The Autumn Equinox and Japanese Knotweed – it’s all about the timing!

By the time you read this, we will already be three days beyond the autumn equinox (22nd September) – the start of astronomical autumn which signals when daylight and night time hours are equal and the plane of the equator is at right angles to the sun. Is this important to how we manage Japanese knotweed? Yes it is! So let’s unpick the reasons why and how this helps us optimise control programmes.

Peak knotweed season: the autumn rush-hour!

Most of our PCA invasive weed group members are more than busy right now – it’s peak knotweed spraying/injecting season after all! One of the challenges of a perennial plant like Japanese knotweed is that it re-grows every year from storage organs – rhizomes – below ground. This causes problems like the fact that chopping it down never works…in fact…it usually makes the plant more vigorous. Also, the huge biomass of energy reserves in the soil means that it usually takes several years (at least 2 – 3, often more) for any herbicide to become completely effective.

But as PCA members are more than aware, there is one ‘upside’ of herbaceous perennials…is that we can tap-in to the plants’ natural bio-rhythms and ensure herbicide treatments achieve maximum impact in the medium to long-term.

Annual spraying programmes – it’s all about the timing…

This is particularly the case with the herbicide Glyphosate, which the plant handles like a sugar molecule. So, if we spray it now (a few weeks either side of the equinox) the chances are that most of the dose absorbed will end up being moved into the plant’s rhizome network. Clever, yes, but the really clever bit is that all this happens a week or so before the green shoots die-back – and a nearly unique feature of Glyphosate applied at this time of year.

For PCA members, this is an important thing to remember when planning your annual spraying programmes. Attempting to ‘kill’ knotweed in the summer can be counter-productive because the most impact dose-for-dose will be in the autumn; less foliage to work with now means you won’t have optimised the treatment – and this is essential if you want to comply with the government’s over-arching National Action Plan for pesticides.

Is the equinox the best time to experiment?

On this theme, perhaps a similar impact can be achieved without using chemicals at all? We ought to consider any reasonable alternatives and using heat and/or electricity seems to be getting some publicity at the moment.

A number of trials are underway with Utility companies and Local authorities here and in The Netherlands and we would expect timing to be a key factor here too. Stopping the acquisition of carbohydrates to the rhizome network will maximise the effectiveness of treatments in subsequent years.

So, once again, around the equinox will likely give the best impact, although this could be a problem as older canes are more likely to be resistant to hot foam etc. and/or be less conductive? Whenever treatments are applied it remains to be seen whether the effects achieved (with these alternative approaches) are any greater than just cutting canes or indeed spraying (safe and effective) herbicides?

Preparing for the winter

Of course, work aside, most of us are not terribly fond of the autumn equinox because it is the first reminder of the coming winter months. This must be a fallow period for knotweed management contractors, surely? The winter is a time of little or no ‘action’ as far as Japanese knotweed growth is concerned, but can we do some useful things between this equinox and the next one (March 21st)? Yes we can! Let’s assume you’ve done all you can to optimise the effectiveness of your control strategies during the summer/autumn. What happens now..?

The PCA’s Code of Practice suggest that most Japanese knotweed management plans can benefit from some cultivation or biomass reduction alongside herbicide spray programmes. The question is when? We don’t necessarily want to disturb the growth cycle during the summer as this will only adversely affect the success of the key autumn spray treatment.

So when is the best time?

But once the winter has started and any glyphosate has been absorbed and translocated into the rhizome network, and before the spring equinox when most areas see the first signs of new growth, some surface digging to remove crowns and break-up (possibly remove some) rhizomes can be beneficial. This will maximise the vigour of any remaining viable sections and at the same time isolate each knotweed cane from the carbohydrate resource of the rest of the plant (below ground).

This might seem counter-productive, to encourage growth by disturbing the soil, but actually in the early stage of a 4/5 year knotweed management plan, it makes complete sense!

There is also a very useful spin-off benefit arising from such a method – the ground under management (the treatment zone) once ‘completion’ is achieved (no growth for two consecutive years), could be handed back to the client as a fully-fledged part of their normal garden….with restrictions of course!

So, to wrap this up, I hope you agree that the equinox is quite a significant date in the knotweed calendar – not once, but twice, a year!