Helena,
Thank you very much for starting this topic.
Apart from the specifics, it reminds us to challenge our beliefs and automatic practices from time to time.
As with many things, deep planting has its scope, outside which it does not work as well. But within its scope, it has merit. I think the trick is knowing how to apply the knowledge: appropriate soils, plants and climate/watering/time of year.
If
my big Jacaranda cuttings take root, they will need to be planted deeply - to keep them firm in windy conditions. In our Mediterranean climate, my concern would be that such deeply-planted cuttings would need regular deep watering - because it takes some while for the winter rains to penetrate deep into the soil - and I would not want the deeper roots to die and cause the bottom of the stem to die as well, since that would defeat the purpose. That is manageable, by using a perforated watering tube as deep as the cutting; it just needs that bit of consideration.
'Watering tube' explanation:
I plant all my trees with a near-vertical 50mm diameter rigid plastic tube going down to the base of the roots. I drill small holes along the lower half of the pipe, to allow the water better access to the soil. An irrigation outlet goes in a small hole near the top, and the top is capped to exclude creatures and debris and to reduce evaporation. The tube can also serve as a place to apply fertilizer and insecticide. By perforating only the lower half, weeds get less benefit. When measuring the tube for cutting to length, allow at least 30cm to protrude from the soil, otherwise the top can get lost in groundcover/weeds. Eventually, when the tree is well established, the pipe can be pulled out with a twisting action (severing any small roots that grew through the small perforations), and the hole filled with soil.
(Paraphrased from my original comment on 'deep planting'
here.)
I would be very interested to hear others' opinions in principle and from experience.
Helena and I appreciate that experiments in this realm can take quite a while (years) to do.