Japanese knotweed is generally referred to as a herbaceous perennial, as although it leaves behind woody stalks over the winter, it is very rare for the plant to grow new leaves from any part of the previous season’s stems. The crowns and the rhizome (root system) of the plant stay alive over winter, sending up new growth in the spring.
However, the dead stalks do continue to provide for the plant; around this time of the year (early October) Japanese knotweed has usually begun to draw nutrients down in to the rhizome, and as it does this, the surface growth dies off. The woody stems provide insulation for the plant over the winter period, protecting it from the worst effects of frost, and over winter, the rhizome enters a dormant state – controlled mostly by temperature, but sensitive to other factors too.
Over the winter, the surface growth may be dead, but the threat is not gone – on the contrary: for prospective home buyers, the risk of a survey missing a Japanese knotweed infestation are much higher over winter, as unscrupulous sellers may cut back and cover-up the evidence of the plant.
When the time is right for the plant, it sends up new growth. When it gets the right signals in the spring, the plant uses its stored nutrients to power its aggressive, early growth, which gives it a head-start on many native plant species. Sometimes we see Japanese knotweed showing signs of growth as early as February.
When separated from the main root system, segments of Japanese knotweed rhizome can remain in the dormant state – potentially emerging up to twenty years later to re-grow when conditions are right. This is one of a multitude of abilities that Japanese knotweed has evolved as one of the best-adapted plants to volcanic slopes – eruptions and lava can kill off the surface growth, but the deep roots survive, waiting for the best opportunity to return, when the coast is clear.
Unfortunately, it is also possible to send a Japanese knotweed plant in to this dormant state when herbicide is applied in excess or in concentrations higher than the recommended formulation. That’s not good. Basically what a contractor (or over-zealous amateur) is doing in these cases is storing up a problem for the future – you can’t see it, but it will be back to trouble you (or whoever you sell your property to).
Beware of any dormant Japanese knotweed contractor who tells you that they can “kill” your Japanese knotweed with a single chemical treatment. You can fool a bear in to thinking it’s winter, but when it wakes up, you don’t want to be living next door…