Biologists have long debated why some plant species produce many seeds synchronously during some years and few during others. One hypothesis is that selection on phenological synchrony of flowering improves pollination efficiency and leads to increased fecundity. In an 11-year study of valley oak (Quercus lobata), we found that, indeed, trees that produced flowers during mid-season tended to produce more acorns. But, is synchrony the key factor? Here, we test the phenological synchrony hypothesis versus the alternative hypothesis that the environment principally shapes flowering phenology and acorn production. At our site, we find that a tree’s microenvironment is associated with both timing of phenology and acorn production. Timing of flowering depends on air temperature and crop size is not related to synchrony but is best predicted by relative elevation – likely a proxy for substrate quality and access to water. We conclude that microenvironment - not phenological synchrony - shapes acorn production.