To be honest scribble - I don't think it matters one jot. Willows are absolutely AMAZING and get going anywhere.
Personally, I would take a length - about 1m long and a centimetre thick at the base - of this year's growth and stick it in the ground with the buds pointing upwards in January. You just need a proddy stick (we use an earthing pole from the electric fencing) to make a guide hole and then just shove it in the ground.
I started with 25 of five different varieties two years ago and I now have three times that.
I have had 100% take rates on all verieties except for a couple of viminalis stems that the rabbits barked whilst we were waiting for more tree guards to come in the post last winter.
I plant mine through mypex to keep the weeds down but after a couple of years the top cover is such that most weeds won't grow underneath. I plant them 50cm apart. Will send pic once I get home this weekend.
It's very rewarding growing willow. I plan to use mine to make bentwood furniture and hurdles plus some basket weaving.
Have fun