Growth Responses of Ornamental Bedding Plants to Sargassum Compost

Peat extraction for horticultural substrates releases stored carbon and degrades sensitive bog ecosystems. However, unprecedented strandings of pelagic Sargassum spp. along Atlantic and Caribbean coastlines present increasing waste-management challenges. To address the environmental impacts of both peat harvesting and the costly disposal of beached seaweed, this study evaluated sargassum compost (SC) as a partial peat substitute for containerized ornamental plant production. Three bedding plant species—petunia ( Petunia × hybrida ‘Easy Wave’), vinca ( Catharanthus roseus ‘Tattoo’), and marigold ( Tagetes patula ‘Janie’)—were cultivated in substrates containing 0% (SC0), 30% (SC30), or 50% (SC50) sargassum compost to assess the effects on substrate properties on plant growth and foliar nutrient composition. Plant growth responses were species specific; germination time was significantly delayed in marigold and petunia with increasing compost proportion, whereas vinca germination was unaffected. The germination rate differed significantly only for petunia, declining from 65% with SC0 to 33.3% in SC30. Aboveground biomass increased significantly for vinca and petunia, with vinca achieving the highest mean biomass in SC30 (0.60 g/plant, 13.2% greater than SC0), while petunia biomass increased from 0.10 to 0.17 g/plant in SC30. Marigold vegetative growth declined with greater compost fraction, with mean plant height reduced from 20.96 cm (SC0) to 15.30 cm (SC50). Compost addition enhanced floral productivity in marigold, increasing flowering incidence from 7.6% (SC0) to 38.2% (SC50), and in vinca at moderate compost levels (54.2% flowering in SC30 vs. 17.4% in control). Higher compost incorporation improved plant uniformity in both marigold and vinca, although 50% substitution suppressed growth in both species and reduced flowering in vinca. The results support the selective use of SC in bedding plant production with 30% substitution offering an effective balance between sustainability and plant performance.