What size container is it in at the present time? 20 gallons seems like a bit of a leap for a 3 foot avocado. Oftentimes, when a typical potting medium is used (like your MG), an overly large container can result in a stagnant root environment. There is considerable information about potting mixes in this forum. If you spend a bit of time reading some older threads, you'll find all sorts of possibilities. However, the majority of us have learned that by adding some simple amendments to a typical potting medium, you can create something that your plants will thrive in. The incorporation of pine bark fines, Turface MVP, perlite, and/or similiar ingredients will help you create something that is fast draining, and conducive to a healthy and vigorous root system.

SeanPez, I note you are a new member, welcome. It would help if you edit your post information so we know where you garden. If you are in California, you have planted your tree in the ground at the right time to take advantage of the rainy season to establish your tree in the native soil. I would hold off on the fertilizer until late winter/early spring. With the exception of citrus and avocado, I do not use synthetic fertilizer, only compost. For citrus and avocado I use Lily Miller as per directions, as it contains the minor nutrients essential for good health. Avocado are shallow rooted and benefit from regular additions of mulch, keeping it an inch or two clear of the trunk. Al

What you would need to do would be to keep the top growth small in proportion to the smaller root system. I recently went to a lecture by a guy from Dave Wilson (wholesaler of fruit trees) and he said you could do that--keep an avocado trimmed down to about 8-10'. Of course, you are going to be out there trimming it frequently. Having said that, you want as big a planter as you can possibly give it, like 24"x24" or bigger, and as Renee said, you need to figure out a way to keep the root system cool and moist, perhaps with an outer white pot to insulate the inner one. Long term, train the tree into an umbrella shape so that the top growth will shade the container. Of course you are not going to get the yield you would get from a large tree, but if a few Avocados is enough, well there you go. And if you can return it and get a Reed instead of a Hass...the Avocado growers have a saying: "Hass you sell, Reed you eat."

It is very interesting to me to see what has happened in the last few years to this particular spot in your garden. I think your garden has done very well under that maple. But gardeners know when their plants are not growing as well as we know they could---and that is disappointing. I have two Jacob's Ladder plants. They are cute and a bit delicate looking. They are in a badly rooted area. I have to have them in spin out bags and that garden bed gets dry very quickly. This year we added soaker hose to that bed. It goes on every second day. I have two pulmonaria there too, but they are also in spin out bags. The only thing not in a spin out bag is variegated Solomon's seal. It grows, but spacing between stems, I believe, is further apart than it normally would be. In my neighborhood there is a house that has several June's around a tree. They may have been there for almost ten years? But let's say 5-10 since I can't remember. How do they look? Cute, small for June, and they have remained the same size in the last few years. What I noticed last year, was that they are now reverting to Halcyon. I do like that look, but we know where that is headed. So I think the hosta can survive in a rooted area, but just survival is not the goal. It's growth and health. I think the pot in a pot method is worth trying. I think I will look to see if I can find two pots that fit together well and try it myself. I wouldn't want to lose any hosta. I will pick a hosta that I have a duplicate of in case it doesn't work out well for me. Good luck.