Invasive plants can use a lot of water, causing problems for local communities and native plants. It's difficult and expensive to measure how much water these plants use, so we built our own sensors using hobby electronics parts and installed them in two sites: a tropical island with invasive coconut palms, and a riverside woodland in a hot desert environment with invasive reeds. We found that our electronics functioned well for as long as 18 months in harsh environments and saved a lot of money. At the tropical site, we found that each coconut palm consumes almost 100 L per day—almost twice as much as the native rainforest trees. Invasive reeds at the desert site also appear to be using water at a very high rate compared to the native species they are replacing. These results highlight how invasive plants can endanger water resources in diverse environments, and how low-cost electronics can help scientists collect much more data than would otherwise be possible.