You may find a lot of hints in the Forums List » The Guides » Plant Tycoon Guide » [SPOILER] The Six Magic Plants-Windows/Mac version

And in the Forums List » The Guides » Plant Tycoon Guide » [SPOILERS] Fan guides to Plant Tycoon especially:
Jazzo's getting started guide to Plant Tycoon
Jazzo's money-making guide for Plant Tycoon §: "Plant Breeding Guide:"

Good luck!