Improving strategies for long-term eradication of rhododendron

This work area involves a series of field experiments designed to test and evaluate three management strategies:

1. Use of alternative chemical herbicides to prevent regrowth and
2. Use of the bioherbicide Chondrostereum purpureum

Regrowth of rhododendron 3 months after cutting in a no treatment control plot
Regrowth of rhododendron 3 months after cutting in a no treatment control plot
Chondrostereum purpureum growing on a birch stump
Chondrostereum purpureum growing on a birch stump

Rhododendron is difficult to treat chemically because the uptake of foliar-acting herbicides through the thick, waxy leaves is low.  Field experiments have been established to examine the efficacy of a range of chemical herbicides applied as stump treatments to control regrowth of cut infected rhododendron plants.  Reducing the amount of infected regrowth and killing infected stumps and root systems would help to reduce inoculum levels on the site.

In addition to the chemical stump treatments trialled, treatments have also been established to investigate the efficacy of the bioherbicide Chondrostereum purpureum (Cp).  Cp is a basidiomycete fungus known to suppress the regrowth of target vegetation when applied to cut stumps. The native strain of the fungus has not previously been tested in the field in Britain but may have potential in controlling regrowth of cut rhododendron stumps.

Eight chemical herbicide and bioherbicide treatments are being trialled on a site in Cornwall:

  • Control (rhododendron cut and stumps untreated)
  • Roundup Pro Biactive® (glyphosate) as a 20% solution in water
  • Tordon 22K (picloram) as a 2.5% solution in water
  • Timbrel (triclopyr) as an 8% solution in water
  • Timbrel (triclopyr) applied undiluted
  • Roundup Pro Biactive® (as a 20% solution) plus growing medium for Chondrostereum purpureum (Cp) but without the Cp (see below for further information on Cp)
  • Roundup Pro Biactive® (as a 20% solution) plus Cp comprising mycelia concentration of > 105 colony forming units (CFU) per ml growing medium / carrier
  • Diluted growing medium only (no Cp present), Cp comprising mycelia concentration of > 105 colony forming units (CFU) per ml growing medium / carrier.

All treatments were applied immediately after cutting.  Initial assessments of the fate of inoculum, presence of Cp fruiting bodies and re-sprouting of cut stumps have been carried out at regular intervals since treatment.

In general, all herbicide treatments from both spray dates appear to be very effective at controlling regrowth. The triclopyr treatments appear most effective, followed by picloram and then glyphosate. The biocontrol treatments, or growth medium only treatments, in general do not appear to have been effective at reducing growth, although there are occasional signs of some apparent growth suppression. Biocontrol treatments have not visibly reduced glyphosate efficacy. Regrowth from the untreated controls is strong, and often forms multiple stems.

Final assessments of above ground rhododendron biomass and Phytophthora inoculum levels two years after application are due to commence shortly.

3. Mechanical stump treatments

Fendt 970 with FAE mulcher
Fendt 970 with FAE mulcher

The third management strategy being investigated is a range of mechanical stump treatment methods.  If stem damage is inflicted below the point at which new buds arise, resprouting is prevented.  This type of damage can be applied with tractor/excavator mounted flails which, following conventional cutting, destroy stumps in situ, minimising regrowth.  

Field trials of five mechanical stump treatments have been carried out on a range of sites with contrasting terrain and vegetation conditions.  Mechanical stump treatments being investigated include:

  • Excavator mulch
  • Excavator rake / tractor mulch
  • Excavator rake & burn
  • Tractor mulch
  • Purpose built mulcher
  • Motor-manual cut & burn (control).

During the efficacy trials, work study has been carried out allowing areas of outputs or work-rates to be determined to derive operational costs for the conditions studied.  The efficacy of each treatment in controlling rhododendron regrowth will be assessed shortly, and outputs will allow users to apply to their own machine / labour costs, to assess the costs of working in their own conditions. Health and Safety, environmental site impacts and regional machine availability will be considered and practical recommendations made.