Absolutely. Can you send me your info?
Is this plausible?
Step-by-Step Breeding Plan for Developing a Zone 5b Cold-Hardy Avocado
Goal: To develop an avocado tree that can thrive in USDA Zone 5b (-15°F to -10°F / -26°C to -23°C) by selectively breeding for cold tolerance and using techniques to speed up growth and flowering.
Year 1–2: Foundation & Initial Seedling Selection
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Gather Cold-Hardy Avocado Varieties
Focus on Mexican avocados (e.g., ‘Del Rio,’ ‘Mexicola,’ ‘Joey,’ ‘Wilma,’ and ‘Aravaipa’).
Acquire seeds or young trees from multiple sources to create genetic diversity. -
Start Seedlings in Warm Conditions
Germinate seeds indoors or in a greenhouse to accelerate growth. Use hydroponics or aeroponics for faster root development.
Grow 100+ seedlings to ensure genetic variation. -
Test Cold Tolerance
Once seedlings are a few months old, begin exposing them to progressively colder temperatures:
First, expose seedlings to 25°F (-4°C) for a few hours.
Gradually lower the temperature every two weeks, aiming for 15°F (-9°C) by the end of Year 1.
Select the survivors based on cold resistance for the next breeding stage.
Year 2–4: Growth Acceleration & Rootstock Selection
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Use Grafting for Early Flowering
Graft the surviving seedlings onto cold-hardy rootstocks like ‘Mexicola’ or ‘Del Rio’ when they reach 6–12 months.
This encourages early flowering in 2-3 years instead of 5-10 years.
Monitor graft success to ensure healthy growth. -
Experiment with Rootstocks
Test different cold-hardy rootstocks like Persea borbonia (Red Bay), Sassafras albidum, or even Persea palustris (Swamp Bay), which can tolerate slightly lower temperatures.
Graft seedlings onto rootstocks and observe growth for the next 2 years. -
Accelerate Growth with Greenhouse or Indoors
Use a heated greenhouse or indoor grow lights to speed up growth rates by maintaining temperatures at 70–75°F (21-24°C).
This helps the trees develop faster, reaching maturity sooner.
Year 4–6: Selection of Cold-Hardy Traits & First Breeding Trials -
Cold Testing of Grafted Trees
Continue testing for cold tolerance in trees grafted onto hardy rootstocks. Test both seedling trees and grafted trees at temperatures around 10°F (-12°C).
Choose the cold-tolerant candidates (the trees that survive and show minimal damage after exposure). -
Start First Breeding Crosses
Cross-pollinate the most cold-hardy trees (survivors from the cold tolerance test) with other cold-hardy varieties like ‘Joey,’ ‘Del Rio,’ ‘Mexicola,’ and ‘Aravaipa.’
You may need to grow trees for several generations to achieve the desired hardiness.
Year 6–8: Hybridization & Genetic Testing -
Growing Hybrid Seedlings
Once you have hybridized trees, grow several hundred seedlings from the cross-pollination efforts.
Test each seedling’s cold tolerance at different stages and document results. -
Grafting for Better Fruit Production
Graft the best cold-tolerant hybrids onto mature rootstocks to see if they produce better fruit in a shorter time. This can also test fruit quality for the long-term breeding program. -
Trial with Biotechnological Approaches (Optional)
If natural hybridization isn’t yielding fast results, consider exploring genetic modification or embryo rescue.
CRISPR Technology might help target specific cold tolerance genes from related species (such as Persea borbonia).
Embryo rescue could help overcome hybridization barriers between avocado and cold-tolerant relatives.
Year 8–12: Refining the Cold-Hardy Traits & Establishing a Marketable Tree -
Continued Cold Testing
Continue growing and testing the cold-hardy hybrids in a controlled cold environment (down to -15°F/-26°C). Select the most robust seedlings that show resistance to cold damage. -
Select for High-Quality Fruit & Hardiness
Test for fruit quality and consistency—focus on fruit size, taste, and yield.
Select hybrids that not only tolerate extreme cold but also produce viable fruit. -
Final Crossbreeding
Continue crossbreeding with any remaining cold-hardy trees that produce the most favorable cold tolerance traits and fruit quality.
Year 12–15: Final Stage & Mass Production -
Establish the Final Cold-Hardy Avocado Cultivar
By Year 12–15, if you’ve been successful, you should have developed a cold-hardy avocado tree that can survive Zone 5b winter temperatures.
Ensure that this tree:
Survives Zone 5b winter conditions without frost or freeze damage.
Produces good-quality fruit (or even marketable fruit). -
Expansion of Trials and Distribution
Begin offering the tree to local gardeners and small-scale farmers in Zone 5b to test its full potential.
Partner with agricultural suppliers or local nurseries for large-scale propagation of the final cultivar.
Protecting Trees Until the Cold-Hardy Avocado is Ready
While breeding is ongoing, here are some ways to protect avocado trees from extreme cold in Zone 5b: -
Use a Microclimate
Plant your trees near south-facing walls to trap heat during the day.
Use thermal mass materials (e.g., large rocks or water barrels) to retain warmth. -
Mulching & Wrapping
Mulch thickly around the base to insulate the roots. Use burlap or foam pipe insulation to wrap the trunk and prevent freezing. -
Cold Frames & Greenhouses
Build a mini greenhouse or cold frame around the trees during winter to keep the temperature in the root zone warmer.
Conclusion: Timeline and Effort Required
Year 1–2: Start with cold-hardy varieties, germinate seeds, and test cold tolerance.
Year 3–4: Accelerate growth using grafting, rootstock selection, and controlled environment.
Year 4–6: Begin the first hybridization trials and cold-testing grafted trees.
Year 6–12: Focus on refining cold tolerance through breeding and testing.
Year 12–15: Achieve a cold-hardy avocado cultivar for Zone 5b.
This project would take 12–15 years, but the success of creating a hardy avocado is possible with a combination of selective breeding, grafting, cold-testing, and potential genetic manipulation.
I don’t mean to sound mean, but this is not within the realm of what’s physically possible.
No. These species are way, way, way too far apart. Avocado do not even hybridize with red bay, which are in the same genus. Sassafras isn’t even in the same clade. Just same family. It’d be like trying to cross corn and bamboo. Or in animals it’d be like trying to breed goats with cows.
It doesn’t struggle, it dies in zone 5.
Again, I don’t want to be harsh, but no, it isn’t.
This will kill all of them.
These are not graft compatible. It’s been tried (well, not with Sassafras, but, again, they’re as closely related as corn and bamboo, so, no, they’re not graft compatible).
Red bay isn’t cold hardy enough, it will be killed by -15 F every time.
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@barkrunner
It is wishful thinking, demonstrating a lack of knowledge regarding avocado botany. For example, there is no need to speed up flowering. Instead, there is a need to produce a crop within the limited number of degree days in your region.
In my opinion, the most realistic goal is a magnolia that produces nuts in USDA zone 5.
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Theoretically, you could use some crazy genetic engineering to transfer a whole bunch of genes for cold hardiness, but by theoretically I mean one could do so in a sci-fi novel.
Actual scientists using state-of-the-art genetic engineering working for Arborgen who are word leaders in GE trees are currently struggling, hard, with cold hardy GE hybrid eucalyptus, and they’re just trying to add about a zone of hardiness, going from zone 9 to zone 8. The world’s experts, a genus that has some species in it that hardy to zone 6, and yet this GE project to bump hybrid eucalyptus up a single zone is likely going to fail. And hybrid eucalyptus is arguably hardier than avocado.
There aren’t even that many broadleaf evergreens in any family that are hardy in zone 5. Ilex, and… that’s about all I can think of.
What’s more, suppose you engineer avocado sap into pure anti-freeze or something and somehow get it to be cold tolerant without killing the tree or anyone who touches it. Ok, the tree will still desiccate and die in the winter. When the ground freezes, roots stop absorbing water. Without a constant flow of water, even in winter, the leaves will pull whatever water is left in the trunk, dry out and die, often taking the tree with them. Regardless of how freeze resistant they are, the ground still freezes. So you’ll have to modify the structure of the leaves themselves, or GE hack deciduousness into a genus with no deciduous members. Hundreds to thousands of genes to change.
Avocado flower and fruit cycles are way, way too long for short season climates. Even in zone 8 it’s hard to get them to flower and ripen in time. There are no super-fast ripening varieties to breed in either, so you’d have to crack the code of fruit maturation and GE a faster flower and fruiting cycle. Good luck, I don’t think that’s ever been done either. We’re talking tens of thousands of genes to modify.
Avocado cellular metabolism is tuned to work in warm temperatures. Keeping the plant too cool for too long will injure or even kill it because it’s metabolism or respiration or hormonal balance will get messed up by sustained low temperatures. There’s a reason why tropical plants eventually fade and die if kept at 40 F even if they never dry out or freeze, and why raspberries and currants and Norway spruce all die if the summer stays in the 90s for months on end, even if they’re watered and in the shade. Plants are adapted to specific temperature ranges, if they spend too long outside those ranges, they die. So, you’d need to GE everything about the plant, from its respiration to its biochemical pathways to it’s mitochondrial activity to its hormonal regulation, etc.
That’s physically impossible.
This is why breeding zone 5 avocado is impossible: because even zone 5 GE avocado is impossible.
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I appreciate the response and discussion. I know this was out there. But sometimes ideas are sparked from conversations like this. Sometimes takes that one person to mention what hasn’t been thought of. I am by no means an avocado expert and do not claim to be. Appreciate all the the avocado enthusiast and those interested in this potential and lovers of avocado!
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You know, for far less money and effort you could grow amazing crops of something regionally appropriate – and likely it won’t produce in my location. 
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I know Richard. Like pawpaw or persimmons. I just love avocados. I eat them everyday. I do appreciate your responses. Maybe someone in the future will see this and be inspired. I am, but I want to know the effort is a sure thing.
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i have a friend in 4b that grows citrus and avocados and bananas in a Underground greenhouse. also another one trying in willow alaska with some success. here is a link to a green house citrus avocado Orchard https://www.youtube.com/watch?v=Q81UF5aWMGo… Please if the link isnt allowable let me know im not sure of the rules on that
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I have several trees from winn and two of my own, all Mexican varieties. I have a greenhouse that stays above 40F year round. these are large pots, not in ground yet. I’m in zone 7a (was 6b).
they live in the greenhouse now but this is the second year of flowering on one and the first flowers on another and if they form any fruit they’ll spend summer outside then come into the house itself to stay warm enough to turn ripe under lights and heat.
the effort of keeping these alive even in my zone, with a warm greenhouse- if you built a warmed greenhouse you could maybe do it.
figs might be more attainable if you’re willing to seriously protect them, warm them a bit, get the right varieties.
this Year’s pair flowering.
last year:
I try to grow all kinds of inappropriate stuff here in that greenhouse. sugar cane, sapodilla, avos, figs etc. I started with a diy hoop house made out of bubble wrap and insulation strips and sheeting! they mostly survived in that too. so I guess I’m saying it’s all about how much effort you want to make to grow a thing. just planting outside? never. with way too much effort? mmmmaybe.
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I ripen Meyer lemons, sudachi and mandarins outdoors in western oregon. Plenty of people do. Only requires covering about 10 nights per winter. There is usually two cold snaps that require covering for 2 days uninterrupted. Greenhouse is not required. It isn’t for the Mexican avocados either. Just slightly more time under the cover than the citrus. I drape plastic from an awning. Very simple. Takes 2 minutes to put on or take off. Christmas lights if we drop below 20 which only happens 2 or 3 nights in an average winter. In recent years that’s become even less common.
This Meyer has been blooming and actively growing since February. That’s standard here. I have yuzu fully uncovered as does my neighbor.
The yuzu
Step back view
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I find Meyer lemons and sudachi very easy. Only covering a handful of nights each winter. Yuzu is almost never protected. Early mandarins in the same boat. My Mexican avocados also thrive with the same protocol as citrus. The amount of time here that is inhospitable to avocados is relatively short. It doesn’t take much to create a 9b microclimate with plastic draped from an awning and if the temps falls below 20 F I activate the christmas lights. That is rare to be below 20. Putting on or taking off the plastic is only 2 minute job, and the lights are automated.
The Meyer grows almost year round here even unprotected most days. Blooms from February thru November. Makes so much fruit, but it does take like 6 months to fully ripen. This tree is 6 feet tall by 4 feet wide. it is currently blooming
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An experienced gardener growing lemon and sudachi with winter protection in Portland OR is very different from a novice gardener dreaming about growing sensitive Citrus varieties outdoors year-round unprotected in zones 6-8 of the PNW.
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This is the first year we have not heated our dome that has our citrus and a lone avocado in.
The citrus still look pretty good, but the avocado looks like heck and we had a mild la Nina winter.
The avocado is a Mexicola and we are in zone 8a.
I will let you know if it recovers. It’s not all dead. It just looks a lot rougher than when I was heating the dome last winter.
I’d like to give a shout out to Moro blood orange and Owari Satsuma Mandarin for shrugging off this unheated winter.
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What were your winter lows? Mexicola and the other cold hardy avocados are good to around 20 but not much lower
If we did get below 20f, I don’t think it would’ve been for long.
I try to pay attention, but I honestly don’t recall the lowest low for this Winter, but we have had it be really cold here before, like 11°, and we had a lantern or two in the dome to keep it warmer.
It’s a struggle; I need to figure something out if I really want to try to get avocados out of this situation.
@resonanteye I love the phrase “inappropriate stuff” because it sounds rebellious and experimental and this is how I try to operate as well. I’ve been going back and forth on potted avocado in our greenhouse and you’ve definitely given me a push to try.
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Which variety is your largest tree?
You are absolutely correct. I didn’t mean to suggest it’s easy. There is a bit of work that goes into the site preparation. At least once it’s established, maintaining it is not difficult or time consuming. That was what I meant to convey.
There is definitely a hobby aspect to this endeavour for me that would not appeal to everyone. When it comes to growing fruit, most people just want to put something in the ground and forget about it. For the most part, those of us on this site are a bit fanatical. lol…
Certain people just want to push limits. When I was a kid, growing up near Borrego Springs, my uncle was committed to growing avocados and mangoes. He battled that interminable sun and heat for decades to make it work. That probably rubbed off on me.
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Tallest is the yuzu. It reached the roof’s gutters last fall so I trimmed it back slightly, trying to encourage a bushier habit. Unfortunately, the yuzu has show a tendency for alternate bearing. Hopefully this year is a bumper crop.
The Meyer is not as tall, only 6 feet, but is much bushier, and so far more prolific than yuzu. I can depend on the Meyer to make lots of fruit.
I’m also trialing a specimen called Lemandarin that has excellent fruit and grows unprotected in Yalta. Crimean Peninsula. The climate of Portland and Yalta are nearly identical, so there is definitely reason for optimism.
The avocados are showing signs of an impressive bloom this spring, and hopefully that translates to fruit. I’ll definitely post an update if any sets. These are the Rincón Valley cultivar from Winn’s trial.