Fertilizer scales by price, everyting above 50 simoleons is good.
For gardening tips, if you have no other plans, I recommend starting in the rightmost neighborhood of Newcrest in summer to be able to harvest the rather valuable lilys. There are two or three wild lilys in this area and the townies will happily grill for you at the public barbecue, no need to spend time on cooking early on.
If you have Eco Lifestyle, flower kombucha generates a crazy payout. I made my fortune in the millionaire scenario with nine distilleries running nonstop and an ocean of lilies and orchids (Lily + Snapdragon, that conveniently also grow in this neighborhood).
A time saver is analyzing your starting plants with the Chemical Analyzer (from GTW) before planting them. If you succeed (some Logic points required), they will instantly be Nice instead of Normal. Warning: Only have 1 single plant in your inventory when doing this, otherwise failing to analyze one plant will mark all plants of the same stash as failed.
