Zone Pushing - What to grow in unheated greenhouse in 8a

[All the fruit]

Hi everybody,

my friends have a huge greenhose (about 20 000 sq feet) every hobby gardeners dream. Single glass, on an open field near Heidelberg/Germany (teaditionally USDA zone 7b, now 8a. They like exotic fruits and with a little help from me they are starting a non herdy fruit collection. We already have Musa vasjoo, sikkimensis, Musella lasiocarpa, Ensete maurelii, Yuzu, Ichang Papeda, some surviving citrus rootstocks and seedlings, pomegrenades...Wondering if strawberry guava, mountain papaya or hardy tamarillo will grow there

My questions:1:  What else can we grow there? Im especially interested in the super rare and tasty fruits
2: How much does such a greenhouse help?Is it like 8b or even 9b?
3: Any specific care tips for those plants in such a place

[drymifolia]

How much does such a greenhouse help?Is it like 8b or even 9b?

Unfortunately, if you aren't heating it at all then it provides very little protection against overnight low temperatures, but the best way to know for sure is to install temperature sensors inside and outside, and compare them just before dawn.

You will likely find that by the end of the night, the greenhouse is (at best) one or two degrees warmer than the outside temperature.

During the day, it can be dozens of degrees warmer, but once the sun is gone, the extra trapped heat will generally all escape within a few hours.

As an example, here's the ∆°C chart (i.e., how much warmer it is compared to the outside) for my greenhouse today since midnight (currently just after 2pm in my time zone). Overnight it was about 2°C warmer, and has about 150 watts of seedling heating pads and about 100 watts of LED grow lights currently turned on. It would be about half that otherwise.



The 8am anomaly is due to the morning sun hitting the outdoor sensor on the outside north wall of the greenhouse and creating a false reading a few degrees warmer than the air temperature. The sudden drop before 11am was the exhaust fan turning on, and that's still running now. Without that, it would probably be 25+ degrees warmer inside (and my plants would be cooked).

[All the fruit]

Thank you. Those are a lot of valuable insights

[Nick C]

You'd be good with hardy tamarillo for sure. I have one in ground under a poly tunnel in 7a

[All the fruit]

You'd be good with hardy tamarillo for sure. I have one in ground under a poly tunnel in 7a

No heating? Also where in zone 7a?

[Nick C]

You'd be good with hardy tamarillo for sure. I have one in ground under a poly tunnel in 7a

No heating? Also where in zone 7a?

Just some frost cloth and black garbage can filled with water, no heating. I'm in New Jersey

[Plantinyum]

How much does such a greenhouse help?Is it like 8b or even 9b?

Unfortunately, if you aren't heating it at all then it provides very little protection against overnight low temperatures, but the best way to know for sure is to install temperature sensors inside and outside, and compare them just before dawn.

You will likely find that by the end of the night, the greenhouse is (at best) one or two degrees warmer than the outside temperature.

During the day, it can be dozens of degrees warmer, but once the sun is gone, the extra trapped heat will generally all escape within a few hours.

As an example, here's the ∆°C chart (i.e., how much warmer it is compared to the outside) for my greenhouse today since midnight (currently just after 2pm in my time zone). Overnight it was about 2°C warmer, and has about 150 watts of seedling heating pads and about 100 watts of LED grow lights currently turned on. It would be about half that otherwise.



The 8am anomaly is due to the morning sun hitting the outdoor sensor on the outside north wall of the greenhouse and creating a false reading a few degrees warmer than the air temperature. The sudden drop before 11am was the exhaust fan turning on, and that's still running now. Without that, it would probably be 25+ degrees warmer inside (and my plants would be cooked).

Wright after i build mine, i monitored the temperature differences between inside and outside, trough the first winter, before i planted my tropicals. The result was that i had a stable 4 degree C positive difference on the inside. Sometimes it was les, sometimes it was more, but 90% of the time the difference was 4 degrees. Thats without heating, and now after it is heated, the gh equals to zone 10- 11, and i am in zone 7.
For a passive greenhouse, the place and how it is build is also important, mine had such a high temp difference, becouse it is on the south side of a building, also i have solid concreete surounding base.

Hi everybody,

my friends have a huge greenhose (about 20 000 sq feet) every hobby gardeners dream. Single glass, on an open field near Heidelberg/Germany (teaditionally USDA zone 7b, now 8a. They like exotic fruits and with a little help from me they are starting a non herdy fruit collection. We already have Musa vasjoo, sikkimensis, Musella lasiocarpa, Ensete maurelii, Yuzu, Ichang Papeda, some surviving citrus rootstocks and seedlings, pomegrenades...Wondering if strawberry guava, mountain papaya or hardy tamarillo will grow there

My questions:1:  What else can we grow there? Im especially interested in the super rare and tasty fruits
2: How much does such a greenhouse help?Is it like 8b or even 9b?
3: Any specific care tips for those plants in such a place

i suggest you to monitor the environment for a winter, just to know what your lowest temperatures are, meanwhile you can plant the more hardy stuff. Mountain papaya is supposedly hardier than regular papaya, i have 3 that are doing wonderful and also setting fruit atm. But i also have 5 regular ones, both are grown in the same greenhouse, i cannot tell you how much hardier mauntain papaya are, as i keep the gh very warm trough winter as i have tender stuff, like the regular papaya.

[drymifolia]

For a passive greenhouse, the place and how it is build is also important, mine had such a high temp difference, becouse it is on the south side of a building, also i have solid concreete surounding base.

I don't think the concrete base helps much, concrete and soil have similar heat-retaining capacity. Being attached to a (presumably) heated building is why yours maintains such an unusually high ∆ vs outside. Even before you added heat inside, your greenhouse is heated, by the waste heat escaping from that building.

The OP already said this:

a huge greenhose (about 20 000 sq feet) every hobby gardeners dream. Single glass, on an open field near Heidelberg/Germany (teaditionally USDA zone 7b, now 8a.

Single pane glass in an open field probably has an R value low enough that it reaches equilibrium with the outside temperature within an hour or so, but maybe it lags the outside low by that much time, so it might be about a degree (Celsius) warmer at dawn, if the outside temperature is continuously falling until dawn. If the outside temperature goes flat, the greenhouse will probably match it to within a fraction of a degree.

But I agree, sensors will answer the question quickly. To give a better example than the one I posted above, here's a pretty typical stretch of winter days here in Seattle, in my detached greenhouse that has twin wall 8mm polycarbonate glazing (a much better R value than single pane glass):





The first day was thin clouds, the next day was partly sunny, then it rained for a few days and you can see on those days with very little sun, the ∆ falls to zero.

[All the fruit]

Pity. I thought a greenhouse of any kind would make much more difference. But i hope they can increase the effect by keeping the inside dryer in winter and do a lot of passive heat retention and insulation.

Where near Sofia do you live. During my childhood in Sofia such exotic plants would have been impossible to obtain.

[drymifolia]

Pity. I thought a greenhouse of any kind would make much more difference. But i hope they can increase the effect by keeping the inside dryer in winter and do a lot of passive heat retention and insulation.

Is there no way to add heat? A wood stove to use during unusually cold weather, at least? That's a very large greenhouse to heat with electric heaters, so even if you have electrical hookups that would likely be too expensive.

[Plantinyum]

Pity. I thought a greenhouse of any kind would make much more difference. But i hope they can increase the effect by keeping the inside dryer in winter and do a lot of passive heat retention and insulation.

Where near Sofia do you live. During my childhood in Sofia such exotic plants would have been impossible to obtain.

about 30 km from sofia, in a mountainous area, 830 m above sea level. Well there are sellers in our country, who happen to cary exotic stuff in summer, also there are the some people, who grow such species and sell them, mostly seed germinated plants, once in a while i do happen to find a species i have been searching for a long time.
Yes those tipes of plants do not grow outside anywhere in BG , you have to go to the botanical gardens or people's personal collections, to see them.

Drymifolia, my gh base has insulation on the concreete , on both sides, that does help alot. It is also not atached to the house, but rather one meter from the wall. But yes , the place where it is located is somewhat wind protected, and is kinda a sheltered location overall.

I would not reccomed wood stoves for greenhouses, too much maintenance and also could be a fire hazard. I was  using one the first winter,  the chimney catched fire a few times, i had to monitor the fire closely each evening, resulting in me staying regularly up to 1- 2 am, in order to be sure the fire wont die off, a single mistace is all it takes for a collection to be lost. BUT i did have a very small gh and also plants closeby enough to be fried from the heat, i quess in a much bigger gh, the wood stove wouldn't be such a pain...

[All the fruit]

Thank you all very much for the professional advice. Also Platinium, can i visit your collection when i go home some of those years?

Also i would be happy about more suggestions what they can plant there that does not grow outdoors in Germany

[CarolinaZone]

Zone 8...almost any thing. You only need to stay above 50 degrees F and almost any thing will survive. Not thrive but survive until spring. Citrus will work. White Sapote will work. Sapodilla will work. Mamey will work. Custard apple will work. Abiu will work. Babados cherry will work. You have to seal it well and you have to make sure you have enough compost stored inside.

[drymifolia]

Zone 8...almost any thing. You only need to stay above 50 degrees F and almost any thing will survive. Not thrive but survive until spring. Citrus will work. White Sapote will work. Sapodilla will work. Mamey will work. Custard apple will work. Abiu will work. Babados cherry will work. You have to seal it well and you have to make sure you have enough compost stored inside.

It would take an impossible amount of compost to heat a 20,000 sq ft glass single-pane greenhouse above 50°F in 8a Germany. I mean by all means start by putting in a large compost pile and add a temperature sensor, but Germany is more like Seattle than an east coast 8a in North America: cold and overcast for much of the winter.  When it's overcast and the outside high is in the low 40s°F with freezing lows every night for months on end, there's no way compost will work to heat it that much.

Look at the average low and high in winter, it's not like the zone 8 in the Carolinas:

[CarolinaZone]

Zone 8...almost any thing. You only need to stay above 50 degrees F and almost any thing will survive. Not thrive but survive until spring. Citrus will work. White Sapote will work. Sapodilla will work. Mamey will work. Custard apple will work. Abiu will work. Babados cherry will work. You have to seal it well and you have to make sure you have enough compost stored inside.

You might be right. My thought was definitely bases on my Carolina experience and the fact that 32 at night  but 40 to 50 in the day. I can get well into the 70's in my hoop house on a 40 degree day. The temp inside may get into the 30's for a short amount of time but most of my plants have withstood it.
It would take an impossible amount of compost to heat a 20,000 sq ft glass single-pane greenhouse above 50°F in 8a Germany. I mean by all means start by putting in a large compost pile and add a temperature sensor, but Germany is more like Seattle than an east coast 8a in North America: cold and overcast for much of the winter.  When it's overcast and the outside high is in the low 40s°F with freezing lows every night for months on end, there's no way compost will work to heat it that much.

Look at the average low and high in winter, it's not like the zone 8 in the Carolinas:

[All the fruit]

I might have an almost unlimited supply of compost for them but somebody needs to shovel those tons and tons of stinky gunk into the greenhouse and they dont look too game 😂
Also i was told to be careful since large piles can spontaneously combust

[Solko]

In my experience an unheated greenhouse helps a lot. But the way it helps is not easily translated in a few extra degrees. For example,I have two big trees of a type of Strawberry Guava (Psidium Longipetiolatum) here in Rotterdam. I had one inside my unheated greenhouse, and one outside the last winter with a full week of night temperatures of -6, -7 C in December. The minimum temperature at night was almost the same inside the greenhouse as outside. Maybe it was 1 or two degrees warmer inside the greenhouse. But that little extra protection against wind, against radiation and the fact that the air heats up in the daytime as soon as the sun comes out made all the difference. One tree is completely undamaged (the one inside the unheated greenhouse) and the other tree is completely defoliated and died back to the main trunk. It will take 3 to 4 years to grow that one back. So the protection inside the unheated greenhouse in the winter makes a lot of difference, but is difficult to translate in degrees.
Over the last ten years I have tried to grow a lot of subtropical fruits from the Myrtle family like strawberry guava, Uvaia, Ubajay, Pitanga, Cerella, and also citrus and avocado in my small unheated greenhouse and I even made some selections from the most successful experiments that I think are hardy enough to grow in an unheated greenhouse in zone 8 in Western Europe. You can check them out on www.bontegout.com/seeds-shop
Good luck!
Solko

[lajos93]

If you place a bunch of water barrels painted black you can plant frost sensitive (or ones that can handle until -5C) plants near to the barrels,

if you had made it sunken that would even give it better protection but thats another story

As for the night lows.. its true that its almost just as cold at night as it is outside, but its for a few hours shorter period, which also helps the sensitive plants

It takes quite a bit of experimentation what you can push, I mean many years.. but I would easily start with citrus, there are many tasty but hardy citrus that can take down to zone 8A, or 8B with ease

[All the fruit]

Thank you, this is good info

[Plantinyum]

Thank you all very much for the professional advice. Also Platinium, can i visit your collection when i go home some of those years?

Also i would be happy about more suggestions what they can plant there that does not grow outdoors in Germany

Yeah , sure, havent been active on the forum lately , thus the late response. Love your youtube channel and videos, especially the ones from tropical countries!

[Rob From Sydney]

There's been a recent hardiness topic, it's worth reading.
https://tropicalfruitforum.com/index.php?topic=56805.msg536766#msg536766