Plant Tycoon does have a speed setting. On the Palm, you simply click on the "Menu" icon to access the Game Menu. From there, select Preferences, and you'll be able to set your game speed. If you don't know where to find the "Menu" icon, you can check out the pictures in this post!

The plants are mature and can be pollinated, or used to pollinate other plants, when they reach age 24. Like the fish in Fish Tycoon, the plants don't die of old age, but you can kill them through neglect. The main benefit of plant food, as you suspected (and the shop description says), is more flowers (which makes the plants more valuable). The effect is most noticeable over several generations of fertilized plants. Cross-breeding plants before you have the better soils will create hybrids that cannot live in the soil that you have at the start of the game (similar to Fish Environment and fragile hybrids in Fish Tycoon). There are (just as in Fish Tycoon) common plants that will sell for good prices (some sell for more than $40). All you have to do is find them and breed them to finance all of those necessary upgrades. There are a lot more varieties of plants to discover in Plant Tycoon!