How hot is to hot for JBP in a Green House?

I’ve been growing JBPs in a greenhouse since the start of the year to give them a head start in my climate and was wondering how hot is too hot for them before they will suffer? yesterday it got to 50 degrees celsius (or 122 Fahrenheit) and 95% humidity. They all seemed fine. they get direct sun from 11:30am till sunset around 9pm

I am surprised they are still alive, must be the humidity. I cannot say what the extreme temperature tolerance is but suspected it is much lower than that if the humidity drops. I shift seedlings outside from the greenhouse as soon as the risk of frost has passed and start adjusting them to the outdoors.

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they are mostly older plants 4/7 year old on pond baskets and a few older ones in bonsai pots

My mistake. I did not notice any pictures on either of the websites you posted this question so i just assumed that the only time JBP would be in the green house at this time of year would be seedlings. But you did say you were trying to give them a head start!
Where are you located?

If you’ve let the pines get to 122F you’re likely in a better position to answer the question than anyone!

I’d be interested to know how long they can be kept inside a greenhouse, and whether the growth produced in a greenhouse is preferable to the growth produced outdoors.

When I said head start i meant a head start in their current growing season. I live in London and the weather (humidity/temperatures and sun light) isn’t as good most other climates I have a 10x25 foot poly tunnel I keep my JBP and other trees in over the winter then I’ve just left the JBPs in there as its alot warmer and humidity is very high… they seem to have grown quite well in there so far tbh… I’m thinking Ill have to bring them out at some stage so was trying to get an idea of when that would be.

The growth is better this year than last year I think but I have kept all my JBP’s inside the poly tunnel so haven’t got anything to compare them to… next year I will put one or two outside and see if there is any difference.

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As you mentioned, one approach is to bring the trees out at different times and see if you notice any difference. In general it’s best to bring out the trees when it starts getting warm out - a challenge when spring remains cloudy and cool. If the trees don’t come out early enough, there can be a risk that the foliage will burn when you do bring it out, though cool summers might make it OK to bring the trees out on the late side.

It sounds like some experimentation will be the best guide considering your moderate climate.

Here are some comparison pics:

spring last year (candles quite yellow and not as long):

Same plants yesterday after being repoted early spring (Alot greener and I think longer extensions):
IMG_2457 IMG_2452 IMG_2458

And a few bigger trees:

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Thanks for the pics - do you have an idea about why the candles were so yellow?

I guessing it was due to them being in pond baskets over winter and not getting sufficient weather protection also the pond baskets I use have a flat bottom and the water seemed to pool under the pot so they were prob staying too wet. I now tilt one side up with a bit of wood to help drainage

Ah, that sounds about right - too much water can turn the base of needles yellow, and apparently the whole candles yellow in some cases.

The longer extensions make sense as shoots will extend with less sunlight. This may be OK for young stock but the goal is shorter internodes for the more mature trees - possibly good candidates for removing from the greenhouse earlier.

Is summer warm enough for you to decandle every year or do you do this every other year?

Good point about shorter internodes, I may look to move the more mature trees out sooner. The younger trees with extensions are sacrifice branches so I don’t mind longer extensions on them. I’ve made sure they all have tight buds lower down I can cut back to in a few years.

I have spoken to a few more experienced people with JBP’s I really like about decandling and what I hear most is that its not a good idea to decandle them every year in my climate… every other year at best and only if the tree is healthy… some people don’t decandle at all and treat them as single flush pines. I haven’t had enough time to experiment much so can only go off what I have been told by locals. I have decandled some of my trees over the years but cut them alot earlier in the season than you would.

It sounds like you’re on top of things - will look forward to seeing how your pines develop!