The tape serves to add a little strength to the join before the wood has callused over and provided a strong union, but perhaps more importantly it also keeps the graft from drying out before the join has healed sufficiently. Take the tape off too early and you could be jeopardising this healing process resulting in a failed graft, but leave it on too late and the tape may restrict the swelling of the wood which will form the trunk of your infant tree.
I'd been keeping a watchful eye on our grafts for some time now, and thought all was well. The two dessert apple varieties I grafted under tuition had already expanded beyond the confines of the rapidly degrading tape, but on closer examination I discovered one of the Dabinett's had become quite restricted by the freezer bag tape I'd used. I decided it was time to remove the tape from all of the trees and hope for the best.
As you can see, the grafts are all looking good, and they seem strong enough. I'll be keeping all these trees in a sheltered spot for the rest of the Summer, just in case of high winds, and they'll also need staking when I plant them out in the Winter. The restriction on the Dabinett is quite pronounced, though I'm sure the tree will recover. The tape we used on the grafting course seems to have done a better job, degrading as the union swells from the callousing. Freezer bag tape needs to be removed earlier, perhaps even as early as late May or early June, as it doesn't seem to stretch in the way I thought it would.
The more careful examination of these trees also helped me to spot a small patch of Woolly Aphid tucked into a leaf axil. Ray & Gail of Torkard Cider have had these for years, and I was starting to feel a bit left out, so I'm pleased to say we now appear to have the full set of Aphid pests to contend with, and something new to bitch about when next we meet.