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Rhododendron - Weed KillerRhododendron:

Rhododendron ponticum ssp. baeticum. Rhododendron ponticum ssp.ponticum.


Common Names
Rhododendron, Common Rhododendron.

Introduction History
Rhododendron ponticum was grown initially as a horticultural exhibit and then widely as an ornamental in parks, gardens and estates. Its toughness and ability to cover the ground soon became apparent, and it was widely planted in the woodlands and heaths of sporting estates throughout the British Isles, to provide game cover. Where not distributed by nurserymen, as a plant in its own right, it was used widely as a rootstock for the propagation of more desirable cultivars by grafting. Such plants were rarely totally satisfactory, as the ponticum rootstock frequently suckered below the graft point to the detriment and often elimination of the desired variety.

It is common in collections of Rhododendrons, planted perhaps up to the middle of the 20th century, to see great branches of ponticum dominating what may be left of a less vigorous but superior cultivar. Such overgrowth of rootstock can provide a nucleus for invasion to begin. Nowadays, easily rooted hybrid Rhododendrons are propagated to provide non-suckering rootstocks, and ponticum is no longer used.

Rhododendron ponticum has two subspecies. The first is Rhododendron ponticum subsp. ponticum, which has a general range from Bulgaria to Turkey and east to Georgia. The second, from which almost all UK populations were derived, is Rhododendron ponticum subsp. baeticum, which is native to Spain and northern Portugal. There is evidence to suggest that its range was much greater in pre-ice age Europe 20,000 years ago, and indeed may have been native here in the UK, under very different climatic conditions, when we were still joined to continental Europe. DNA analysis of wild-type ponticum, in the UK, indicates not only that it originates from populations native to the Iberian Peninsula, but that some interbreeding with two similar but distinct species, R. catawbiense and R. maximum (which are both native to the USA, not Europe) has occurred. This hybridity is widely, if not universally, distributed throughout invasive UK populations. As with the x bohemica form of Japanese Knotweed, beware the hybrid; it may be stronger and more resourceful than the parent!

In moist woods and heaths in the north and west, given moderate soil acidity, Rhododendron frequently grows unchecked. Plants throw up suckers from surface roots; branches root where they touch the ground. Gradually a tangle of near-horizontal branches builds up to a height of over 10m, and an individual plant may cover upwards of 100m2 of ground. Streams may be bridged as part of this process, and the canopy of thick evergreen leaves excludes all light from the banks and the water, as it does the woodland floor. Nothing grows, and the whole system of herbivorous invertebrates and vertebrates and the animals which feed on them vanishes. Even the water of the engulfed streams dies. There are no plants left on the banks, therefore no invertebrates, therefore little food for fish. There is no light for aquatic plants. Only the trees taller than the Rhododendron survives. When they die, there is no chance of seedling regeneration in the dry darkness beneath the Rhododendron.

If any light or moisture gets through on the margins of an invasion, a thick, toxic mulch of decay-resistant chemical-laden leaves prevents all but a very few species from germinating. Above this Tolkien-esque gloom, the spectacular flowers in their season attract insects so strongly, that there are fewer available to pollinate native plants in neighbouring areas. By this means of indirect competition, areas outside a ponticum monoculture can likewise suffer declines in native plants and their dependant species.

Rhododendron leaves are tough and unpalatable to livestock, though may occasionally be eaten, resulting in poisoning caused by the phenol and diterpene constituents in the sap.

Rhododendron has also been found to be the host plant for two very serious and increasingly frequent diseases of hardwood and coniferous trees, Phytopthora ramorum and Pyhtopthora kernoviae.

Honey containing a large amount of converted Rhododendron nectar, may also cause poisoning to human beings, though fatalities are rare.

A handsome evergreen shrub or tree of 5-10m height at maturity. Leaves, oblanceolate to broadly elliptic, 10-20cm x 2-6cm, sometimes larger. Dark green above, paler below, glabrous. Inflorescence with up to 15 flowers, and stem densely pubescent in the baeticum subspecies. Calyx, short, to 2mm; glabrous with 5 blunt teeth. Corolla, campanulate, pinkish-lilac to blue-mauve, 3.5-5cm long with greenish yellow spots in the throat. Style glabrous. Stamens, 10, with filaments pubescent at base. Fruit, a dry capsule to 25mm, containing up to 7,000 seeds.

As has already been stated, Rhododendron has great powers of vegetative spread. It also seeds prolifically, with a large plant producing a million or so wind dispersed seeds per year. Wherever these germinate, they quickly form intimate mycorrhizal associations with specific bacteria which facilitate the absorbtion of nutrients from infertile soils, and encourage a typically densely branched system of fine roots, with the maximum capacity for dissolved mineral uptake. This feature gives Rhododendron an immediate competitive advantage over other species present. Bacteria of this kind also live in symbiotic relationships with other members of the family Ericaceae, such as Heather and Ling, allowing Rhododendron to rapidly overrun fragile heathland habitats. This advantage is maintained by an imperfectly understood allelopathic suppression of germination and growth of other plants, probably involving the inhibition of their own mycorrhizal bacteria.

Even when Rhododendron is cleared from an area, the root-mat, mulch layer and soil (which will contain much rhododendron-derived material) hinders the germination of most other species for a period of several years. This problem can be addressed by removal of the whole of this layer, which by reason of thin soil or other practical considerations, is often impossible. My childhood recollections of finding abundant foxgloves (Digitalis purpurea) growing in the company of Rhododendron, is well borne out in fact, as this species is one of the few that is able to resist the inhibitory effects of Rhododendron.

Control Methods
Chain saws, flails, mechanical diggers or hand tools can be used to physically remove Rhododendron. The huge costs in time and manpower of these types of operation can be partially mitigated by the willingness of conservation volunteers to go "Rhody-bashing". Such techniques, nevertheless, are obviously impracticable in many cases. It should also be noted that the ground disturbance often involved, whilst not removing the inhibitory litter layer, does provide the ideal conditions for the germination of Rhododendron seeds from the persistent seed bank.

Chemical controls, likewise have, in most cases, limited efficacy. It is easy to understand that the size of plants can impede foliar spraying, and penetration of the tough leaves, even with suitable adjuvants, may not allow lethal doses of herbicide to be absorbed. The reduction of the range of herbicides available is also a handicap; with one of the more effective chemicals, Imazapyr, being no longer permitted in the UK

The method mainly recommended until very recently was cut stump application with Glyphosate. Recent field experience suggests that stem injection with Glyphosate may kill a larger proportion of treated plants than the cut stump method. I also consider that earlier trials with Ammonium Sulphamate should be repeated using the cut stump technique, or by stem injection with solution or crystals. back to top

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