence

In Fermanagh, R. ponticum has been recorded in 74 tetrads, 14% of those in the VC, chiefly around Upper and Lower Lough Erne. Here it often forms very dense evergreen thickets from 3 to 5 m tall. Although, as the distribution map indicates, it is widely scattered throughout the county but, in high density, it remains centred on the old estate woods where it was first planted and in a few other upland peaty areas, eg within the Lough Navar Forest Park.

While the species has reached significant weed status in some lowland woods in Fermanagh, it is still comparatively rare on both the lowland bogs and in most upland woods and peatland areas of the VC.

Toxic properties

The leaves, flowers, pollen and nectar all contain several very poisonous diterpenoids (or grayanotoxins) and the shrub has been recognised as poisonous since ancient times (Cooper & Johnson 1998). In 400 BC, Xenophon reported poisoning of Greek soldiers from honey made by bees from wild rhododendrons − a thing virtually unknown since then due to better apiary management (Allison & Day 1997). Fortunately, stock animals are only likely to graze evergreen shrubs, including R. ponticum, under very adverse conditions when other food is extremely scarce. Poisoning is most common in sheep for some unknown reason (particularly in rams) and losses are annually reported in the N & W of Britain (Cooper & Johnson 1998). Roots of R. ponticum also poison the soil, releasing short-chain aliphatic acids which deter the roots of other plants − the phenomenon known as allelopathy. Its own roots are mycorrhizal, having an association with soil fungi and in nutrient poor acidic soils they may starve competing plants of vital nutrient requirements.

Invasive nature and control

In Fermanagh, while the species has reached significant weed status in some lowland woods, it is still a comparatively rare species on lowland bogs and in most upland woods and peatland areas of the VC. In view of the invasive behaviour of the species elsewhere in these islands in wet, cool oceanic climatic conditions, eg in W Ireland, N Wales, the Lake District and the W Highlands of Scotland (Cross 1975, 1982; Abbott et al. 2003), every effort should be made to keep these types of habitats in Fermanagh and elsewhere completely free from R. ponticum, by the immediate uprooting and burning of any plants that are found spreading.

Numerous studies around the world have proven how difficult or impossible control is once Rhododendron (often R. ponticum) gains a real hold, particularly in a wood or forest situation where its presence significantly decreases tree regeneration and growth (Esen & Zedaker 2003). A comparative field study of various manual and herbicide 'control' measures in Turkey by the latter authors found that low rates of foliar-applied Arsenal SL (imazapyr) (ie 3.0 kg active ingredient/ha), sprayed when translocation is downward to the roots (ie between mid-July and mid-September), gave significantly greater Rhododendron control than other herbicides including Garlon 4 (triclopyr ester). Physical grubbing (ie manual uprooting from the uphill direction) also gave a great potential for sprout crown reduction. From what has been said above, grubbing up bushes and burning them on site provides an ideal seed bed if other Rhododendron bushes remain nearby to provide a source (Thomson et al. 1993).

Names

The genus name 'Rhododendron' comes from two Greek words meaning 'Rose tree' and is thought to have originally been applied to the rose-flowered form of Oleander, Nerium oleander, now transferred to this genus (Gilbert-Carter 1964). The Latin specific epithet 'ponticum' is geographical, meaning 'belonging to the south shore of the Black Sea' (Stearn 1992).

Threats

Although R. ponticum is a serious invasive weed of woodland and peatland, it has limited powers of dispersal and thus its spread can be predicted. Once established, it is very persistent, steadily increasing in density. Control is best achieved by preventing spread, which requires constant vigilance to restrict the shrub to existing sites by avoiding soil and vegetation disturbance near seed sources (Rotherham 2003). In 2003, the fungal pathogen Phytopthora ramosum responsible for a 'Sudden Oak Death' epidemic in N America was introduced to a garden plant nursery in Cornwall, very probably on a rhododendron cultivar. The pathogen has quickly spread across most of southern Britain and all of Ireland, and is now capable of attacking a wide range of woody species including bilberry (Vaccinium myrtillus), larch (Larix spp.), ash (Fraxinus excelsior) and chestnut (Castanea sativa).