nursery stock pines -- timing for development ?

greetings from zone 3a.

i picked up a couple of nursery stock pines last weekend: a mugo ['slowmound' cultivar] and a stone pine [pinus cembra].
both show healthy vigour, but are impossibly bushy for bonsai training --- i'd post pics, but all you'd see are two very dense mounds of green.

i know they need to lose at least half their branches and old growth needles, before i can even think about further styling.
my question is: when is the best time of year to give a nursery stock pine its [first] hard pruning?

as indicated, i'm in zone 3a boreal forest. winters are long and cruel here, but summer makes up for her short run with [often] intense heat and humidity.
we're into the second week of a 28C to 33C heat wave [82F to 91F] with no sign of let up on the near horizon.
i'm guessing this is a bad time to do much of anything to these new charges, but appeal to other members' experience.

can i get away with hard pruning [removing] some branches now?
if not, would autumn, late winter or spring be the advisable season?

thanks

hi,
I personally think if the trees are growing strongly they bleed too much sap if very hard pruned - this makes a real mess when it turns white and sticks to the bark etc . I have summer pruned and wired many pines though when needed, but I cut off the branches leaving a long 3-4" stub - then the sap drips off rather than running down the trunk - even from under cut pastes it seems to seep out. over the next few months the sap crystalises in the stub and it can be cut back or jinned.

I would say if you have no other projects to play with go ahead an play with the pines - garden center material is always great for practice, and can turn into something very good over future years

have fun,

Marcus

I picked up a Mugo 'mops' 2 weeks ago, and gave it a hard trim right away, for the same reasons you mentioned. As mentioned above, cut-paste does not stop the bleeding, and I have some white traces (Which I do not really mind, as the tree is still years away from being anything decent, and will get a number of cleaning runs; A bit of terpentine does wonders if you really want to remove it, be carefull to not saok the tree though).
The tree has already started to backbud ferociously and all weak branches in the middle of the canopy have benefitted & grown stronger.

I would shield the plant for a week or two from the midday sun, and extreme dry winds; I've kept the tree reletively moist.

Note: We have had temps in low 20-ies & very high humidty..

thanks for steadying my nerves & walking me through this.

since my initial post, I have:
a) removed the top nursery soil [about 2 to 3 cm] to expose what there is by way of nebari [mercifully, there was no evidence of that cussed 'upward growth' in the roots]
b) lifted the trees out of their garden centre pots long enough to hack the pots themselves down about 8cm [with a jigsaw].
c) removed some soil and a very few bottom roots from the ball.
d) returned them to the shortened pots, with a cm or two soil on the bottom..
e) removed between 30 to 40% of the existing tangle of branches / folliage --- with nothing like 'design' in mind, just opening matters up.
f) parked them in dappled shade, where they will sit for a few weeks.

come winter, i'll find a sheltered corner & stick the pots into the ground. then [assuming they make it through our often harsh winter] I'll get them into training pots and free draining soil come spring.

turns out the pinus cembra has some lovely bark and impressive taper, much better than i had first anticipated. the needles may be large for bonsai culture, but the trunk makes me deteremined to find out what can be done.

thanks again.