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Japanese knotweed, a noxious weed

Jun 25, 2011

These noxious weeds go by numerous other names, including Polygonum cuspidatum (one of its scientific names). But no matter what you call them these plants deserve the title of "killer bamboo." It is effectively a giant herbaceous perennial which grows at an alarming rate of up to 10cm per day in any type of soil. It forms dense clumps of up to 3m (10ft) in height. Japanese Knotweed thrives on disturbance and spreads by natural means and by human activity. Very small fragments of rhizome (underground stems), as little as 0.7g - approximately the size of a fingernail - can produce fresh new plants. But it is below ground where this invader causes the biggest problems, as each stand creates a rhizome network that can extend to 3m in depth and 7m in all directions. This makes it a serious threat to construction where it can have devastating consequences damaging foundations, drains and other underground services. Japanese knotweed plants were introduced from Japan first into Ireland the U.K., then into North America in the 19th century as a landscaping ornamental (alas, there is no accounting for some peoples tastes!). Japanese knotweed soon spread like wildfire, a mad killer taking on a life of its own, independent of its human sponsors. A common habitat for Japanese knotweed is sunny, moist areas, including riverbanks, roadsides and, yes, your lawn and garden. Japanese knotweed is often spread via landfill: all it takes is one fragment of one root, furtively submerged within a pile of fill dumped onto an unsuspecting ditch and, next thing you know, a burgeoning menace is gaining a foothold. Before you know it, all other plants are crowded out by this highly successful competitor for landscaping space, resulting in a monoculture. Luckily, Japanese knotweed is not inclined to occupy forested areas. Rather, it typically takes advantage of areas disturbed by humans, areas affording not only ample sunlight but also friable soil for its invasive roots. If you already have a patch of Japanese knotweed at the edge of the woods on your property, whatever you do, do not begin clearing the wooded land until you have completed the eradication of this killer bamboo. You'll only invite it to spread, if you clear land adjoining the currently infested area. Eradication of an entrenched stand of Japanese knotweed, however, is easier said than done. You can slash Japanese knotweed to the ground, but they come back. You root them out and destroy by fire on a would-be funeral pyre; but it is only you who feel lifeless, fatigued from all your labours. Why, Japanese knotweed even mocks concrete, bursting up through any available crack in a driveway or path with its incredible tenacious strength. But is killing knotweed a realistic goal? Is it possible to kill the Japanese bamboo and reclaim your lands? Well, for those who would like to free up some landscaping space for a garden by killing an well-established stand your hope resides in different tactics, as part of a multipronged strategy, carried out diligently over a long campaign. There is hope for your garden, but you'll have to be persistent with your tactics and wage a smart war. And if you'll settle for just suppressing the enemy at first, using landscape fabrics or even old carpets, you can at least reclaim the war-torn land for the short term, while you maintain the siege that will (hopefully) kill Polygonum cuspidatum for the long term. Begin by investing in some plastic , with which you'll cover your patch of Polygonum cuspidatum and smother it. Invest the money in the biggest you can find, the outlay will save you a lot of labour. If the landscape area from which the Japanese knotweed emerges is covered in the early spring with plastic or carpet, the Japanese knotweed's growth is immediately impeded. The covered knotweed will still make a protest. It is not for nothing that in Japan, home of this killer bamboo, Japanese knotweed is referred to as itadori, which means "strong plant." With their Godzilla like strength, the new shoots will act like tent poles, pushing your coverings up. But you can then simply trample them down by walking over them. What growth does occur under these will be unsuccessful, deprived of sufficient sunlight. Make sure your covers overlap each other considerably, and are weighted down all along the seams and the outskirts; or else the sun-seeking shoots will be pushing through the gaps in no time. This is why buying the largest covers you can find, is a good investment. One reason why I would say that this covering tactic is perhaps the most powerful of your options is the fact that, during carrying out this, the piece of your lands can be reclaimed for above-ground gardening uses. For example, you could apply an attractive mulch on top of the covers, and display containers in this area. You could even build raised-bed gardens right on top of the covers. No matter how long it takes the Japanese knotweed down below to be smothered, your raised beds will be safe: the covers act as a protective barrier against incursion. Knotweed can be suppressed (but not eradicated) by cutting it back all the way through the summer, so that its photosynthesis is never allowed to operate at high levels. Since cuttings will easily sprout new roots and re-entrench themselves in the landscape or garden, pick up the cuttings and bag them afterwards. Don't rely on the cutting method in isolation, though. Cutting back Japanese knotweed regularly is a tactic only meant to be used in conjunction with injections of weed killer into the cane stumps. But this is a lot of work, and certainly not my preferred method. Finally, dig into the ground where the bamboo shoots come up most vigorously in your garden. In these areas you will probably discover the rhizome-clumps from which spring the knotweed's roots and shoots. In stands of Japanese knotweed that have flourished for many years, these rhizome-clumps are very woody and quite easily reach widths of a foot or more. The rhizomes can be dug up and bagged. Do not, however, expect immediate results from implementing this tactic. For no matter how careful you are, some of the rhizome roots will snap off. And from even the tiniest root of Japanese knotweed left in the ground, a new plant will eventually emerge. But remember: this is a long-term war. In this case, the nourishment your enemy requires to fight you most vigorously is stored in its rhizomes. Think of the rhizomes as strongholds. Although enemy soldiers will fan out and hide after their stronghold has been destroyed, the loss of the stronghold makes their long-term success more difficult. There is hope on the Horizon though: “A tiny Japanese insect that could help the fight against an aggressive superweed has been given the go-ahead for a trial release in England.” Since Japanese knotweed was introduced to the UK it has rapidly spread, and the plant currently costs over £150m a year to control and clear. But scientists say a natural predator in the weed's native home of Japan could also help to control it here. The insect will initially be released in a handful of sites this spring. This is the first time that biocontrol - the use of a "natural predator" to control a pest - has been used in the EU to fight a weed. Wildlife Minister Huw Irranca-Davies said: "These tiny insects, which naturally prey on Japanese Knotweed, will help free local authorities and industry from the huge cost of treating and killing this devastating plant." A little known fact is though that it is edible and likened to rhubarb! Here is a link to a site, I may just try some! Why not, it’s very nutritious, has many medicinal uses and also contains an organic pesticide; this is well worth a visit! Highly interesting!

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