the family I'm playing atm has a big garden with plants that are almost all 'perfect' quality, if I leave them for a while to play another family in the same world, what will happen to the garden? will all the plants die with no one to care for them?
I have only the base game and seasons (and its forever summer in my game right now, no other seasons).

I don't have Seasons, but in my game, any plant that is not on the home lot stays tended automatically. This applies to community gardens, gardens on another residential lot, or even wild plants in the world. And sending your sims from the new household to harvest perfect plants from your other sim's house fills the "Harvest a Wild Plant" wants that they sometimes roll.

Ghost sdoj is right. The plants will stay as they are

thank you for clearing this up!

I don't know if this aspect is handled by Nraas Story Progression or not, but on my residential farm lots I've seen sims tending their gardens. I don't know if they sell the produce, though; an occasional check of the objects in town reveals hundreds of harvestables in inventories, so I'm guessing they just hoard tons of watermelons in their ass :P

You can check what happens to plants by checking the ones on other lots, like fishing spots or the graveyard. The death plant dies occasionally but revives itself, I think.

They sell some of the produce because when you go to the consignment store or the produce stand (not sure about which it is) some of the produce will be marked as coming from this or that person's garden.

From my experiences, though. Don't send your active sims to that location or the plants will start aging. Because the game will manage those crops for you by not allowing it to age unless you are there. After I learned this, it kind of discourage me from playing for a bit, I like the fact the game kinda helps me out, but still...

^^^ That's not normal, however. Once a lot is not actively played or is a community lot, none of the plants on it will age or die. The only exception is the death flower plant: this will be dead once you harvest from it, no matter where you have it.

But other than that, plants on community lots or inactive residential lots will grow and mature in a random fashion but they will not die or become barren.

There are mods that change that, though. AwesomeMod, for example, has an option to change plant growth such that inactive lots' plants will have the same growth cycles as plants on active residential lots.

I've seen random townies come onto my lots and raid my garden so I would imagine that you could do so with anyone's house.

Mmmm, yes. Brings back so many memories from Sunset Valley, i always raided Gobias Koffi's garden to get them sweet flame fruits.

Gobias Koffi grows flame fruits? I did not know this!

Thanks for the info!