The present study determined how additives and harvesting periods affect the quality of silage from common reed (Phragmites australis) and cattail (Typha spp.). Both species were harvested separately and together before and after flowering (June and August). Shredded plants (1–3 cm) were ensiled in four groups: one control and three experimental groups (addition of 5% molasses, 5% crushed barley, or 0.5% formic acid). For each harvest period and treatment, 11 jars were filled and incubated for 3 months. The silages were examined for their physical and chemical characteristics, volatile fatty acid and mycotoxin contents, gas production, and organic matter digestibility. The interaction between harvest period, species, and treatment significantly affected (P ≤ 0.007) crude protein content, Flieg score, NH3-N/Total N, and in vitro gas production in silages. Specifically, the August harvest combined with molasses treatment significantly reduced acid detergent fibre content in silages (P ≤ 0.019). Cattail silage had significantly lower contents of crude fibre (P < 0.001), acid detergent fibre (P ≤ 0.006), and neutral detergent fibre (P < 0.001) compared to common reed silage. The addition of 5% molasses to common reed and cattail silages significantly increased lactic acid levels (P < 0.001). The effect of harvest period on gas and methane production proved to be insignificant regardless of the treatment-species interaction. In conclusion, treating Typha harvested in June with 5% molasses can be recommended as the most optimal wetland plant silage. Furthermore, the addition of 5% molasses positively affected the nutrient content and Flieg score for silages from both species harvested in August.