Backscatter enables wireless transmissions at dramatically lower power compared to mainstream Internet of Things (IoT) transmitters. This significantly improves battery life or even eliminates the need of batteries for backscatter-based 'tags' operating on energy harvested from the environment. However, trading off complexity for low power consumption exposing backscatter tags to a multitude of security risks. A significant challenge is imposter tags that mimic legitimate tag behaviour, compromising the system data integrity. In this work, we argue that tag simplicity and operation on harvested energy can help identify imposter transmissions, thus safeguarding data integrity. Our experimental study reveals that commonly used low-power tag oscillators demonstrate unique fingerprint patterns when exposed to the dynamics of harvested energy. Based on this observation, we design and propose HarvestPrint, which leverages these fingerprints to differentiate authentic backscatter transmissions from imposter transmissions. Experiments with a tag powered through a small solar cell show the potential of HarvestPrint.