Heterogeneity exists in biology, which poses a challenge because often outcomes or responses need to be robust despite this heterogeneity. This challenge is of particular importance during morphogenesis because organ size and shape are important for their function. For example, organs need to scale to be the correct size for the organism, whether that organism is a mouse or an elephant. Organ shape can also allow for specialized function, like how beaks of birds are specialized for their diet. The size and shape of sepals, which are the four leaf-like organs on the outside flowers, allows them to enclose and protect the developing floral organs until the flower blooms. The sepals of Arabidopsis thaliana have uniform size and shape within a flower and between flowers despite heterogeneity in cell growth rates. Thus, heterogeneity is averaged spatially in the tissue and over developmental time to result in a uniform or robust outcome, and this process can be referred to as spatiotemporal averaging. The ftsh4-5 mutant has decreased heterogeneity in cell growth rates, a loss of spatiotemporal averaging of growth heterogeneity, and variable final sepal size and shape. This implies that heterogeneity in cell growth rates is necessary for robust sepal development rather than a challenge to overcome. Here, I study the organ-scale mechanism of spatiotemporal averaging through live imaging and quantitative analysis. I find that fluctuations in cell growth rates that are uncorrelated in time and space are necessary for growth to accumulate or average evenly over time and space. Decreased growth heterogeneity with correlated fluctuations, as in ftsh4-5, causes growth to accumulate unevenly, which is a loss of spatiotemporal averaging. Previously it was found that the ftsh4-5 phenotype is caused by elevated reactive oxygen species (ROS), and that microtubules are also involved in growth heterogeneity. Therefore, I examine the relationships between ROS, microtubules, and growth heterogeneity. We find that ROS inhibits microtubule dynamics and growth heterogeneity. Together, this identifies more factors involved in growth heterogeneity, and that growth heterogeneity must have uncorrelated fluctuations in order to accumulate evenly and robustly into a uniform size and shape. Altogether, my research provides insight into how heterogeneity is averaged in biology so that outcomes are robust.