I use Superthrive too to soak my bare roots. A tablespoon or so per bucket but I combine it with Liquid Karma. If I'm out of Liquid Karma I just use Kelp solution,
Yesterday, I planted 11 bare root roses. 9 from David Austin and 2 monster roses from EuroDesert, English Miss and Star of the Nile.
Star of the Nile is one of those in my 'must have list'. I was not expecting EuroD to have any left and I could have settled for a smaller plant, but I was pleasantly surprised (and lucky) when they sent me a freshly dug, matured plant (red bucket in photo).
Edited to add - Toni - don't discard that soaking solution with superthrive. Use it to water your roses after you planted them.
