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.
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.