Assessing seed pellet formulations to improve native plant restoration under water-limited conditions

Water scarcity, a challenge expected to worsen with climate change, significantly hinders native plant community restoration. Enhancing seed-based restoration requires methods to increase the water availability for seeds and seedlings. Surfactants and superabsorbent polymers (SAPs) can improve soil water-holding capacity and infiltration, but their use in seed enhancements remains underexplored. We investigated whether pellets containing surfactants or SAPs could improve seedling emergence of two native species (Rytidosperma caespitosum and Chrysocephalum apiculatum) common in temperate grasslands in south-eastern Australia under different watering treatments. We used a randomized block design with five watering treatments to simulate predicted changes in precipitation for south-eastern Australia: ambient, two reducing overall water volume, and two reducing watering frequency while increasing watering volume to maintain ambient treatment water volume. We explored four enhanced pellets (two containing surfactants and two containing SAPs) and non-pelleted seeds. Our results showed that watering events with larger volumes but reduced frequency increased seedling emergence. Under these conditions, block co-polymer surfactants further increased seedling emergence of C. apiculatum, while synthetic SAP pellets promoted emergence of R. caespitosum. Block co-polymer surfactants decreased R. caespitosum emergence, and both SAPs reduced C. apiculatum emergence under reduced watering frequency. A 50% reduction in overall water volume significantly reduced seedling emergence for both species, regardless of seed enhancement. These findings suggest that surfactants and SAP pellets can improve the success of seed-based restoration under ambient conditions and when rainfall events are larger in volume but reduced in frequency, but not when the overall volume of rainfall is reduced.