Control or eradication? 

Stem injection is a relatively new technique whereby concentrated herbicide is injected into the lower part of each stem of knotweed using a stem injector. There is some concern within the knotweed industry that the concentrated herbicide induces rhizome dormancy. This means that at the surface it appears that the knotweed is dead, whereas in reality the rhizome is far from dead, simply waiting for the right conditions before it starts to grow again.

Our research confirms that stem injection can kill rhizome near the surface, but does not kill the rhizome at greater depth. This is because the shock dose of herbicide stops the translocation process. In our opinion therefore stem injection provides "control", but not eradication. 

Is there any situation where stem injection is the preferred method of application?

Yes. The advantage of stem injection is that a measured dose of herbicide can be applied directly into the plant, without the risk of drift prevalent with a foliar spray method of application. It is a time consuming method, so for small amounts of knotweed growing in amongst other valued plants where control is acceptable it has it's place, as risk of collateral damage to non-target plant species is minimised.

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