Cut astilbe (Astilbe × arendsii) inflorescences have many small florets with low ethylene sensitivity, and a short vase life. The senescence of flowers is complex and probably regulated by programmed cell death, but specific senescence-related genes have not been widely studied, apart from ethylene-sensitive species. In this study, we investigated the effects of continuous treatments with 4% sucrose (Suc), 2% trehalose (Tre), and a combination (Suc+Tre) on postharvest quality and senescence-related gene expression in cut flowers of the astilbe ‘Gloria Purpurea’. Vase life was extended from 3.7 days to 10.3, 9.4, and 12.4 days by treatment with Suc, Tre and Suc+Tre, respectively. Florets were sampled at 0 days (0 d) and 2 days after harvest (2 d). De novo RNA-sequencing (RNA-seq) of floret tissues produced 89,705 unique sequences, and 2,517, 979, 609, and 1,846 differentially expressed genes were identified from the 0d_Control (Cont) vs. 2d_Cont, 2d_Cont vs. 2d_Suc, 2d_Cont vs. 2d_Tre, and 2d_Cont vs. 2d_Suc+Tre libraries, respectively. Gene Ontology (GO) analysis indicated that expression of many photosynthesis-related genes was upregulated in 2d_Cont florets, suggesting the photosynthesis system is activated in senescing florets. Kyoto Encyclopedia of Genes and Genomes analysis showed that WRKY22, related to leaf senescence, was more highly upregulated in 2d_Cont compared with 0d_Cont, 2d_Suc, and 2d_Tre in the Mitogen-activated Protein Kinase (MAPK) signaling pathway. Using Fragments Per Kilobase per Million reads (FPKM) values from RNA-seq, candidate genes associated with senescence including the no apical meristem, ATAF1/2 and cup-shaped cotyledon transcription factor 29 (NAC029), senescence-associated gene 12 (SAG12), and peroxidase 21 (PER21) were selected. Suc+Tre in particular, the most effective treatment for prolonging vase life, downregulated NAC029, SAG12, and PER21 expression at 2 d. Reverse transcription-quantitative polymerase chain reaction (RT-qPCR) validated the RNA-seq results, i.e., similar expression trends were found in the three genes, although relatively weak expression levels were detected compared with FPKM values from RNA-seq. The effect of Tre alone on the three genes’ expression levels was relatively small compared with treatments containing Suc, suggesting that Tre may delay floret senescence by altering genes involved in water transport such as Aquaporin tonoplast intrinsic protein 1-3. This study revealed several candidate genes and GO terms involved in the senescence of cut florets, but further study is needed, especially on key genes including NAC029 and WRKY22.