According to the Rhododendron Species Foundation, there are more than 850 different varieties of rhododendrons, including azaleas and gardenias among others. The plants grow wild in Asia, North America and Europe, and killing them can be a challenge. If you have cut down a rhododendron shrub, you must treat the stump as well to complete the process. If left untreated, the stump continues to grow and produces new top growth in the spring.
Wait until the after the rhododendron stops blooming for the year before attempting to kill it. The best time to kill it for most locations is between October to February.
Cut off the stumps and any remaining branches as close to the ground as possible using a saw. If the stump is less than 2 inches in diameter, you may be able to use loppers instead.
Fill a garden sprayer with an herbicide containing the ingredient glyphosate and add enough water to dilute the herbicide to 20 per cent. Close the garden sprayer and shake it gently to mix the solution.
Spray the herbicide solution onto all cut surfaces of the rhododendron stump until it drips off.
Watch the stump closely to see if any new foliage develops on the top. If new growth is seen, saturate all leaves with the same solution of the glyphosate herbicide to finish killing the rhododendron stump. The mixture is absorbed through the leaves and carried into the stump.
Herbicides containing tryclopyr (8 per cent solution) and ammonium sulphamate (40 per cent solution) also kill rhododendron stumps. Apply tryclopyr anytime and apply ammonium sulphamate between April and September. If desired, you may also dig out the entire stump of the rhododendron stump with a shovel. Once removed from the ground, burn the stump to kill the root system.
Do not apply the herbicide when rain or wind is expected.