Adjusting relative allocation between sexual and clonal reproduction is part of how plants respond to a variable environment, but we know little about how the allocation strategies of plant populations vary over time as abiotic conditions change. We studied correlations between sexual and clonal reproduction using 23 years of data on the clonal salt marsh plant Spartina alterniflora at eight sites in coastal Georgia, USA. The relationship between sexual reproduction and clonal reproduction varied over time. Within years, sexual reproduction was negatively related to clonal reproduction. These relationships were stronger in cooler years and weaker in warmer years, with slopes ranging from -0.202 in cool years to -0.013 in warm years. The trade-offs were also affected by river discharge, with stronger (more negative) slopes as river discharge increased. In a random forest model, temperature had the greatest influence (58%) on the relationships compared to other global change variables (precipitation, river discharge, sea level, and tide range). Overall, our study demonstrated that climate warming gradually disrupts the negative correlation in allocation between reproductive modes in a common salt marsh plant, shifting the affected populations toward a near-total reliance on clonal reproduction, potentially limiting their spread and the generation of new genotypes.