For the occasion of Germany’s reunification, the people of Japan donated a generous number of flowering cherry trees that line both sides of the Mauerweg right below the Bösebrücke bridge. The cherry trees are in full blossom at the moment, and their presence is duly appreciated by the locals as well as the growing number of tourists who wander this far north following the path of the former Berlin Wall. Yet the ostentatious beauty of the cherry trees overshadows the presence of a more recent botanical immigrant from the East: Japanese knotweed (Polygonum cuspidatum)*.
Much has already been written about the edible and medicinal virtues of this plant, which also happens to be an incredibly tenacious, invasive species with powerful root systems that can cause widespread structural damage. The young shoots are typically harvested in the spring when they are less than a foot tall and can be eaten either raw or cooked. Later in the year, the stems become hard and the tough outer shell needs to be pealed for consumption. Bees are fond of the plant’s small white blossoms which appear in the late fall, and it is said that they produce a mild but flavorful honey after pollinating these flowers. A quick Google search on Japanese knotweed recipes will give you many suggestions for how to eat the shoots, from simply sautéing them in butter to making rhubarb-like jams or pies. And, as Wild Man Steve Brill mentions in his description of this forager’s classic, the plant has myriad nutritional benefits.
However, enthusiastic foragers will quickly notice that the raw shoots taste quite sour and this is because the plant contains considerable amounts of oxalic acid, with the highest concentrations found in the leaves. Since oxalic acid can exacerbate medical conditions like kidney stones and rheumatism, only small amounts of Japanese knotweed should be ingested. (For more detailed information on the oxalate contents of different parts of the Polygonaceae family plants, take a look at the following abstract from the Food Chemistry journal.)
However, for foragers in heavily industrialized areas, oxalic acid indicates the possible presence of a much more threatening substance – namely lead. In the last few years, Japanese Knotweed has showed promising results in reducing lead levels in contaminated soils (the process is known as phytoextraction, which is a specific form of the phytoremediation already mentioned in our post on cattails). The high oxalic acid makes the lead more water soluble and hence the plant can absorb it more readily. In 2006, several Japanese “inventors” filed an application with the US Patent Office for a method of cleaning up lead-contaminated soil using Japanese knotweed.
Obviously, this should invite extreme vigilance on the part of the urban forager. After reading this rather disturbing news, we continued researching and found out that the area where we took these pictures has an average of 150-200 mg\kg of lead in the soil – about three times the naturally occurring level (although still considered relatively low in an industrialized setting according to US standards). We encourage all foragers in Berlin to use the public information made available by the Senate Department for Urban Development and Environment to check lead concentrations in their local foraging spots.
While this post may seem a bit discouraging to urban foragers, it is important to remember that our search for edible plants is hardly driven by hunger or immediate necessity; rather, it is a slow and deliberate movement towards a stronger relationship with the natural world despite the inevitable distance created by our urban habitation!
*There are two closely-related species of Polygonum in Berlin so it is also possible that the species in our photographs is Giant Knotweed (Polygonum sachalinensis). Everything we discuss about Japanese knotweed is likely valid for Giant knotweed.