Microplastics (MPs) are emerging contaminants with uncertain effects on plants, particularly ornamentals. We tested how polyethylene (PE), polylactic acid (PLA), and a constant-total-dose 1:1 (w/w) mixture of PE and PLA (MIX; 25 μm) at 0.1% (w/v), 0.5% (w/v), and 1.0% (w/v) influence seed germination and early growth of white clover (Trifolium repens) in Petri-dish assays. Although these exposure levels exceed typical background levels in bulk soils, they were used to represent worst-case/high-exposure scenarios to elucidate potential mechanisms and effect thresholds in a 7-day assay. Endpoints included germination potential and final germination rate, root traits (radicle elongation inhibition rate and root-to-shoot ratio), seedling biomass (fresh/dry mass), and water content. Responses generally followed a dose-dependent hormetic pattern, with low-dose PE increasing final germination rate, while higher exposures impaired root development and altered biomass allocation. Notably, the high-dose MIX treatment reduced the root-to-shoot ratio by ~37% relative to the control, suggesting disrupted allocation and/or disproportionate root impairment under intense MP stress. Water relations were polymer-specific: PE reduced seedling water content across all tested levels, consistent with physical blockage and/or impaired root water transport, whereas PLA and MIX caused significant declines mainly at higher concentrations. Overall, polymer identity and concentration jointly regulated early establishment, supporting the “low-dose stimulation, high-dose inhibition” framework and underscoring the need to consider both biodegradable and conventional MPs in ecological risk assessment.