Seeds represent a stage of a plant’s life cycle that is extremely vulnerable to predation, which, unlike most vegetative herbivory, is fatal to the individual. As such, understanding the distribution and abundance of plants may rely upon understanding seed defenses: characteristics of seeds that make them more difficult for granivores to locate, make them less beneficial for granivores to consume, or both. Seeds that produce mucilage are widespread and found across many families and species. Although short-term (single-day) studies indicate that mucilage may be a fundamental seed defense found throughout the plant kingdom, it is not clear whether mucilage provides long-term benefits to seeds via reducing granivory. Moreover, it is not clear whether this long-term defense occurs because mucus reduces seed apparency because substrate-coated seeds are more difficult to detect, because substrate-coated seeds are of less value, or both. In this field experiment, we factorially manipulated sand coatings, background sand color, and granivore community using feeding depots in order to test the mechanistic basis of mucilage-bound sand as a seed defense against diverse granivores. We found that the sand coating significantly extended seed survival over the 101-day trial, during which rodents were the primary granivores, and our coloration manipulation suggests that the effect was almost entirely physical. We found that whether or not a seed’s sand coating matched its background did not greatly affect seed survival, leaving the crypsis mechanism unsupported. A follow-up experiment to test the background-matching hypothesis in more detail, using 20 sand colors and two colors of flax seeds again found no support for crypsis, corroborating the finding that mucilage-bound sand provides a primarily physical defensive benefit. The totality of the results from this and previous studies across granivore taxa and plant species suggests this highly effective defense may be ubiquitous.