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The invasion of <i>Spartina alterniflora</i> (<i>S. alterniflora</i>) has posed serious threats to the sustainability, quality and biodiversity of coastal wetlands. To safeguard coastal ecosystems, China has enacted large-scale <i>S. alterniflora</i> removal projects, which set the goal of effectively controlling <i>S. alterniflora</i> throughout China by 2025. The accurate monitoring of <i>S. alterniflora</i> with remote sensing is urgent and requisite for the scientific eradication, control and management of this invasive plant. In this study, we combined multi-temporal WorldView-2/3 (WV-2/3) and Sentinel-1/2 imagery to monitor the <i>S. alterniflora</i> dynamics before and after the <i>S. alterniflora</i> removal projects in Zhangjiang Estuary. We put forward a new method for <i>S. alterniflora</i> detection with eight-band WV-2/3 imagery. The proposed method first used NDVI to discriminate <i>S. alterniflora</i> from water, mud flats and mangroves based on Ostu thresholding and then used the red-edge, NIR1 and NIR2 bands and support vector machine (SVM) classifier to distinguish <i>S. alterniflora</i> from algae. Due to the contamination of frequent cloud cover and tidal inundation, the long revisit time of high-resolution satellite sensors and the short-term <i>S. alterniflora</i> removal projects, we combined Sentinel-1 SAR time series and Sentinel-2 optical imagery to monitor the <i>S. alterniflora</i> removal project status in 2023. The overall accuracies of the <i>S. alterniflora</i> detection results here are above 90%. Compared with the traditional SVM method, the proposed method achieved significantly higher identification accuracy. The <i>S. alterniflora</i> area was 115.19 hm² in 2015, 152.40 hm² in 2017 and 15.29 hm² in 2023, respectively. The generated <i>S. alterniflora</i> maps clearly show the clonal growth of <i>S. alterniflora</i> in Zhangjiang Estuary from 2015 to 2017, and the large-scale <i>S. alterniflora</i> eradication project has achieved remarkable results with a removal rate of about 90% in the study area. With the continuous implementation of the “Special Action Plan for the Prevention and Control of <i>Spartina alterniflora</i> (2022–2025)” which aims to eliminate more than 90% of <i>S. alterniflora</i> in all provinces in China by 2025, the continual high-spatial resolution monitoring of <i>S. alterniflora</i> is crucial to control secondary invasion and restore coastal wetlands.