There is a reason that San Diego County, California (known as "Avocado Country" to industry insiders), produced nearly 150 million pounds of avocados in 2003. Its almost always warm there—in fact, the average temperature consistently hovers near 70°F.
Unfortunately for those of us Zone 8 and above, "Avocados will not survive in northeast Texas or anyplace else in Texas, except right along the coast", says Gary Bender, Ph.D., subtropical horticulture expert at the University of California—Davis San Diego County Cooperative Extension. Jacksonville's tendency toward extreme heat in the summer and occasional frost in the winter, including freezing rain and even snow, makes your corner of the world inhospitable to avocado trees.
Avocados are subtropical trees native to southern Mexico, and they grow best in semihumid climates with moderate temperatures (60° to 80°F). Even the most coldhardy varieties ('Mexicola' or 'Topa Topa') freeze at about 25°F, Dr. Bender says.
You can, however, grow avocados in containers and move them inside during cold weather, but there are no dwarf varieties available, so the trees tend to get too big within about three years.
Organic Gardening contributing editor Willi Evans Galloway wanted to be really sure that avocados would not survive in Zone 8. So she called up the local Jacksonville, Texas extension horticulture agent, Joseph Daniel. "I'm afraid that we're a little too far north for avocados," says Daniel. "We can have fun germinating the seed and growing the plant for a while, but that's about as far as we can take it." So it seems the only avocados you'll be picking will be from the supermarket. But if you still want to grow your own indoor baby avocado tree, the California Avocado Commission has helpful instructions online at www.avocado.org.