In the late 1800s, America imported Japanese hops for medicine and gardens; escaped vines can now form 4-foot-thick blankets with thousands of plants per acre

A plant that arrives as decoration does not always stay decorative. This is what marks the interesting story behind Japanese hops in America. Introduced from eastern Asia in the late 1800s as an ornamental plant and for potential medicinal uses, Japanese hops was initially valued for its decorative appeal and possible therapeutic applications rather than viewed as the invasive threat it would later become. With time, the vine, which might have been initially embraced in the garden, started spreading in a takeover sort of behavior, growing over other natural plants and spreading in moist and sunny places. As stated by the Missouri Department of Conservation, Japanese hops can occur in thousands per acre and cover as much as four feet thick in vegetation.

The transformation from useful foreign plant to invasive menace is what makes Japanese hops so interesting in terms of how plants spread. They were not introduced as a pest. They were introduced as a useful plant for ornamental and medical purposes. Once released in the wild, they found ideal conditions in which to flourish: moist and exposed soil, bright sun, and disturbed ground where other native plants are endangered.

From ornamental vine to aggressive spreader

Japanese hop, or Humulus japonicus, is an herbaceous vine that is usually annual, which means it completes its life cycle in a single growing season. That might sound like a limitation, but the plant makes up for it with speed and volume. After seeds germinate in early spring, the vine grows quickly and produces many seeds that contribute to its spread. Those seeds can ride wind, water, machinery, and animals into new places, helping the plant spread far beyond the spot where it first took root.

The climbing nature of Humulus japonicus also makes it difficult to overlook this species once it starts growing in some place. Growing rapidly and climbing around other plants, it covers their surface quickly enough to overshadow existing vegetation and block any light coming from above. Given suitable conditions, this plant is capable of colonizing stream banks, ditches, roadsides, railroad embankments, pastures, and any other disturbed areas with exposed soil where the competition is limited. The preference of the species to direct sunlight and inability to tolerate significant shade make it particularly successful in such habitats. This rapid growth, high reproductive rate, and adaptability to disturbed environments make the plant problematic in management.

Why a thick vine blanket matters

The most striking detail about Japanese hop is not only that it spreads, but how it changes the places it invades. This vine can form mats reaching up to four feet thick. This is not just an annoyance because such a mat can block the light and growth space of the vegetation below it, causing it to die off. The vines can also drag down smaller trees and shrubs due to their growing mass.

This has implications for habitat because those plants are an essential part of the habitat in which they live. The stream bank vegetation serves as a way to hold soil, shade water, and provide cover for animals. Habitats such as bottomland prairie, moist forests, and river corridors require diversity of plants in order to remain healthy. What happens with Japanese hop is that the plant does more than introduce another type of vine into the area. It alters the entire landscape. This is why invasive plants are assessed based not only on their range, but also on how they alter the landscape they have invaded. Japanese hop does this by being able to grow quickly, produce many seeds, and come back each year.

A plant that favors the edges of disturbance

The Japanese hop appears to thrive in environments that have been changed by human activity. The species is an aggressive invader of roadsides, railroads, ditches, pastures, stream banks, and disturbed moist sites. This is precisely the kind of environment where there will be sunlight at the soil level and possibly sparse native vegetation. The Japanese hop does not need pristine wilderness in order to prosper; it grows best in the disturbed environments.

This goes some way toward explaining the difficulty of control. A vine that reproduces through seeds and through disturbance is impossible to eradicate all at once. If the plants are destroyed without stopping their seed production, there may be enough seeds in the soil to re-establish the problem next year.

What actually takes control

Control methods vary depending on the severity of the infestation. In light infestations, manual removal will be successful only if the roots are completely removed at an early stage of the growing season. In case of severe infestations, herbicides might prove more effective, particularly when used in conjunction with post-application to inhibit seed formation. Mowing or cutting also proves useful in controlling invasive weeds but only if done early enough to prevent the formation of seeds. It should be noted that within three successive years of controlling measures that prevent seed formation, the seed bank will be depleted. That detail is a useful reminder that invasive-plant control is not a one-time project.