Cash-crop diffusion in colonial Africa was uneven and defied colonizers expectations and efforts, especially for cotton. This study investigates how agricultural seasonality affected African farmers cotton adoption, circa 1900-1960. A contrast between British Uganda and the interior of French West Africa demonstrates that a short rainy season and the resulting short farming cycles generated seasonal labor bottlenecks and food security concerns, limiting cotton output. Agricultural seasonality also had wider repercussions, for colonial coercion, investment, and African income-earning strategies. A labor productivity breakthrough in post-colonial Francophone West Africa mitigated the seasonality constraint, facilitating impressive cotton output growth post-1960.