Firstly, what is plant dormancy?

Plant dormancy is when a plant slows down its activity in response to tough conditions. This helps it save energy and survive challenges like cold weather or lack of water. Different types of dormancy exist, like winter dormancy when trees lose leaves, and summer dormancy when plants in dry areas slow down. Plants can also delay seed sprouting or bud development to stop new growth to cope with bad weather. Dormancy is controlled by factors inside the plant, like hormones, and external factors like temperature and light.

How does dormancy work for Japanese knotweed?

Japanese knotweed, like many plants, goes through a phase of reduced activity in winter to survive. It stores energy in underground stems called rhizomes, allowing it to grow quickly in spring. The huge reserves of energy stored in the root system allow it to grow rapidly, outcompeting smaller, slower growing species. What’s unique about Japanese knotweed is its apparent ability to stay dormant for years, even a decade or more (Environment Agency research), not just because of the weather but also due to factors like lack of light and stress from herbicides.

What features help Japanese knotweed stay dormant?

As one of the most recognised invasive plants in the world, Japanese knotweed has been subject to a huge amount of research in recent years. Discoveries about its origins, biological form and chemical structure all reveal possible answers to its resilience and longevity.

Origins: Japanese knotweed’s native range is Japan and some parts of China. Here, it is known as a pioneer species in mountainous regions, surviving extremes of temperature and relatively poor soil conditions. It has also been observed as one of the first species to reappear following volcanic activity. It may be that over time, the hardiest plants, that could survive long periods of dormancy beneath lava or volcanic ash, only to re-emerge through cracks and weaknesses months, or years later survived, and therefore affected the genetics of the modern-day plants.

Biology: Japanese knotweed produces extensive networks of underground rhizome, packed full of sugars. This underground storage means that the plants have little need to emerge when conditions are sub-optimal.

Chemistry: Another possible reason that knotweed can survive unaffected below ground, could be that the rhizome system is packed full of Resveratrol. This wonder substance is an antitumor, antioxidant, antiviral compound, that protect the plants from attack by pathogens. This could explain how knotweed survives for such long periods of time in a dormant state, without suffering decomposition.

The dangers of herbicide-induced dormancy:

Using the wrong herbicide and/or incorrect dosage (particularly for glyphosate), can make Japanese knotweed go dormant temporarily. The plant may look like it’s gone, but the underground system stays alive and recovers. When conditions improve, the knotweed starts growing again. So, using herbicides alone isn’t a reliable way to get rid of it. This is why reputable specialists only promote herbicide treatment of knotweed as a control measure, rather than an effective eradication strategy. Of course, in certain situations, inducing long-term dormancy may be all that is required to successfully manage the risks associated with knotweed.

Can you treat Japanese knotweed while it’s dormant?

Managing Japanese knotweed during dormancy is tricky because herbicides need active growth to work. If you have identified knotweed, and it’s in a dormant state, the best way to deal with it is to get a professional to dig it up and remove it. Leaving it in the ground during dormancy can result in it coming back when conditions change. Removing it completely is a smart way to prevent it from causing problems later, especially when you plan to disturb the soil through building, landscaping or significant replanting.

Have you have identified Japanese knotweed this winter, and feel you need some professional help controlling or removing your infestation? Simply contact Environet’s team on 01932 868 700 or email us at [email protected]