Though there are over 900 varieties of avocado in the world, the majority of the avocados you find in the supermarket are Hass avocados. The Hass avocado is an avocado variety with a purplish-black skin color and a nutty flavor that ripens after it is picked. All Hass avocados are descended from a single mother tree of an unknown subspecies. You can grow a Hass avocado tree directly from the pit of a store-bought Hass avocado.
Scrub your avocado seed to remove the outer coating.
Insert three toothpicks into the seed at equal points around the center circumference of the seed of the way down from the narrow end of the seed.
Place the seed into the mouth of a quart jar with the wide end pointing downward so that the toothpicks prevent the seed from falling into the jar. Fill the jar with water so that the bottom 1/3 of the seed is covered with the water.
Wait until roots form from the lower part of the seed and the top begins to split open and sprout. Pot the avocado seedling so that the pit is halfway buried in a 6-inch container with well-drained potting soil. Place the container in a sunny windowsill.
Re-pot the avocado in a slightly larger container after the plant becomes root bound. Avocados do well when they're slightly root bound. Transplant your avocado into a sunny location with well-drained soil if you live in zone 9 or warmer. If you live in zone 8 or cooler, leave your avocado tree in a container.
Check the soil weekly by inserting your finger into the soil up to your second knuckle. Water the plant anytime the soil appears dry. Avocado plants like soil that is as damp as a wrung-out sponge.