Japanese knotweed mercilessly overgrows any surrounding vegetation. The Federal Agency for Nature Conservation strongly advises against planting the immigrated neophyte. Read here how you can rid your garden of this all-displacing perennial and prevent an invasion in an emergency.

Japanese knotweed spreads extremely quickly

Table of Contents

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  1. Which location is suitable?
  2. What soil does the plant need?
  3. When is flowering time?
  4. Japanese knotweed in a pot
  5. Is Japanese knotweed poisonous?
  6. What are the characteristics of Japanese knotweed?
  7. How does mechanical combat succeed?
  8. How does black foil control work?
  9. Are there any effective herbicides against Japanese knotweed?
  10. How do I prevent invasive Japanese knotweed?
  11. Is Japanese knotweed edible?

    12. Which location is suitable?

    In its homeland, Japanese knotweed often grows on volcanic ash fields. In Europe, the habitat extends over areas with moist soil, such as along water bodies and wooded riparian zones. But the plant also settles on ruderal areas, on the roadside, along embankments and forests. The Japanese knotweed is flexible in terms of light conditions and grows equally excessively in sunny and shady locations. The perennial is also stable, so that wind-exposed locations do not stop it from spreading.

    What soil does the plant need?

    Japanese knotweed develops resolute growth in almost any soil condition. The perennial thrives mainly in nutrient-rich, moist soil; nevertheless, it has no inhibitions about establishing itself in dry, sandy-loamy substrate. Only in acidic bog bed soil and permanent waterlogging you will hardly find Japanese knotweed.

    When is flowering time?

    One of the unmistakable characteristics of a Japanese knotweed is the pure white flower. From August to October, the panicle-like inflorescences rise above the leathery, broadly ovate leaves.

    Japanese knotweed in a pot

    Cultivation in a pot still harbors the slightest risk of becoming a victim of the Kamchatka knotweed's vehement urge to spread. If you want to dare the experiment, the bucket should have a volume of at least 30 liters. Any commercially available compost-based potting soil is suitable as a substrate. Before planting, spread a drainage made of gravel or broken clay over the water drain, because the plant does not tolerate waterlogging in the long term. The care program is limited to the following measures:

    • Water regularly when the substrate surface dries
    • Fertilizing is not necessary if the plant is repotted into fresh substrate every year
    • Radical pruning in autumn or late winter

    Since Japanese knotweed sprout from even the smallest parts of the plant, the clippings should not be disposed of in the compost. Immediately after pruning is the best time to repot the vigorous plant into a larger bucket.

    Is Japanese knotweed poisonous?

    As an invasive neophyte, Japanese knotweed threatens ecosystems around the world. Despite this, the plant does not pose a toxic hazard to humans or animals. On the contrary, the young sprouts can be prepared like rhubarb or asparagus. In Japanese medicine, the root provides a tincture that has a beneficial effect on blood pressure and is said to lower high cholesterol levels.
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    What are the characteristics of Japanese knotweed?

    During the flowering period from August to October, Japanese knotweed is easy to recognize by its white paniculate flowers, which are quite pretty to look at. Outside the flowering period, it is the leaves and shoots that serve as a distinguishing feature:

    • Leathery green leaves, 12 to 18 cm long
    • Ovate pointed and 8 to 13 cm wide
    • Arranged alternately on hollow shoots up to 300 cm long
    • Knotweed shoots often appear red streaked

    There are tiny hairs on the leaf veins on the undersides of the foliage that are barely visible to the naked eye. If you run your finger over it, you can at least feel the hair. After the first frosty night, the above-ground parts of the plant die off within a short time. The huge rhizomes in the ground, on the other hand, survive the winter unscathed and sprout again with vehemence at the beginning of the following vegetation period.

    How does mechanical combat succeed?

    If you have discovered a Japanese knotweed in the garden, there is an immediate need for action. In the early stages, the following mechanical combat methods still give you good chances of preventing an invasion:

    Spreading area 1 to 4 square meters
    Once the rhizomes of the Japanese knotweed have conquered a small area, you can defeat the invaders by consistently digging up and tearing them out. Be careful not to leave any bits of root in the ground. Go to work with a spade or a pickaxe until there are no more sprouts to be seen.

    Spreading area 4 to 20 square meters
    If the knotweed already inhabits a larger area, digging up will not achieve your goal. Now the plant should be mown close to the ground every 2-3 weeks from April to October/November. Then clean the devices thoroughly so that no parts of the plant are carried over to other regions of the garden. Experience has shown that this control method requires patience for up to 5 years.

    In order to properly dispose of the excavated or mown parts of the plant, small amounts end up in the household waste. A large quantity of rhizomes, shoots and leaves are in good hands in the municipal composting plant or are transported to the bio-power plant for incineration. Kamchatka knotweed has no place in domestic compost.

    How does black foil control work?

    Cut off Japanese knotweed from sunlight using a sturdy, black film. Since photosynthesis primarily comes to a standstill, the perennial dies over time. Of course, a long line of patience of 1 to 2 years is required to triumph over the invaders. How to do it right:

    • Cover the entire infested area with thick, opaque film
    • Lay the edges at least 150 cm beyond the visible edge
    • Lay out the film sections with an overlap of 10-20 cm and fix them in the ground

    In this way you at least achieve a significant weakening of the stock, so that digging up all the remaining rhizomes promises the best chance of success.

    Are there any effective herbicides against Japanese knotweed?

    The use of chemical herbicides in private ornamental and kitchen gardens is being viewed more and more critically. The negative consequences for the surrounding flora and fauna are too great. Only when all mechanical control methods against the Japanese knotweed fail do hobby gardeners resort to this means. Broad spectrum herbicides, such as Roundup or glyphosate, are not without reason: they are suspected of contributing to insect mortality and various diseases in humans are attributed to the use of broad spectrum herbicides. Thus, the herbicide affects our gut health and can lead to infertility. Skin diseases and cancer are also associated with it. Do yourself and the environment a favor and refrain from using such harmful substances.

    How do I prevent invasive Japanese knotweed?

    An effective prevention against Japanese knotweed is not to plant the perennial in the garden in the first place. The multifaceted plant family of the knotweed family offers a wide range of beautiful species and varieties that adorn the garden without completely subjugating it. The distinctive candle knotweed is one of them, as is the majestic bush knotweed.

    Create your garden on a new plot, do not use excavated soil that could be contaminated with pieces of Japanese knotweed rhizome.

    Is Japanese knotweed edible?

    If you're struggling with Japanese knotweed in the garden, you shouldn't close your eyes to its benefits. Leaves and roots of knotweed are edible. The young shoots can be prepared in refreshing salads and make aromatic side dishes with meat or fish. When cooked, the leaves turn into a vegetable reminiscent of spinach.

    Don't just dig up the roots to destroy Kamchatka knotweed. Peeled and boiled in salted water, they taste like asparagus. But be careful when disposing of the uncooked plant remains, because they do not belong in the compost. The plant sprouts happily from a 1 cm piece of rhizome.

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Herbs