The literature is ambivalent about the relationship between urban greening and health but generally reports positive associations for mental health and wellbeing (MHWB). It also notes that how street greening – a commonly used urban intervention – is experienced and impacts MHWB in real settings remains less studied and the evidence basis that guides urban planning is limited. Meantime, urban policy and practice treat urban greening as a near-universal solution for better quality of life. This paper studies street greening in situ and examines such policy claims empirically. It does so through a field experiment at two matched intervention-control site pairs in London. Each intervention site received planting and public-realm improvements in 2025. 215 participants were surveyed before and after the intervention, six months apart, using a WHO-5-aligned tool to assess experiential indicators associated with subjective MHWB. This is the first field experiment designed to document street greening's experiential effects in real settings. Based on a paired comparison, the paper finds that (1) street greening was experienced differently at the two sites, and (2) its effects were correspondingly uneven: at one site, greening was associated with improvements in experiential indicators linked to MHWB, while at the other the association was negligible. The paper suggests that the scale and design of interventions, their connectivity to the green infrastructure network and their streetscape are candidate explanations for such difference. The findings suggest that street greening may not consistently translate into MHWB benefits and point to the importance of context-sensitivity in policy and practice.