Rhododendrons: Rhododendron ponticum

Discussion in 'Ericaceae (rhododendrons, arbutus, etc.)' started by Douglas Justice, Aug 1, 2003.

  1. Douglas Justice

    Douglas Justice Active Member UBC Botanical Garden Forums Administrator Forums Moderator VCBF Cherry Scout Maple Society 10 Years

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    The following was received via email:

    In a book on rhodos which I recently purchased in the U.K. there is the following mention of R. ponticum:

    "As well as being used as an ornamental in its own right, R. ponticum was widely planted as game cover throughout Britain and Ireland, and was extensively used as a rootstock for grafting less vigorous but floristically superior forms. Its vigour and tendency to spread rampantly, however, with its ability to render the soil beneath it poisonous to other plant species, is a legacy that can still be seen."

    The part that surprised me was the fact that it renders the soil poisonous. Is this true?

    Secondly, this spring I noticed a couple of plants purchased from reputable nurseries via the VanDusen plant sale (Christmas Cheer & Taurus) had obviously been grafted onto R. ponticum, as they both had strong growth of that species which, after consulting with a rhododendron society member, I cut off to the base.

    So my second question is how many of our local plants are grafted onto R. ponticum - do you have any idea? With our mild climate I am puzzled as to why this is necessary unless plants are sold further afield in British Columbia where the winter temperatures dip lower than here.

    I guess these 2 queries are linked as I wonder if the roots of R. ponticum contaminate the soil as the book I have states they do, in which case should I dig up my Christmas Cheer & Taurus? Also, is there any way of checking in future which plants are grafted, because it certainly isn't apparent to my inexperienced eye.
     
  2. Douglas Justice

    Douglas Justice Active Member UBC Botanical Garden Forums Administrator Forums Moderator VCBF Cherry Scout Maple Society 10 Years

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    Rhododendron ponticum is a vigorous grower that is easy to propagate and grow in a variety of garden situations. As a rootstock it easily accepts scions of other rhododendrons and is reasonably well-behaved. This makes it an ideal grafting stock. Rhododendron ponticum was used commonly by nurseries locally until the 1960s or even later, until better propagation technology and easier to root cultivars changed nursery practices. Apparently the older practices haven't died out.

    Many older rhododendrons in the landscape (particularly hybrids) have been grafted. Eventually, the understock usually shows itself and given time, will overtake the scion. Rhododendron ponticum is inherently more vigorous than most other species and hybrids. This is why it is prudent to immediately remove any growth that appears from below the graft union. Once the ponticum starts manufacturing food, it will build up strength below the graft and compete with the scion for nutrients.

    A tour around the more "established" neighbourhoods in Vancouver, for example, shows numerous 'Roseum Elegans', 'Cynthia', and 'Fastuosum Flora Pleno' plants (usually in the middle of the front lawn) with understock showing. Notice that turfgrass grows well in these situations, and doesn't appear to be adversely affected by the R. ponticum. A number of older plants in the botanical garden collection are grafted. These are growing alonside other non-grafted plants, without apparent negative effects.

    Nevertheless, outside of container and garden culture, there is plenty of evidence that R. ponticum has allelopathic effects (i.e., prevents the growth of other plants). This suggests that the effects are slight except where accumulation of leaf litter is considerable, such as in the kind of thickets that develop in the UK and in the drier, milder parts of the Pacific Northwest. See the following websites for more information.

    webpage from Oliver Merrington
    Offwell Woodland and Wildlife Trust webpage
    American Journal of Botany article on R. maximum (pdf)
     
    Last edited by a moderator: Oct 5, 2005

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