That water wicks in soil - if you put a pot of earth in a saucer or tub of water, the water will rise near the top of the pot. It really depends on all sorts of things but it is a surprisingly ill-founded rule of thumb.
Using broken-up polystyrene is a good substitute for rocks or broken pots to provide drainage at the bottom of your planters. (The key feature of polystyrene is that, unlike rocks, it floats, so if your planter is flooded with rain, the polystyrene floats to the top)
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In the sticks near Peterborough