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There are literally hundreds of olive tree varieties around the world. Some are similar with only slight differences in olive color and flavor. Others are extremely different and have growing preferences which impact the ripening time.  For example, the four main types in California are: Mission, Manzanillo, Sevillano and Ascolano. Even though these are grown in the same state, the diverse climate, coupled with a particular olive strand, yields a range of results. Significantly research your location to determine which type of olive will prosper. Growing from seed, you will get more of a wild variety than any of the parent trees it came from. This may sound easy, but the fruit must be picked directly from the tree so the seed remains alive. Olive trees prosper in climate zones 8-11. These areas have a subtropical climate and mild winters. Pick them in early autumn, after the fruits ripen and are green. Leave the black ones. Don't pick any off the ground and make sure the ones you choose don't have any holes from insects.  Using olives purchased from a grocery store will not work. These have been processed for consumption which means they've been cooked. This process kills the seed inside the pit, so they are no longer viable. Raw olives from the produce section may be viable, though. If you don't have access to an olive tree, many large orchards will ship pits/seeds directly to you. Once you have the olives, gently smash the flesh with a hammer to loosen the fruit around the pit. Cover the crushed olives with warm water and soak them overnight. Stir the water every few hours or so. Jostling them will encourage the fruit to loosen further.  If a hammer isn't available, use a wide knife and smash with the flat side. If you notice a few floating to the surface, gather these and discard. They are likely rotten. Gather the pits and use a scour pad to rub the excess skin off. This is the same pad you probably use on pots and pans. After you've rubbed the skin, thoroughly rinse the pits in warm water for several minutes. If a scour pad is not available, try sandpaper. There is a blunt end and a pointy end on every olive pit. With a knife, nick the blunt end. Do not break through the hull completely or the seed will be useless. Instead, create a tiny hole about the size of a pen tip. Soak these for 24 hours in room temperature water.
Determine the type of tree you want to grow. Gather olives. Place the olives in a bucket. Drain the water. Cut the ends of the pits.