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Control of Rhododendron (Rhododendron ponticum) with Roundup Pro Biactive


Rhododendron (Rhododendron ponticum), brought to the UK by the Victorian plant hunters, was widely planted throughout upland Britain in gardens, and in woodlands on sporting estates, to provide cover for game birds, particularly pheasants. From this beginning the species has escaped into woodland, both natural and commercial, scrubby areas and even out onto open country.High densities can greatly reduce biodiversity and the invasion is so great in some areas, especially in wet western Britain, the decision has been made to establish management programmes.

More recently Rhododendron plants have been identified as major reservoir for tree diseases Phytophtora ramorum, and Phytophtora kernoviae (sometimes known by the US term of Sudden Oak Death). Since 2009 the spread of the fatal disease to Japanese Larch, (Larix kaempferi) has been widely linked to Rhododendron. These diseases are notifiable pathogens and destruction of infected plants is a statutuory requirement. Contact your local Fera plant health and seeds Inspector for more details.

The sheer physical size of the plant, up to 8m in places, together with its tough, waxy leaves make Rhododendron a difficult weed to control. In addition, it will quickly re-grow if it is not completely killed.

Control with Roundup Pro Biactive

There are several methods of controlling Rhododendron using Roundup Pro Biactive. Overall spraying is not suitable for larger plants or for thick stands, as it is difficult to spray and the older plants tend to be less susceptible. These taller bushes must first be cut back to ground level before treatment. They can either be hand cut using power tools, such as a brush saw or chainsaw, or by using a hydraulic flail typically mounted on a crawler or other armed tractor base. The cut stumps may be treated immediately using the cut stump method or the foliage left to re-grow for 2-3 years and sprayed.

Foliar Sprays

An overall spray applied to the foliage will provide effective control of young bushes up to 1.3m high or as re-growth 2-3 years after cutting back.  The best time for application is from early May to late September but in milder areas applications earlier and later than this have in the past proved successful.  A high dose of Roundup Pro Biactive is required for this species.  Addition of Mixture B NF, a non-ionic surfactant/spreader, at 2% of the spray volume can improve control and allows the recommended dose rate to be reduced.

All parts of the foliage should be sprayed since the plant exhibits transcendental translocation, which means spray droplets falling on one side of a bush will be translocated down to roots on the same side but will not be distributed across to the opposite side. A small missed area can result in re-growth and rapid re-colonisiation.

Cut Stump

Cut stump treatments are best made from November through to March or April, and not during the period of active sap flow.

Application of Roundup Pro Biactive must be made to a fresh cut so that uptake into the phloem is maximised. A two-man operation is often the most practical for a quick post-felling treatment. If more than a few minutes of time have elapsed, recutting the stump will restore the effectiveness of the application. (Uptake is almost immediate from a fresh cut and will be rainfast within 10 minutes. Application to a cut that has partially sealed means absorption is slow and rain within 6 hours will wash some of the product off.) 

Chemical Thinning

This method works from slightly earlier in the season, late summer, which may be useful to use before stump painting could commence in November. It has the benefit of not requiring the stump to be freshly cut, so it can be used where they have already been cut back, or where the plant is to remain in situ. It is also easy to see which areas have been treated. 

Neat Roundup Pro Biactive is introduced straight into the phloem through a hatchet cut into the bark of the standing Rhododendron or stump. The cut can be made using a small axe - it is advisable to make a second cut under the first to catch any surplus herbicide.  Alternatively a small hole can be drilled at an angle downwards towards the centre of the trunk. A spot gun with a solid stream nozzle is recommended.

A cut/hole is needed for each 10cm diameter of the trunk. Work out how many are needed and space them round the girth. e.g. trunk of 30 cm diameter requires 3 cuts.

Remember to plan to revisit the site and spray out any new seedlings. These will rapidly germinate after the removal of the canopy and following soil disturbance. A visit 2-3 years after the initial treatment would be appropriate to control both seedlings and re-growth.

Rates, Timings and Water Volumes

MethodDose Rate of Roundup Pro BiactiveDose Rate of Roundup ProBiactive450Application Advice
Overall Spray 10l/ha or 8l/ha with Mixture B NF at 2% spray volume 8l/ha or 6.4l/ha with Mixture B NF at 2% spray volume Ensure effective coverage of all foliage but avoid run-off
Cut Stump 20% solution of Roundup Pro Biactive and water 16% solution of Roundup ProBiactive450 and water Apply using knapsack sprayer, spot gun or paintbrush
Chemical Thinning 2ml per 10cm diameter of trunk 1.6ml per 10cm diameter of trunk A second cut below the first one will catch any surplus herbicide

CDA application and hand held Weedwipers can also be used for foliar application at rates according to the instructions.

Spot Treatment in Knapsack Sprayer

Most knapsack sprayers are supplied with a set of 4 deflector nozzles giving different swath widths but all delivering 200l/ha of water at 1 bar pressure and a walking speed of 1 metre per second.

Area SprayedRate of Roundup Pro Biactive for 10l/haRate of Roundup ProBiactive450 for 8l/haVolume of Water
50 Square Metres 50ml 40ml 1 litre
500 Square Metres 500ml 400ml 10 litres
1,000 Square Metres 1,000ml 800ml 20 litres