Rhododendron, an Invasive species

RhododendronRhododendron is a large evergreen shrub, with a spiral stem, woody trunk and branches. Its flowers are pink and purple in colour and the shrub often grows in an oval shape. It was introduced in Britain in the early 18th century on many Estates, Parks and woodlands and was widely used for game cover. As part of the heather family, it thrives on acidic, peaty soils growing in ecologically sensitive areas such as woodlands, heathlands, roadsides and riverbanks.

Rhododendron re-generates by two methods. The first is stem laying. When the shrub is growing in open areas, the large leafy canopies get incredibly heavy especially when wet. When the branch can’t support the weight any longer, then the branch lowers to the ground beneath. The branch then grows its own roots and eventually becomes a separate plant. This is a very slow procedure, and in woodland areas were the plant is sheltered and rainfall is light, this is a less likely method.

The second method of regeneration is by seed production. Rhododendrons don’t flower until they reach a maturity of a minimum of 10 years. Regrowth can flower within two years. Flowering takes place between May to June, however the seeds do not ripen until December ready for dispersal in February to March. Mature plants in the perfect condition produce up to 1million seeds per year. These seeds are the smallest, lightest of any plant and can travel up to 500m in strong winds.

Rhododendron stemsOnce the seed has germinated initial growth is relatively slow, however this rapidly speeds up as the shrub ages and the canopy fills blocking sunlight to the ground below. This is detrimental to other flora habitat on the ground below and the Rhododendron dominates the environment.

There are four main methods of control. However to completely eradicate is both a lengthy and expensive procedure. Young, freshly germinated plants and regrowth can be sprayed with herbicide typically between March and October. The process will need to be repeated to ensure total irradiation over number of years. Young plants can also be pulled and removed from the site for burning. Larger plants will need to cut down and removed for burning or chipping. The stumps will need treating and any regrowth needs spraying in the forthcoming years. On suitable sites, where access is available, mechanical flails can be used to cut up and mulch the shrub. The stumps must then be treated, however finding them all can be a problem due to the mulch material on the ground. Again, a herbicide treatment is required in the forthcoming years.