Posted: 10/01/2013 at 14:10
I have a bank just the same, couldn't get a fork it in, so hard- that was summer of 2011,, so used a spade to chip away at it and then added a scak of sharp sand and had a bucket of home-made compost which made it easier to dig. Then planted up with Red hot pokers and Cristophiii Allims - both which flowered for the first time this year- 2012-lovely. Also planted a couple of Kniphofias in 2010 on the level rather wet clay and they are growing huge and had flowers for the first time 2012. they were only young plants about 6in when I planted them- now 2-3 ft across. They don't need any care, have survived the frost & ice of 2010 which was down to -17 in York sometimes.
This autumn I have added some ornamental grasses so hoping they will flourish. In the more shady area I have added hostas, ferns and primroses. The soil is now very good and as on a slope does drain well even though clay. Would swear by adding a sack of sharp sand- £2 ish from B& Q in the building section (not builders sand has to be Sharp sand) I can't believe the difference in the soil! On the same sloping bank where I didn't add sand the soil is currently claggy and wet after all the rain and frost.
PS. Hemoracalis (Day lillies) grow really well if they can have some summer sun on my North facing, clay soil.
Hope this helps...