There isn’t much widespread knowledge about growing Avocado trees, especially outside of the few regions they’re grown commercially. I’ve been pretty singularly focused on growing Avocados in the last few years, but in sub-optimal climates, and wanted to share some of the hard-to-find information I’ve come across. (Disclaimer: I don’t claim any special knowledge beyond a serious interest in the subject and I don’t want to duplicate much of what’s already written in the Avocado wikipedia entry among other sources.)
There are a number of reasons I think Avocados are worth growing: they’re one of the few sources of plant-based fats (and non-starch calories), they have a broad spectrum of nutrients, they can be harvested over a long period and don’t drop off the tree that easily, they’re relatively pest free, and, of course, they’re delicious. Also, a new and somewhat sad reason has come up: Citrus greening disease, which is wiping out Citrus trees worldwide. Avocados can grow well in most places that Citrus grow today; Citrus that succumb to greening could be replaced by Avocados.
So suppose you want to want to grow Avocados. Well first let’s examine the optimal climate, and then see how far we might be able to stretch it. Some of the best climates for growing Avocados in the United States have December average daily low temperatures in the mid-high 40s F (8 C) and average daily high temperatures in the mid 60s F (18 C), and August average daily highs in the high 70s F (25 C). (Since they get little rainfall in Southern California, Avocado orchards are of course irrigated at great expense.)
My interest has been growing Avocados outside of that optimal climate. So there are a few things to know. First, the three types have different properties both in fruit and in growing conditions. Second, it’s possible to give them what they need outside of their optimal climates.
There are broadly speaking three types of Avocado: Mexican, Guatemalan, and West Indian (there are of course numerous crosses between them). Avocado trees, like many other fruit trees, don’t grow true to type if grown from a seed — start an Avocado from seed and you never know what you’ll get (and it’s generally thought to be unlikely you’ll get a tree that produces tasty fruit or any fruit at all, though you will most certainly get a beautiful tree). If you have land to spare and live in a marginal climate, I would recommend starting many Avocado trees from seed to find what survives in your region, but this approach can take many years of patience. So most Avocados come from grafted trees of a specific cultivar. Hass, the most common cultivar grown today, was discovered in Southern California in the early 1900s, and is thought to be mostly Guatemalan with a bit of Mexican genetics. But there are many, many more cultivars, some of which I’ll talk about below.
West Indian Avocados grow well in the Caribbean, Florida, and similar areas that have hot, humid, and wet climates. However the large light green-skinned fruit they produce aren’t generally as flavorful as the Guatemalan / Mexican varieties I’ve come to love — West Indian Avocados are low in oil content and more watery. I have to admit that I don’t have much to say in their favor, and will focus on the other two types.
Guatemalan Avocados are most similar to the rich, nutty, creamy varieties available at the store — Hass and its relatives. Guatemalan cultivars are the most sensitive to frost, and when young can barely handle any time below freezing without protection, but as mature trees can handle a few degrees below freezing before succumbing. They do best in regions with a smaller temperature range, such as along the coast. Other Guatemalan or mostly-Guatemalan cultivars include Reed, Gwen, Queen, Kona Sharwil, and Pinkerton.
Mexican Avocados are also quite tasty though not quite as rich in flavor as Guatemalan varieties — Mexicola Grande is one of the better known cultivars — and are able to thrive in locations with more winter frost and more summer heat. They have been known to survive temperatures of 20 F (-7 C) (some claim even lower). Their fruit is generally smaller than the other two types, and has very thin skin (that is often deep purple-black). Other Mexican cultivars include Mexicola (non-Grande), Bacon, and Zutano.
Mexican-Guatemalan hybrids thrive in in-between climates — regions with some coastal influence but more heat in the summer and cold in the winter. Fuerte is one among many such half and half cultivars, and Ettinger is another such hybrid.
In their optimal climates, Avocados are split into type A and type B, which indicate their flowering pattern. It’s thought that you’ll need a type A and a type B for cross-pollination to get fruit. However outside of regions that are as warm and temperate as Southern California, the trees get confused about their flowering schedule and having the two types is less important. In general, though, Avocado trees produce better when they have cross-pollinators.
The trees don’t appear to be too picky about soil, but they don’t like waterlogged soil (which can cause root rot) and have shallow feeder roots so do best with a thick layer of coarse, weed-free mulch underneath (e.g. leaves, tree trimmings, wood chips, etc.). Planting them a little above natural grade can help avoid waterlogging and keep them slightly warmer. A deep watering once per week when there isn’t rain seems to be sufficient, though there are plenty of mature Avocado trees I’ve seen that are growing and fruiting in Northern California without any care at all, surviving year-round just on the rain they get during rainy winters. While they will grow in containers, it’s unlikely they will fruit. Generally they need full or near-full sun to do well, though cloudiness doesn’t seem be an issue as they grow in plenty of cloudy / foggy coastal locations in California.
Most Avocados are grown in tropical, subtropical, and Mediterranean climates, but I suspect that it’s possible to grow Avocados much further North than they are currently grown. And beyond that, as climate zones slowly shift pole-ward, places that an Avocado might just barely survive now might be able to get fruit in a few decades.
As it stands, I know that it’s possible to grow Avocados in most parts California, much further North than most people are aware — virtually to the Oregon border. Within the U.S. I’d bet that Mexican cultivars (or hybrids that are mostly Mexican and part Guatemalan) could grow in a number of spots along the Oregon and Washington coast (and/or on hillsides of near-coastal sheltered valleys — assuming the hillsides get less frost than the valley floor, which is usually the case). Outside of the U.S. it might be possible to grow them in regions like Southern England, along the West Coast of France, and maybe even protected coastal areas of Belgium and the Netherlands; also much of coastal Japan could likely grow Avocados. Elsewhere they can be grown in large greenhouses, but you’d need to ensure a moderate temperature range and that (for Mexican and Guatemalan varieties) the humidity doesn’t get too high. When living outside of the right climate it can be hard to get grafted trees of specific cultivars; however I’ve found that local fruit tree nurseries will often do a special order if you ask.
When grown outside of their optimal climate, Avocado trees will sometimes lose their leaves when young or when stressed by cold or over-watering. Sometimes the leaves will just turn brown and hang on the tree for months through the winter, only to drop in the spring when new leaves come in. So don’t assume the tree is dead just because it’s lost its leaves in the winter.
Beyond selecting the right cultivars, it’s also fairly easy to create a microclimate that is a few degrees warmer than the surroundings by adding more temperature buffers (e.g. rocks, water features, concrete, etc.), cold air drainage (e.g. downslopes from the Avocado so cold air can flow away on cold nights), and placement so that the tree gets sun as early as possible on winter mornings. Growing against a South-facing wall (in the Northern Hemisphere) can help. With these steps, plus protection while the tree is young, and I think Avocados could be grown far and wide.