One of my biggest foraging crushes is the Japanese Knotweed, Fallopia japonica.  Another garden escapee, brought to the UK in 1825, it is quite invasive and found in urban areas and along waterways.  Just like with the equally invasive Allium paradoxum,  there is no need to feel any guilt harvesting it; if there is any ecological impact from using it (which is doubtful) it will be positive.  Its flavour is deliciously tart, lemony, similar to a mild rhubarb when raw, cooked maybe reminiscent of gooseberries, and can be eaten cooked and raw and used in sweet and savoury applications.  It’s packed with health (as fresh, foraged things tend to be) containing Vitamins A and C, potassium, zinc, phosphorus and manganese, as well as rutin and resveratrol, which may lower bad cholesterol.   Like rhubarb and sorrel, Fallopia japonica contains oxalic acid, especially raw, so moderation is indicated, no matter how delicious the stuff is.  

A word of warning about how to treat waste from your knotweed harvest.  It reproduces vegetatively and could take over your garden if you compost raw pieces of it.  If you have left over bits, I recommend heating them in the microwave or pouring boiling water over them before composting.  Also take care not to drop leaves along your walk, since spreading this invader outside your own property is prohibited.  Also, make sure the site you collect from has not been treated with herbicides.  People often attempt to eradicate this plant, so urban collection sites can be contaminated.

I harvest the young shoots while they are still tender (soon they will become too woody, like asparagus), though later in the year, rhizomes, leaves, and seeds can also being utilised.  

Use the young Fallopia shoots for juice, wine, compote, jam, chutney, sorbet, cakes, and anything else you can think of.  Start with your favourite rhubarb recipes and go from there.

If you are curious and haven’t tasted japanese knotweed before, look for the beautiful pink spears poking out in troops along rivers and on urban waste sites.