Thanks everyone for all the great info / ideas. I do agree with
california that the spades on the wheel idea could be a bit hit and miss, and ultimately take longer than using the excavator or auger.
Woodland Farms video shows just the sort of thing I'm thinking of... guess it is a difference in translation, as looking at that vid I'd say some of those roots were at least 20cm diameter, so not much larger than what I'm thinking.
Now for the post / tree auger debate.... :stirthepot:
I'm not disagreeing that post / tree borers are used in europe - but it is definately not common practice here on commercial farms. I have lived and worked in agriculture all my life, laterly here in Spain but also in the UK and France (where my best friend now farms) and personally have never seen a field planted using that method - although I conceed smaller farmers may well resort to that method on occasion.
It's pure maths to work out that the time taken with anything which has to stop to form each hole is longer than a constantly moving tractor, and given that agricultural incomes are increasingly under a lot of pressure and margins the tightest they've ever been - time & productivity is very much key, hence why farmers here prefer a constantly moving machine with minimal labour.
I have seen fields in the UK planted with christmas trees and more recently here in Spain with vines using an attachment like the one in your video - in both cases no additional labour was needed other than the driver and planter riding behind. As the Olive & Almond trees are not too large, I hope that at most I'll only need one person following - although I will also have a crew installing the irrigation, so guess they can help if needed.
As said in my OP - what I'm really wanting is ideas / suggestions on the best and cheapest way to build a planter. Ultimately if the cost is too high I will use our excavator, but I'm hoping this could be an opportunity to build a piece of kit that can be hired out onto future jobs as well.
So keep the ideas coming please

..... maybe someone will have a simple (but rapid) solution?
:thumbsup: