Tropicals in the pacific northwest

I'm in zone 8b near portland and I was wondering what it would take to grow tropicals in a greenhouse here. My inspiration comes from https://www.lowtechmagazine.com/2020/04/fruit-trenches-cultivating-subtropical-plants-in-freezing-temperatures.html. They're talking about growing subtropicals there. I can practically grow subtropicals here without any protection. It rarely although sometimes does his freezing in the winter. Last winter we had a low of 20F for 1 night. If I wanted to grow something like mango, avocado, and coffee in a greenhouse what type of design would I be looking for? Mango's in particular have a habit ( i think ) of flowering in the winter. So frost free nights would be necessary. Citrus would also be nice but I've mostly solved that problem with a combination of seaberries ( orange flavor ), yuzu (lemon/lime flavor) and kumquats (snacks). They're all hardy enough.
In the past I've looked into geodesic, used windows, etc but I'm having a hard time figuring out how much of a temperature boost the greenhouse would provide in the winter. Our winters are mostly cloudy and rainy so the solar gain is likely not that great. I don't need much of a gain here though

Some digging led me up this post: https://permies.com/t/64334/Tropical-fruit-Pacific-Northwest
Verge permaculture also posted this recently: https://www.youtube.com/watch?v=kDJ7E5SdBZ0 which is great. He has the same problem in canada as we do here in the northwest. The nights get cold enough that the plants don't advance quickly during the season.

I know people experimenting with cold hardy mangos but I'm not sure how it's going. Also, Loquats are a thing people are growing. I think you could grow them in the ground if you wanted. They flower in winter. Bananas are also being grown in greenhouses here, too. I've got some cold sensitive citrus in a greenhouse. We'll see how it does, the greenhouse is unheated except for potentially some compost.

Hi James, I actually put a loquat in the ground last year :). It made it through the winter just fine. I don't know about fruit yet because it's tiny but it seems promising.
I did some research into the cold hardy mangos. The consensus seems to be that they'll survive but the flowers won't.

Some plants just will not do well in the shade. A related problem is growing degree days. The pacific northwest doesn't have enough growing degree days, so some plants just will not be able to give you an harvest. Your zone 8b is different from north Las Vegas zone 8b or Texas or panhandle florida.

All that said here is a nursery from the PNW, that offers alot of subtropical/tropical cultivars that will most likely do well in your zone 8b.
https://onegreenworld.com/product/nikita-2/

I agree about the recommendation for one green world, but I will warn you to take their information with a grain of salt, and ask very specific questions about how they keep things alive through the winter. For example, they once told me of a grapefruit they were growing successfully outside. Suspicious, I grilled them about how they did it, and they revealed that they had to use techniques like carefully timed christmas lights and such.

Another thing to note is that a lot of the things they market that are hardy for our area need to be grown for several years in a greenhouse before they're big enough to be hardy for the outdoors. Yuzu and pineapple guava are examples, as is loquat. Though if you're growing in a greenhouse long as you mentioned it's a moot point.

It's surprising how much of a boot even an unheated greenhouse can give in this region, even if it's not that much warmer in winter than outside. A lot of things are just barely out of reach in this region and a greenhouse helps tip the balance. A composting station (for heat) can boost the temperature a lot though.

I built a greenhouse out of lumber and salvage windows with a friend, and it works very well.

Chris Holcombe wrote:

I did some research into the cold hardy mangos. The consensus seems to be that they'll survive but the flowers won't.

Is it that the flowers don't survive, or that they don't set fruit?
For example, I've read that some pepper plants only set fruit when the temperature hits a certain high, but also needs a certain low at night as well. I think that's the problem I'm having with my tomatillos this year. They aren't far from where they were last year, but I planted them earlier and we've had a really hot summer. The plants are a great size, they are covered in flowers, I regularly see at least two types of bees harvesting pollen from them, but hardly any fruit. I'm hoping that now that we're getting cooler fall weather, that will suddenly change. I don't think it's a day length issue, as a friend has had fruit on hers for the last month. Her plant may be getting less west sun than mine. Problems like this can be so hard to figure out!

I'm even further north, but I've been toying with trying that Russian method of only letting a lemon tree grow about 1 ft high and then all lateral from there so it's really low to the ground and using rock mulch. I need to choose the right spot so it doesn't drown in winter or shrivel up in the summer drought.

I haven’t tried growing mangoes here yet so it’s hard for me to say what their issue would be with fruiting.
I agree with the thought that in zone 8b we’re on the edge of so many things 🙂. I’ve overwintered yuzu, loquat and Chilean guava just fine in the ground but I picked my spots carefully. After 6yrs living at this property I’m familiar with the warm micro zones and I plant accordingly. I’ve done the Christmas lights technique with my olive trees when they were first planted. That seemed to work well. The winters since have been pretty mild so I haven’t bothered and the trees are older so presumably more cold tolerant.
I good point was raised about us not having enough solar radiation in the winter to ripen tropicals. I hadn’t considered that. I suppose there’s not much that can be done about that except some strategic use of mirrors to multiply your light 😁

I don't know the reason for the rule, but I've been told that white walls reflecting light onto plants is good, but mirrors in a greenhouse is a no-no - maybe fire risk? or maybe it's too intense for the plants? or maybe they tend to blind people when the sun hits them directly? or maybe it's an urban legend?

Opinions anyone?

Yeah I can see the reasoning there Jay. Plants that evolved to grow under 1 sun might get burned to a crisp under 2 suns worth of light. Especially on cloud free days.

8b isn't that cold. I would expect you to be able to grow certain citrus and sub-tropicals with no problem. I'm about the same zone, I think, maybe 8a, but I'm further south so that helps. Lack of light is a big problem the further north you go particularly with winter flowering plants. I don't like much tropical fruit so it isn't a problem for me but I love citrus. Meyer lemons are cold tolerant enough for you as are many other citrus, just look around. It definitely helps to get them established before a brutal odd winter, like we had in 2019.

Looking into olives, many should work as well. I'm planning for some in my Mediterranean garden. Pomegranates can work as well. Great for the dry summer, cool winter conditions that we have in the PW Northwest.