Urban green spaces are increasingly recognised as critical reservoirs of plant biodiversity in rapidly growing cities of sub-Saharan Africa, yet comparative data on species richness and community composition across contrasting management types remain scarce for Central African cities. This study presents findings from a quantitative comparative analysis of plant species richness and diversity in managed and unmanaged urban green spaces in Lusaka, Zambia. Using a stratified random sampling design, a total of 60 vegetation quadrats (10 m by 10 m) were surveyed across two contrasting site categories: the formally managed Lusaka Botanic Garden in Longacres (n = 30 plots distributed across three management zone strata) and abandoned lots in three residential townships, namely Kalingalinga, Chilenje, and Matero (n = 10 plots per township). A structured questionnaire was administered to 35 respondents comprising garden management staff (Version A, n = 21) and lot occupants or owners (Version B, n = 14). Botanical garden plots recorded a significantly higher mean species richness of 20.9 species per plot (SD = 3.87) compared with 14.4 species per plot (SD = 1.87) in abandoned lots (Welch t-test: t(58) = 5.21, p < 0.001). Across both site types, 269 unique vascular plant taxa were recorded from 84 families. The botanical garden harboured a higher proportion of exotic and cultivated species (58.3 percent) while abandoned lots contained a significantly larger proportion of native Zambian plant species (67.9 percent). Shannon diversity indices were significantly higher in botanical garden plots (mean H\' = 2.70, SD = 0.15) than in abandoned lots (mean H\' = 2.21, SD = 0.13). Non-metric multidimensional scaling and PERMANOVA confirmed highly significant compositional differentiation between the two space types (R² = 0.310, F = 26.4, p = 0.001). Questionnaire data revealed that 85.7 percent of garden staff perceived funding constraints as limiting management intensity, and 71.4 percent of lot occupants expressed willingness to participate in community-led conservation programmes. The findings highlight that both managed and unmanaged urban green spaces in Lusaka contribute complementary and non-redundant plant diversity values, and that conservation policy must recognise both categories as components of an integrated urban biodiversity strategy.