As summer fades, Japanese knotweed (Reynoutria japonica) undergoes distinct changes that can help with identification. Recognising the transformation of Japanese knotweed in Autumn will allow management measures to be put in place as soon as possible . This guide covers best management practices Autumn knotweed, but starts with key features to look for during this transitional season.
2. Browning Knotweed Canes
3. Rhizome Characteristics
4. Knotweed Flowers & Seed Pods
5. Early-to-Late Autumn Transition
What does Japanese knotweed look like in Autumn?
When temperatures drop, Japanese knotweed prepares for winter dormancy, displaying several changes. Here are the key Autumn knotweed identification characteristics:
1. Yellow Knotweed Leaves
Speckled and yellow leaves of autumn knotweed
Knotweed leaves in early autumn remain green and spade-shaped, resembling a shield. By mid-September, running into October, they start turning yellow or orange and may develop brown spots before wilting. As the season advances, the creamy-white flower clusters transition into heart-shaped seed pods. This colour change makes knotweed more noticeable, especially when compared to other plants that may still retain green foliage. When walking through infested areas, you might notice an abundance of fallen leaves, which provide a clue to the extent of the infestation.
2. Browning Knotweed Canes
Close up of the brown canes of Japanese knotweed in autumn
Knotweed canes or stems, which are hollow and bamboo-like, transition from a fleshy green to reddish-brown by late autumn. Initially marked with purple speckles, the stems become more prominent as the leaves fall. Despite the onset of winter dormancy, the plant can still reach heights of 2-3 metres, making it easy to spot in areas where the stems have clustered together. The brittle nature of the stems sets knotweed apart from other woody plants. As autumn progresses, these hollow, dried stems become more vulnerable to breaking, often snapping under their own weight or from wind.
3. Rhizome Characteristics
Close up of a snapped piece of Japanese knotweed rhizome © 2024 PBA Solutions
Beneath the soil, Japanese knotweed rhizomes (underground stems) play a critical role in the plant’s lifecycle. They appear tough and woody on the outside and can be identified by their distinctive orange interior when broken. The rhizomes store energy during the autumn months, ensuring that the plant can survive through winter and regrow in spring. Even tiny fragments of rhizome can give rise to new growth, so disturbing the soil around knotweed should be avoided without professional guidance.
4. Knotweed Flowers & Seed Pods
Close up of Japanese knotweed flowering in late September
By early autumn, knotweed may still display remnants of its late summer blooms. The creamy-white flower clusters give way to heart-shaped seed pods as the season advances. Japanese knotweed seeds, whilst visible, do not contribute to the plant’s spread, as knotweed primarily reproduces through its rhizomes. Nonetheless, the presence of seed pods can aid in confirming the plant’s identity. For identification purposes, it is helpful to look for any remaining flower tassels, which may linger on the plant even after leaves have fallen.
5. Early-to-Late Autumn Transition
Close up of the yellow and green leaves of knotweed in early autumn
Japanese knotweed undergoes noticeable shifts from early to late autumn. In early September, untreated knotweed will appear lush with green foliage and may be in flower. Through October and into late November the plant transitions, its appearance becoming more desolate as leaves drop and stems turn brown. This gradual die-back helps identify the plant before it goes dormant for winter, making autumn a useful time for spotting and managing knotweed infestations. The stark transformation provides visual pointers that help when mapping out the affected areas and planning remediation efforts.
Comparing Autumn Identification to Other Seasons
Each season offers unique characteristics for identifying Japanese knotweed:
Spring knotweed: Emerges with reddish-purple shoots resembling asparagus tips. Young leaves are rolled, and growth is rapid as the plant strives to reach full height. This is often the season when initial infestations are first noticed, especially when shoots appear in unexpected areas.
Summer knotweed: Will usually acheive the full height the plant will reach during the growing season. The dense green foliage and creamy flower clusters provide a visual display that attracts pollinators. This lush growth can overshadow nearby vegetation, making knotweed easy to identify by its prominence. This is typically the time when the plant’s invasive nature is most apparent.
Winter knotweed: Dies back to ground level, leaving behind hollow brown canes and crowns. Winter makes identification more challenging because the above-ground parts are mostly dead. However, the skeletal canes and crowns can still provide visual clues, especially in areas where knotweed has been established for several years.
Understanding these seasonal differences helps ensure year-round knotweed management. In autumn, the plant’s transition toward dormancy provides a window for Japanese knotwed treatment before winter sets in.
Yellow and green leaves of autumn knotweed on the bank of a lake
Autumn Japanese Knotweed Management
Autumn is not only a time to manage existing Japanese knotweed but also the last opportunity to apply herbicide treatment prior to a new growing season starts. Here are some activities undertaken by invasive plant specialists between October and November:
Knotweed Monitoring
Often takes place in the autumn months. Even if no visible knotweed is present, monitoring areas where knotweed has been previously spotted or treated can help detect any signs of regrowth early on. Autumn is a good time to conduct thorough checks before the plant goes into winter dormancy.
Soil Disturbance Precautions
If planning construction or landscaping projects during autumn, take extra care not to disturb the soil around known knotweed locations. The plant’s more subdued appearance can cause soil disturbance, which activates dormant rhizomes and leads to new growth. The autumn provides a good opportunity to mark out the knotweed treatment area.
Herbicide Treatment
Early autumn is an ideal time to apply herbicide treatment for Japanese knotweed, making it an effective part of an ongoing treatment plan or as preparation for excavation later in the season. During summer and autumn, Japanese knotweed transfers resources to its rhizomes to support its growth and spread. This natural process presents a perfect opportunity to deliver a herbicide treatment that targets the rhizomes, potentially slowing or halting the plant’s resource transfer. The systemic herbicides used during this time can penetrate deep into the plant, maximising the impact on the rhizome network.
Professional Excavation
Autumn is an excellent time of year for professional Japanese knotweed removal. The plant still retains enough above-ground visibility to allow the excavation area to be marked out, but there is less foliage to deal with compared to earlier in the year. September presents an opportunity to apply herbicide at an optimal time before completing the excavation process later in the season. The combination of herbicide treatment followed by excavation ensures a more thorough removal by weakening the rhizomes and reducing the risk of regrowth.
The Impact of Autumn Weather on Knotweed Management
Autumn weather significantly impacts Japanese knotweed management, but UK seasonal weather often remains unpredictable. Understanding how different weather conditions affect the plant’s behavior helps plan management tasks effectively.
Temperature: As temperatures drop, knotweed’s growth slows down, but it remains active until the first hard frost. Targeting treatments during this period can disrupt nutrient storage.
Rainfall: Dry calm weather is required for herbicide treatment with a knapsack sprayer. Increased rainfall in autumn means that weather checks are required prior to treatment. This ensures that the application is not washed away and doesn’t impact other plants.
Wind: Strong winds in autumn can break brittle knotweed canes, potentially scattering fragments and trapping rubbish. For this reason, it’s advisable that large infestations are cut when the canes have died back and either broken down in situ or removed from site.
Close up of autumn Japanese knotweed that retains flowers and seed heads
Unlike many other plant species in autumn, Japanese knotweed has identification characteristics that remain visible and distinctive. Autumn is still a time when action can be taken to treat or remove this most notorious invasive species.
The transition in leaf colour, changes in stem characteristics, and preparations for dormancy provide clues that are representative of autumn knotweed. Effective management during this season can set the stage for long-term control, making it easier to tackle knotweed in the coming year.
If you are concerned about the presence of winter knotweed, call and talk to a member of our team on 0203 174 2187 or 01202 816134.
All images © 2024 tintac unless otherwise stated
