The gardens situated the furthest south that I've visited aren't far from Penzance - Trewidden and the National Trust garden - Trengwainton - unfortunately there wasn't time to see these this time but thought I'd give them a mention here anyway as they're fabulous!
Moving up towards Helston direction another favorite is Trevarno which I did go to - I particularly enjoyed the kitchen garden where I did get the chance to have a lengthy chat to the gardener there who has all sorts of interesting ideas with planting and providing support to plants and natural archways for his sweet peas - a progressive area and one that has changed quite a bit over the last few years. 
The other gardens we managed to get to this time round was Caerhays - with its magnificent collection of magnolias. It's set right by the coast and for those of you who enjoy a good walk there's a fair bit of up and down in this garden with some stunnning views.
Burncoose Nursery and Gardens was also well worth a visit - I must admit I spent most time in the nursery- they've got a fantastic selection of plants and the roses in particular are very good value! 
We also managed the National Trust's garden at Trelissick which was lovely, but not one I think I'd rush back to but was definately worth seeing though.
The other two which are always interesting at most times of year are both the National Trust's Glendurgan garden and Trebah - both valley gardens on the Helford River - they are similar in some respects but I'd recommend going to both.
Last but not least is Trewithen - another gorgeous garden - full of natural planting - wonderful Camelias and Azaleas as well as an interesting house which is also open to the public (I must admit I haven't been inside myself -although I've heard good reports). There are many more that I'm planning to visit on our next trip which I've not been along to yet - but hopefully this should keep you going in the meantime!
I also wanted to mention a couple of excellent self catering places to stay - a wonderful cottage right near the coast near Porscatho - within striking distance of Caerhays, Heligan and the Eden Project - Rosevine Farm - lovely owners who are really keen to promote everything "green" - extremely comfortable cottages with everything you could want in a holiday home. For those wanting to stay nearer to the Helford River and be within easy distance of Trevarno, Penzance, St Ives and also the Lizard peninsula - Higher Bowgie is brilliantly placed too, equally great and also very comfortable.
ENJOY!
