Also, if you plan to have a big garden, the sprinkler found under outside stuff (under the football, I think) will be a great aid in watering the plants, especially if the sim (by aquiring a certain level of handiness) can upgrade it to auto watering.
If I remember correct, the auto watering starts at around 4 or 5 am, and the weeds or plant degradation goes off around 5 or 6 am. I make sure I've got at least one sim with a high gardening level (I usually max it out, often by a combo of gardening and reading the garden skill books), preferably with "green thumb" trait and "super green thumb" aspiration. These sims will up the level on plants they plant with at least one level (say, outstanding instead of very nice on a very nice seed level), and you get perfect plants much faster. Also make sure you've got enough space in the garden. Two tight lines of plants works fine, but three or more without some space between will result in annoyed sims who can't get to the ones in the middle.
The collector aspiration reward is a great tool for finding seeds. Death flower and life plant seeds are often to be found around the graveyard. Seeds come as common (apples, grapes, lettuce...), uncommon (lime, onion...), rare (garlic, bell pepper...) and special (Death flower, life plant, money tree). If you've got adventures, you can look around for cherries in China, pomelo (China, I think), and several new grapes in France. The special merchant also sells some rare grapes, which you can buy if you've got a high visa level on a sim.
I don't have any mods, but I've been playing a bit with the "testingcheatsenabled true" cheat, which lets the sim teleport if you shift-click on the ground (no need for the teleporter aspiration reward, in other words). Nice to have it when looking around for seeds (as walking between them takes ages). Too bad it's not also possible to set plants to "harvest" in sims 3, like you can in sims 2...
Here's a bit more about gardening.
