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Premise: Previous studies have suggested a trade‐off between trichome density (Dt) and stomatal density (Ds) due to shared cell precursors. We clarified how, when, and why this developmental trade‐off may be overcome across species. Methods: We derived equations to determine the developmental basis for Dt and Ds in trichome and stomatal indices (it and is) and the sizes of epidermal pavement cells (e), trichome bases (t), and stomata (s) and quantified the importance of these determinants of Dt and Ds for 78 California species. We compiled 17 previous studies of Dt–Ds relationships to determine the commonness of Dt–Ds associations. We modeled the consequences of different Dt–Ds associations for plant carbon balance. Results: Our analyses showed that higher Dt was determined by higher it and lower e, and higher Ds by higher is and lower e. Across California species, positive Dt–Ds coordination arose due to it–is coordination and impacts of the variation in e. A Dt–Ds trade‐off was found in only 30% of studies. Heuristic modeling showed that species sets would have the highest carbon balance with a positive or negative relationship or decoupling of Dt and Ds, depending on environmental conditions. Conclusions: Shared precursor cells of trichomes and stomata do not limit higher numbers of both cell types or drive a general Dt–Ds trade‐off across species. This developmental flexibility across diverse species enables different Dt–Ds associations according to environmental pressures. Developmental trait analysis can clarify how contrasting trait associations would arise within and across species. [ABSTRACT FROM AUTHOR]