The most common invasive knotweeds include a group of tall, rhizomatous, perennial plants that range in height from about 5 feet to more than 20 feet. These include Japanese knotweed (Fallopia cuspidatum [japonica]); giant knotweed (Fallopia sachalinense); Bohemian knotweed (Fallopia x bohemicum), which is a hybrid between giant and Japanese knotweed; and Himalayan knotweed (Persicaria wallichii).
Less common to land managers are ornamental cultivars of knotweed that can become problematic in garden and wildland settings. The following descriptions include several knotweed cultivars that are available for sale on gardening websites, making it important for invasive plant managers to be able to identify these species:
Variegated Japanese Knotweed or Speckled Mexican Bamboo, Fallopia japonica 'Variegata’
Variegated Japanese knotweed has dark green to reddish stems leading to broad, oval leaves in light green and deep green accented with creamy white. Creamy white to white, small trumpet-shaped flowers bloom in late summer from branch tips. Height 4 ft; perennial that spreads by seed and rhizomes.
Grower comment: “This is a beautiful carefree shrub...this variety is large…attracts bees and butterflies. The only problem is that it tends to be very invasive…” (davesgarden.com)
Japanese Knotweed, Fallopia japonica 'Crimson Beauty'
This cultivar looks almost identical to Japanese knotweed except that sepals turn a red color giving the trumpet-shaped flowers a reddish appearance; bloom occurs in late summer. Height 6 to 8 ft; spread by seed and rhizomes.
Grower comments: “This plant stays in its place. But wherever the place is make sure it has a lot of room.” “ ….spread VERY easily when tiny bits of roots are moved.” (davesgarden.com)
Japanese knotweed, Fallopia japonica 'Compacta'
This cultivar is a clumping perennial, which spreads by rhizomes and is often grown as a ground cover. It typically grows only 1 to 2 ft tall. Features loose clusters of pale pink flowers in summer which mature to red and are followed by red seeds. Leaves are heart-shaped (up to 2 inches long), red-veined, green leaves turn reddish-brown in fall. Spread by seed and rhizomes.
Grower comment: “For those of you that don't like invasive plants, but want to grow one just for the experience, I would recommend this plant. It only gets 3 feet high, and has fantastic red flowers.” (davesgarden.com)