Q.transplanting avocados
I started this from a Haas avocado seed. Once rooting took hold I transferred to soil in a 6″ diameter pot. Now the plant is nearing 12″ tall but the trunk is rather spindly looking. It has plenty of healthy leaves but need to know when to transplant to a larger pot then into the ground and the best soil to use. Also, do I need to be concerned about this being a self-propagating plant?

Certified GKH Gardening Expert
Be sure your pot is only a couple inches larger than its existing pot. If this is an indoor plant you can repot it at any time, but to transfer it to the ground, wait until spring or early summer when it's warmed up outside. You may want to take a look at your root ball once you've removed it from its current pot; you can trim any dead roots at this time, and gently untangle roots if needed.
Here's an article that will help you understand its flowering habits: http://ucavo.ucr.edu/Flowering/FloweringBasics.html
and another article about potted avocado plants:https://www.gardeningknowhow.com/edible/fruits/avocado/indoor-avocado-plant-care.htm