A key principle of taking maaser, or tithes, is that the produce needs to be ready for market, or 'on the threshing floor', in the words of the Mishnah. For grain this is obvious because it passes through the threshing floor on its way to market. But what about vegetables and fruit?
For a set of slides which summarises the Mishnah, go to the presentation here: https://tinyurl.com/SlidesMaasrot, scrolling to the end.