Satellite measurements known as vegetation optical depth (VOD) measure how much water is in plants. Because plants with more mass can hold more water, changes in VOD are used to determine biomass changes, ignoring the fact that changes in how dry the plants are also affect VOD. We use an alternative data set for biomass to test this approach. At a given location, VOD and biomass do not vary in the same pattern from year to year and for many types of plants, VOD seems to follow soil moisture (which should co-vary with the fraction of water in plants) better than it follows biomass itself. These results help clarify when VOD can and can’t provide information about biomass.