Japanese knotweed can grow from a piece of root the size of pea. And it can flourish anew if disturbed after lying dormant for more than 20 years.
(All parts of the plant are considered as controlled waste under the Waste Regulations. You can be fined for letting it grow.
Fallopia japonica var. japonica, was introduced from Japan in the early 19th century as an ornamental plant. William Robinson refers to it under a previous name of Polygonum cuspidatum in The English Flower Garden (John Murray 1883, 1907 edn. cited) as “easier to plant than to get rid of in the garden”.)