raised bed

I would worry that it isn't deep enough for the plant to be firmly anchored in windy conditions - which can mean that the sprouts 'blow' rather than keep tightly shut.

I'm not sure about minimum depth though.

Thanks lads. I'll stick to something a bit lower to the ground I think.

Or ladies.

Indeed. 

As vietta said, if your beds are on earth than there should be no problem.

I have PSB that is well over a metre tall growing still, and our beds are only six inches deep...... plus the soil where they are built 

Sprouts were fine too... they are finished now though 

It's actually 9 1/2" deep: that extra inch and a half could make all the difference!

I was intending just putting it on a piece of lawned ground with some weed fabric in the bottom. That should give good drainage, and I would have thought the tap root would get through it ok. Perhaps best to remove the turf? My original intention was to use it for onions next year, but I hate to see it go to waste when I can bang something in it this year.