Many sites here get below 20F often enough that a plant hardy only to that minimum better not be something the planter wants to keep alive for very long. Another thing about comparatively low minimum temperatures noted for warm climate plants is that these are based on brief exposures ordinarily preceded and quickly followed by much higher temperatures - the specimens surviving them are not subjected to significant penetration of frost into the soil, unlike here in the north.

Supposedly there was a fruiting avocado tree many years ago in a mild Seattle neighborhood but I have not seen a single successful specimen myself over many years of driving by, walking past and working in gardens of this area.

Thank you. =) It's useful to know you've never seen an example in Seattle; that tells me my plans are probably doomed. Oh well!

(As a side note, I am still confused by zones and temperature requirements. I have had plants outside for consecutive years that shouldn't survive outside zone 9, yet they keep coming back. And I don't even mulch or protect them.)

You'd be much more likely to have a chance of a fruiting avocado if you grew it as a container plant and brought it inside for the winter. There are grafted dwarf varieties available here in California that are suitable for large containers, (think half wine barrel size). I know of no avocado varieties that are hardy to 20F, it is more likely that there have been survivors here in California that got that cold and may have regrown from the trunk, but rest assured, the foliage and smaller branches almost always get fried at 25F, and would suffer serious die back at 20F. To give you a better idea, they are not usually prolific fruit barers even in San Francisco, especially if they are not cool summer adapted grafted varieties. I would guess that Sunset zone 9, 14-17 are the range of hardiness for any Avocado to be grown outdoors year round.

They keep coming back until it gets too cold for them. Very commonly planted Zone 9 hardy Cordyline australis is an excellent indicator of winter temperature patterns. It grows for varying intervals of years in this area, then freezes down to the crown each time we drop outside of its minimum temperature range. Places too far from salt water will not be able to keep it going for even short intervals from year-to-year, even the roots freezing and dying.

Thanks, bahia. A long time ago I lived in SF, so I know the kind of weather you're talking about. And, yes, I've seen citrus die back and then regrow in California; avocados would likely be similar. I guess you're telling me what I already knew but didn't want to accept: I need to buy a specialized (AKA more expensive) dwarf variety.

Heh, Sunset zone 9 ... I could plant one with my parents and just show up once a year to steal some fruit.

Thanks. =)

"They" being those Zone 9 plants that don't die immediately. Something in the next zone up isn't likely to die the first winter or two, but could - it just depends on how cold it gets each year.