Japanese knotweed (Fallopia japonica) is a perennial plant known for its rapid growth and potential to cause structural damage. It does produce blooms late in the growing season. Although its most feared characteristic is its underground system, the plant flowers when its energy reserves are being moved. Understanding both the flowering process and the non-flowering mechanisms is necessary to grasp why this species is difficult to manage once established.
Characteristics of the Blooms
Japanese knotweed reaches its maximum height, often up to 11 feet, before it begins to flower in late summer or early autumn, typically around August and September. These blooms appear as dense, fingerlike clusters, known as panicles, that sprout from the ends of the stems and the points where leaves meet the stalk. The clusters are generally 3 to 4 inches long and consist of many small, aromatic flowers.
The color of the individual flowers is usually a creamy white or pale greenish-white, providing a distinct contrast to the deep green foliage. These small flowers are only about 0.5 centimeters wide, but their sheer number in the dense sprays makes them noticeable. Flowering marks the point in the plant’s life cycle when it is preparing to draw energy back into its root system for the coming winter.
Reproductive Role of the Flower
Japanese knotweed is naturally dioecious, meaning individual plants possess either male or female reproductive organs. The vast majority of established populations across the United States and the United Kingdom are clones of a single female plant. These female plants can still produce flowers and subsequent seeds, but the seeds are often sterile because they lack viable pollen from a male of the same species.
The plant can sometimes produce fertile seeds through hybridization with closely related species, such as Giant knotweed (Fallopia sachalinensis), creating the hybrid Bohemian knotweed (Fallopia × bohemica). Bohemian knotweed produces a large number of viable seeds with high germination rates in some areas. Sexual reproduction through seed dispersal is generally a minor contributor to the spread of established stands compared to the plant’s vegetative capabilities.
Non-Flowering Spread: The Rhizome System
The primary mechanism of spread is its underground rhizome system. Rhizomes are specialized, horizontal underground stems that function as a nutrient-storing and spreading network. This extensive system can spread laterally up to 23 feet (7 meters) from the parent plant and can extend to depths of about 6.5 feet (2 meters).
The rhizomes are woody, with a bamboo-like appearance, and they are highly regenerative. The plant can sprout into a new clone from tiny fragments of the rhizome, sometimes as small as 0.7 grams or 1 centimeter in length. Human activity is the most common vector for this spread, as the movement of contaminated soil during construction, landscaping, or gardening can disperse these viable fragments to new locations.
Control and Eradication Strategies
Effective control must focus on eliminating the underground rhizome system, as simple cutting or manual removal is ineffective and can exacerbate the problem by creating more fragments. Systemic herbicides, such as glyphosate, are the most common chemical treatment because they are transported from the leaves down into the rhizomes, poisoning the network. This treatment is most effective when applied late in the season, after the plant has flowered, because the plant is actively moving sugars and nutrients down to the roots for winter storage.
Eradication often requires multiple applications of herbicide over several growing seasons due to the resilience of the rhizomes. In cases of large infestations or when construction is planned, professional excavation and removal of the entire contaminated soil mass may be necessary. Any soil or plant material containing knotweed fragments must be handled as controlled waste in many jurisdictions to prevent further spread.