Functional composition of perennial herbaceous mixtures drives vegetation stability, weed control and biodiversity in lowmaintenance urban green spaces

Introduction: Urban green spaces require planting strategies that simultaneously enhance biodiversity, reduce maintenance inputs, and improve resilience to climate change. Herbaceous perennial mixtures represent promising alternatives to conventional lawns and simplified ornamental plantings, although their long-term performance under low-input management remains insufficiently documented. This study evaluated the suitability of different perennial mixtures for sustainable urban landscaping, focusing on vegetation performance, management requirements, ornamental value, and insect biodiversity. Methods: Twelve mixtures of ornamental perennial species were evaluated over three consecutive years under field conditions in northwestern Italy, without irrigation or fertilization. Plant performance was assessed through ground cover, weed suppression, ornamental quality, and flowering dynamics. In 2023–2024, carabid beetles and butterflies were monitored as bioindicator groups to assess the potential contribution of the experimental area to insect biodiversity. Results: Perennial mixture significantly affected vegetation performance, with marked differences in establishment and persistence. Mixtures dominated by Stachys byzantina, Sedum reflexum, Carex flacca, Festuca glauca ‘Elijah Blue’, Phlox subulata, Sedum spurium ‘John Creech’, and Geranium × cantabrigiense ‘Karmina’ rapidly achieved stable canopy closure (>89%), maintained high ornamental quality (>81%), and effectively suppressed weeds. Conversely, mixtures lacking persistent canopy-forming species showed poor establishment, limited ground cover, and greater weed biomass. Flowering was strongly seasonal, peaking in late spring, while mixtures containing complementary flowering taxa provided more continuous floral resources. Biodiversity monitoring recorded 6,897 carabid beetles and 2,271 butterflies, with insect abundance showing pronounced seasonal variation. Species of conservation interest, including Lycaena dispar and Lucanus cervus, were also recorded. Discussion: Functional composition, rather than species richness alone, emerged as a key determinant of perennial mixture performance by influencing canopy development, weed suppression, ornamental quality, and habitat structure. Combining flowering-rich mixtures that provide seasonally complementary floral resources with structurally persistent, weed-suppressive vegetation may therefore represent an effective strategy for simultaneously supporting pollinators and ground-dwelling arthropods. Trait-based perennial mixtures can provide resilient, biodiversity-friendly, and low-maintenance solutions for climate-adapted urban green infrastructure.