Avocado tree is approx. 24" with numerous leaves at the top.
How should I plant it outside (big pot because they say Seattle soil isn't good)?
Do I need to bring it in in the winter?
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There is advice at California Avocados:
Remember that avocado trees do best at moderately warm temperatures (60-85F) with moderate humidity. They can tolerate temperatures, once established, to around 32-28F with minimal damage. Avoid freezing temperatures.
Plant your tree from March through June. If you plant during the summer, there is always the risk of sun damage, because the avocado trees can't take up water very well when young. Plant in a non-lawn area, and away from sidewalks. And if you can, plant your tree in a spot protected from wind and frost. Remember, full sun is best.
Dig a hole as deep as the current root ball and just as wide as the width plus a little extra so you can get your hands into the hole to plant it. The avocado is a shallow-rooted tree with most of the feeder roots in the top 6" of soil, so give it good aeration. Its root system is very sensitive and great care should be taken not to disturb the root system when transplanting. If the tree is root-bound, however, loosen up the soil around the edges and clip the roots that are going in circles.
Avocado trees like the soil pH around 6-6.5. If you have a heavy clay soil, elevate the tree in a mound for better drainage. Make the mound 1-2 feet high and 3-5 feet around.
Don’t put gravel or anything else like planting media in the hole. The sooner the roots get out into the bulk soil, the better the tree will do.
Typically trees need to be watered 2-3 times a week. As the roots reach out into the bulk soil, more water can be applied and frequency of watering diminishes to about 1 time per week by the end of a year.:
When watering the tree, soak the soil well, then, allow it to dry out somewhat before watering again. Of course, like most plants, you don't want the tree to get too dry! The rule of thumb for mature trees is about 20 gallons of water a day during the irrigation season. Seedlings will require quite a bit less water, of course. Check the soil before watering each time to make sure it has dried somewhat. If the soil from around the roots can still hold the impression of the hand when squeezed, it still has enough water.
Mulching and Fertilizing
Mulch with coarse yard mulch. Redwood bark will work or cocoa bean husks and shredded tree bark. Choose something that is woody and about 2 inches in diameter. Coarse yard mulch is available at some garden supply centers or through tree trimming operations. They sometimes have material that has been pruned from the tops of trees and don’t contain any diseased roots. Use the yellow pages to find a local tree service.
Put 20 pounds of gypsum spread around the tree base and mulch the area with 6 inches of the mulch, keeping the material about 6-8 inches away from the tree trunk.
Fertilize your young avocado trees with1/2 -1 pound of actual Nitrogen per tree per year. You can spread it out over several applications as long as it totals 1/2 to 1 pound of Nitrogen. The other important nutrient for avocado trees is Zinc. Ordinary home fertilizer for houseplants normally should work. Back to top Other growing tips
Be patient about seeing fruit. If you have purchased and planted a tree, you can probably expect to see your first fruit 3-4 years after planting. If you are growing from seed, it can take anywhere from 5 to 13 years before the tree is mature enough to set fruit. When the tree does flower, expect a lot of flowers to fall from the tree without setting fruit. This is natural.
However, I suspect you are around Zone 8b (plants hardy down to 15 to 20 degrees) and that is too cold for an avocado to stay outside. Yet your avocado won't appreciate being dug up every winter. So you would really need it to be in a pot so it can be relocated, without much root disturbance, to a warmer location, such as a greenhouse. Back indoors might not suit both because of the climate change (eg possibly too dry), likely reduction in sunlight and, possibly, sheer size.
On balance I fear growing avocados in Seattle does not promise much prospect of success.