I've only made a few rose arrows but what I'd do(I do this with everything, cane or hardwood shoots) bundle them together with rubber bands. I like rubber bands because as the shoots dry they shrink and the rubber bands keep the bundle tight. I think this helps to keep the shoots straight. I also make sure the shoots are all lined up straight in the bundle too to keep everything straight. I usually place them in our utility room next to the hot water heater for at least 2 months. Ideally, a year or more is better but usually 2 months will give you workable shoots.