Can Native Shrubs Help Stop Invasive Plants From Reclaiming Gangwon’s Riverbanks?

Clearing invasive plants can leave a riverbank looking restored, but the exposed ground may soon welcome another wave of unwanted growth. A study titled "Managing invasive alien plants in Gangwon State via alternative planting," published in Frontiers in Ecology and Evolution, explores whether planting native vegetation after clearance can help interrupt that cycle.

Researchers Jin-Young Kim, Hye-Mi Park and Sang-Deok Lee combined government records from South Korea's Gangwon State with two field experiments, finding early signs that Korean forsythia could slow herbaceous plant regrowth at one location, with little clear benefit at the other.

Years of Removal Leave a Persistent Problem

The researchers examined invasive plant distribution and removal records covering all 18 cities and counties in Gangwon State from 2009 to 2024. Local officials compiled these records through field surveys and visual assessments of invaded and cleared areas, providing a regional picture of management activity and the continuing presence of unwanted plants. Removal efforts increased substantially across the full period, but the recorded distribution of invasive plants showed no corresponding sustained decline; neither distribution nor removal area showed a statistically clear trend between 2018 and 2024.

In 2024, invasive plants occupied an estimated 12.28 square kilometres, with three species accounting for about 97% of the total. Giant ragweed covered 52%, bur cucumber 29.6% and common ragweed 15.4%, making them the principal management priorities. Their growth habits help explain the challenge: giant ragweed can reach around three metres, bur cucumber climbs over surrounding vegetation and blocks sunlight, and common ragweed produces seeds that can survive in the soil for several years.

Giant ragweed accounted for 77.1% of invaded land within the Civilian Control Zone near the northern border, where roads, military facilities and disturbed ground provide suitable habitats. Bur cucumber accounted for 50.8% outside that zone, reflecting its association with rivers and riverbanks. Cheorwon, Inje, Chuncheon and Hongcheon had the largest invaded areas. These species patterns describe the 2024 survey; the records could not establish how individual species' dominance changed over the preceding years.

Testing What Grows After Clearance

In 2025, researchers tested three ways to manage cleared land at two places in South Korea where invasive plants kept returning: a riverbank in Hwacheon and land near farms in Yanggu. They removed the existing plants, then left some plots to grow back naturally, planted common reed in others, and planted Korean forsythia shrubs in the rest. Each approach was tested in three plots at each location, with every plot measuring three metres by three metres.

The team chose common reed because it can quickly cover riverside ground, and Korean forsythia because its branches and leaves could shade other plants and leave them less room to grow. The shrubs were planted about 30 centimetres apart using standard landscaping guidance; the study did not test whether this was the best spacing for controlling invasive plants. From May to October, researchers checked plant growth, ground coverage, soil moisture, temperature, sunlight and soil acidity.

On July 9, other plants covered about 35.5% of the ground in plots planted with forsythia, compared with 70.4% in plots left to recover naturally and 61.7% in reed plots. This suggests that planting shrubs may slow the return of other vegetation after clearance, although it does not show that shrubs can stop invasive plants from returning over time.

A Slower Return, Not Lasting Control

Vegetation continued to increase under every treatment at Hwacheon, and the early advantage did not prevent extensive regrowth. By September 2, recorded cover reached 119.9% on initially bare ground, 113.9% in reed plots and 104.1% in forsythia plots. These figures exceed 100% because researchers added the cover of overlapping species and layers, including climbing vines growing over other plants.

Statistical checks made the results less clear-cut than the early cover figures alone might suggest. The overall comparison did not establish different growth rates across all three treatments, although a separate comparison detected a difference between reed and forsythia. Plants took longer to cover half the ground in forsythia plots, but only one of two statistical tests supported this finding. The small number of plots makes these results uncertain.

Lower cover did not mean that forsythia selectively removed the most troublesome invaders. Giant ragweed and bur cucumber together represented 45% of counted plants in forsythia plots, compared with 35.6% on initially bare ground and 38% in reed plots. The number of herbaceous species did not differ significantly between treatments. Forsythia appeared to suppress early herbaceous growth broadly, rather than controlling the target invaders or demonstrating biodiversity recovery.

Soil and light conditions were similar across the Hwacheon treatments, suggesting that competition for space and resources could help explain the differences in plant cover. Thicker vegetation allows less sunlight to reach the ground. At Yanggu, all plots were almost completely covered by late June, so planting reeds or shrubs offered no clear advantage. Seeds arriving from nearby areas, seeds already in the soil and local growing conditions might explain the different results, but researchers did not directly test these possibilities.

Restoring Habitats Takes More Than Clearing Land

The authors recommend linking invasive plant removal with vegetation recovery, choosing responses that match local habitats and dominant invaders. Shrubs such as Korean forsythia may deserve further testing where slowing early regrowth is useful. At locations resembling Yanggu, expensive planting could offer limited benefit over allowing vegetation to recover naturally, and removal before peak invasive growth may be a more practical use of resources.

Success needs to be judged through the return of invasive plants, recovery of vegetation and establishment of planted species, rather than the area cleared alone. The field evidence covers one growing season at only two sites, with three plots per treatment, so it cannot establish lasting control or justify province-wide planting recommendations. Monitoring across more locations and several years is needed to learn whether the early delay seen at Hwacheon becomes a durable ecological benefit.