The confirmation of the change was announced by Dow AgroSciences, which produces the product.
Nomix Enviro said it has stocks of the herbicide ready and expects to be able to sell the product throughout most of 2012.
Managing director Mark Phillips said: "This is excellent news for our customers as they will have a further 12 months to use Timbrel once the final revocation date comes into effect. We expect that use will be able to continue throughout most of 2013."
He added that the control of rhododendron has become more important with the spread of Phytophthora ramorum. Organisations including the Forestry Commission and the National Trust have used the product.
Timbrel contains the active ingredient triclopyr and can be used to control problem woody weeds such as brambles, buddleia, rhododendron, sycamore and ash as well as difficult-to-control weeds such as giant hogweed and Himalayan balsam.
It works by moving down through the plant, getting right to the root system, leading to root destruction and the prevention of regrowth.