Raingardens are a more established idea in Australia and the US than they have so far been in the UK. The expression has existed in two-word form (i.e. rain garden) since the mid-nineties, but with increased exposure and usage the closed form is now becoming more common (a process linguists often describe as lexicalization, and which over time can render the two-word form obsolete, compare e.g. blackboard, football, airport).