Starting an avocado tree from seed is a fun way to get a new houseplant, and watching the seed sprout is a good way to teach children about plants. Avocado seedlings bear fruit in 5 to 7 years, but container-grown, indoor plants rarely blossom or fruit. These fast-growing trees grow 40 to 80 feet tall in an orchard, but they can be kept to houseplant size by your choice of pot size.
Avocado seeds sprout easily both in water and in soil. To start a seed in soil, choose a 6- to 8-inch pot filled with a mix of potting soil, humus and vermiculite: keep the soil moist. Germination takes 1 to 3 months. Once the seed sprouts, induce branching by cutting off the top 2 inches from the main stem when the plant is 6 inches tall, and then again when it reaches 2 to 3 feet tall. The avocado tree can stay in the 6- to 8-inch pot until it reaches 2 to 3 feet in height.
Avocados need to be repotted every two or three years, once the tree's roots fill the current container but before they start circling around inside the pot. Choose a new pot 1 or 2 inches bigger in diameter and depth. Good drainage is vital, especially in large or plastic pots that do not dry out quickly. When repotting, inspect the root system and prune off any broken or diseased roots. Continue to move your avocado to larger pots as needed, but remember that if you summer your plant outside, it needs to be brought inside during the winter unless you live in a frost-free area. Very large containers are heavy and difficult to move.
Avocados can be kept in the same pot indefinitely by carefully removing the top 2 inches of soil from the container and replacing it with fresh soil every spring. However, once the tree's roots have filled the pot, root-pruning will be necessary to control the plant's growth.
Cornell University Extension Service's website recommends root-pruning of container-grown trees over a two-year period. In the first year, knock the plant out of its container and loosen the soil around the roots with your fingers, then prune off 1 inch of the outer roots from half the rootball. Use fresh potting soil in the container when you replace the tree. Lightly prune the top growth to compensate for the root loss. In the second year, follow the same procedure but prune the other side of the rootball. Root-pruning is best done in early spring, before the tree starts active growth for the year. Root-pruning can be repeated whenever the plant's roots fill the container again or when the plant reaches the maximum size desired.