Stem injection is a method of applying herbicide which, as you might guess from the name, involves injecting a concentrated dose of systemic herbicide (usually Glyphosate based) directly into the stem of the plant. This is done either by cutting the stem and injecting into the resultant well or using an injector fitted with a needle which is inserted through the stem. It is the ideal method of applying herbicide in sensitive locations because it completely removes the risks of spray drift thereby ensuring that you only kill the target plant and don’t impact other nearby vegetation or sensitive receptors such as watercourses.
Japanese Knotweed is particularly well suited for this treatment method because it has hollow stems which present a nice resevoir to put the herbicide into. It is crucial to only inject plants which have sufficient intenal volume to accept the required volume of herbicide, this means that a minimal external width of 10mm is required otherwise the herbicide doesn’t go in or comes squirting back out when the needle is removed. As with a foliar application of herbicide timing is crucial the systemic herbicide is going to impact the Rhizome much more effectively at the end of the growing season and stems will have reached their maximum width at this point meaning that you can treat more of them than earlier in the season. Injection should be done between the 1st and 2nd or 2nd and 3rd nodes (the knobbly joints between bamboo like sections) from the ground and no higher. The stem is sometimes woody and difficult to access or filled with water close to the base of the plant but injections above the 3rd node risk disrupting the movement of internal fluids before the herbicide has been effectively translocated to the Rhizome.
If done at the correct time of year and done correctly the kill rate from this treatment method can be spectacular but there are some other considerations that it’s worth keeping in mind. It’s much more effective as a treatment if you inject every stem but it can be quite time consuming to inject every stem. On a large very dense stand it may actually be quite difficult to get access to every stem. In that circumstance cutting and injecting might be a solution but a spray treatment with an extended lance is likely to present a more efficient option. In our experience stem injection is most effective when combined with weed wiping on the smaller plants otherwise you risk blowing the middle out of the stand and leaving an untreated fringe.
In short Stem Injection is a great method for controlling Japanese Knotweed but is most effective when used in conjunction with other methods.
Guide to Japanese Knotweed
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