They can be grown here in Arkansas, in fact I am starting four right now which will get planted in the orchards next spring.
They can handle some cold (we get down to around 10 degrees (what we call brutal cold here)) but frost can kill leaf buds so you need to be able to at least put a cloth cover over the trees if frost is going to occur.
Kent Washington may work out nicely for them Avocado trees do well in zones 7,8,9,10
Rene Nijstad wrote:Avocados are pretty easy to grow, but they cannot take frost. I've heard of people keeping them indoors in colder climates, which will work for some time, but they're big trees and they can get up to 20 meters tall. I don't know what climate you're in, so just put it somewhere where it won't freeze
Contrary to Rene's observation, when I lived in avocado country, I had three avocado shrubs in the back yard. Not trees, but shrubs that weren't as tall as I am (6ft), and they bore lots of nice little avocados. Yes, in the commercial groves, 20 meters is pretty typical, but it may be possible to keep them small by being root-bound (bonsai, anyone?) or by continued pruning back.
Now I have 3 potted avocados (in 20 and 30 gallon pots), and I have to keep them in the greenhouse in my zone 8 winters (which is probably what you have up in WA). They got nipped a couple winters ago when the greenhouse heater wimped out, but they have come back and I am hoping that one year they will flower and set fruit.
I raised mine in the same way yours started, with a pit from a store-bought avocado. I would move that seedling into the largest size pot you can work with and keep coaxing it along. But do it soon. Avocados send down a tap root and that little seedling could have a root that is 3-4 times as deep. And I would vote to cover the exposed part of the pit with some mulch.
In the Avocado orchards of Southern California that I worked at they sprouted new trees and then potted them in 24" tall, 8" diameter containers for the first six months from there they go to their permanent home and the seed is just at the soil surface.
A mulch around the base of the new tree is beneficial but it should be very thin right at the stem, it can cause issues if the growing, new tree trunk can't get air right to the seed junction.