Japanese knotweed growth season starts early…. or is this the ‘new normal’?
South Wales is arguably the ‘home' of Japanese knotweed in the UK so when we see our members and others in that region sharing photos of ‘early’ Japanese knotweed growth (any time from mid-February onwards) we find ourselves asking would Tom Jones, a man from the valleys himself, be surprised or just think “It’s not unusual”? I’ll try to avoid any more awful puns and attempt to unravel all the various factors, which may be behind the precise date when Japanese knotweed starts growing each year. No surprises to those that ‘know’ Japanese knotweed but it reveals once again what a remarkable plant it really is!
Signs of Spring...and knotweed!
If managing Japanese knotweed and other non-native invasives is your primary business interest, you could be accused of being a bit excitable when the first signs of a new year’s growth season are spotted. Every year even the most gnarly old stands of knotweed start growing initially only as tiny specks of pink/purple buds arising on the surface of previous years’ rhizomes/crowns. In good conditions (warm and wet) the tiny buds will rapidly elongate to produce asparagus-like shoots and from these the first leaves will appear. At this stage the plant is responding almost entirely to the environmental conditions – soil temperature and sunlight hours/daylength – so it is no surprise that the warmer south west of the country is always the first to report the ‘knotweed spring’, frequently coinciding with the appearance of daffodils and crocuses etc. This spring ‘event’ is an evolved pattern of growth which enables non-hardy/non-woody plants to survive the winter frosts, whilst conserving energy below ground. It is no surprise that the other early spring plants/flowers we’ve already mentioned are ones which over-winter as bulbs. The scales of the bulb are able to drive spring growth at a time when photosynthesis is difficult/unreliable due to the prevailing conditions. So it is with Japanese knotweed, but the rhizomes are a little different. They are modified underground stems. Rather like tubers they are storage organs with many meristematic ‘nodes’ and from these new shoots can arise. This feature of rhizomes is the underlying reason for the incredible potential for re-growth from contaminated soil – even ‘screened’ soil can contain tiny fragments of less than 1g in weight which, if they include a node, can give rise to a new plant at a remote location.
Random weather patterns or global warming?
So back to the question. We’ve established that geography and physiology is important but still, is it normal for Japanese knotweed to grow in early-mid February? Or is it a new phenomenon brought about due to long-term and permanent changes to the weather? The simple answer is that “it’s complicated” – meteorologists work with statistical models using data collected over decades not just the last few years, and whilst they are unanimous in their conclusion that globally the world is warming, this doesn’t necessarily mean this will always be reflected in the weather patterns in, say, Swansea in 2022! Add to this the enormous and frustrating variable that is the UK climate generally and you soon begin to realise that placing bets on when Japanese knotweed will start growing next year would be a folly. Perhaps we can all agree that the textbooks are correct when the say Japanese knotweed starts growing in the spring and that spring can be anytime from February to April, depending on where you are and a number of other huge variables!
Recent history: Disturbance and Dormancy
So we’ve talked about what I would call the natural cycle of growth of Japanese knotweed but as I said, it’s a remarkable plant which is adapted in a number of ways that make it particularly difficult to manage/control. One of these adaptations is that it responds directly to disturbance and this appears to be due to its ecological niche (preferential habitat) in its home territory – the slopes of volcanoes! So just to make things more complicated we can add the fact that knotweed which has been recently displaced may be hormonally ‘primed’ to start growing early. Perhaps this has a bearing on some of the reports of early growth we see, but this adaptation of knotweed has far greater implications at other times of year too. When Japanese knotweed grows in the spring it is ‘breaking dormancy’ - a natural process; genetically hard-wired as it were. But Japanese knotweed (rhizomes) can become dormant for other reasons too. In nature a sudden change in growth conditions (a fresh lava flow!) may trigger dormancy which can last decades and only be unlocked by a further change in conditions, such as the sudden introduction of light/water or minerals needed for growth.
Site monitoring should be maintained
In the context of managing Japanese knotweed as a non-native invasive weed, we need to remember that this means it is hard to ‘kill’ rhizomes by deep burial, incineration or herbicide treatment. Particularly in relation to the latter, when used as the sole means of control, it is prudent to assume that even during Guarantee periods (starting after cessation of growth for at least two years) that some degree of site monitoring should be maintained, as the period of dormancy achieved is difficult to predict (and the reasons for that are so numerous as to need another blog!). What we can say categorically is that disturbance of ‘dormant’ Japanese knotweed is the strongest possible stimulus to this resilient plant!
Is it unusual, or not?
So, in the end, do we agree with Tom Jones (“it's not unusual”) or not? This probably depends on how long you’ve been a knotweed watcher. Some of our members have been managing knotweed for over 25 years and over those sort of time frames they will have seen Japanese knotweed appear anytime between early February and late March. But there is no doubt that recent weather trends have been dramatic (eight of the warmest ten years in the last century were in the last decade), enabling an earlier start for a plant already classed a ‘super invader’. Early spring growth of Japanese knotweed is NOT, in my honest opinion, unusual anymore!!
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