Avocado trees can grow to more than 50 feet tall in some situations, but generally stay under 20 feet. They bear low, stretching branches and dark, leathery leaves, and produce their fruit bounty in early spring to midsummer. These are subtropical to tropical trees from South America and Mexico, and are not hardy to cold. Even the hardiest avocado tree dies in temperatures lower than 20 degrees F. If you want to grow an avocado tree in the cold USDA Plant Hardiness Zones 2b, 3 or 4 of Minnesota, do so indoors in a large pot.
Start your avocado tree seedling in spring so you can take advantage of the Minnesota summer season. In Minnesota, spring arrives from mid-May to early June, depending on your location.
Prepare a large pot for the avocado tree. Use a 20- to 40-gallon pot with a drainage hole to give the tree room to grow and to give it good drainage. Line the pot with 1/2 to 1 inch of gravel to increase drainage. You probably will want to restrict its growth to 12 to 15 feet since it will be indoors in the winter.
Fill the pot half full of quick-draining potting soil or loam. Avocado trees don't need nutritious soil, but do need soil that drains quickly and maintains moisture content. They sustain damage or die in standing water.
Lay the avocado seedling's root ball over the soil and cover the roots with more potting soil. Fill the pot until it's three-fourths full and the soil level reaches the seedling's root junction. Press the soil firmly around the base of the tree to secure the planting, and stick your finger into the soil in several spots to eliminate air pockets.
Put the pot outdoors in summer, in a spot that receives full sunshine for eight hours every day. Avocado trees won't bloom or bear fruit if they don't receive full light. Choose a spot that is protected from drying winds.
Fill the pot to the brim with water and allow it to soak in, then repeat. Water the tree with this much water once a week to keep the soil moist.
Move the tree inside before the first frost of the season. Frost occurs between late August and mid-September, depending on growing zone. The tree will suffer in frost and die in cold temperatures. Put it in an indoor spot where it still gets full light from the sun and use artificial lights if needed. Don't put it near a fireplace, as it will dry out.
Feed the avocado tree with 1/2 cup of 21-0-0 fertilizer every month during its first year; increase fertilizer to 1 cup a month in the second year, 2 cups a month in the third year, adding a cup to the monthly feeding each year. Always follow manufacturer's directions in regard to application, and water the tree immediately after feeding.