18" of topsoil is quite a reasonable depth for most stuff, though as others have suggested, some carrots and/or parsnips may suffer a little. Using the term "hardcore", do you mean the natural gravelly stuff that is often compacted down for vehicle hard standing, or tarmac road shavings etc? If it's the former I wouldn't anticipate a problem, it's quite free draining and much less of a hindrance to root penetration than you might expect ... I dug out (salvaged) a hawthorn seedling from a car parking area at a previous accomodation that I wanted to put in a gap in the garden hedge ... it went straight down through over a foot of hardcore ... and it filled the gap in the hedge once I'd uprooted it ;)
Tarmac shavings, builders rubble etc. type "hardcore" may be a bit more of a problem if there's any contamination, excessive alkinity from cement etc., but again, drainage shouldn't be a problem. Indeed, I suspect your main problem may be that the drainage is too good, but that will depend to a degree on the nature/quality of the overlying topsoil.
Good luck :)