Anne, if you're growing one of the extremely vigorous rootstocks with inedible fruits, like Maxifort, I would recommend making the cut below the cotyledons, to lessen the chances of the rootstock throwing a shoot. A couple years ago, I intentionally topped the Maxifort rootstocks and let both axillary shoots grow so that I could graft different varieties onto each and have two different scions on each plant. I grow in cages, so when the scions filled the cages, having the rootstock grow a sucker and sneakily send it up through the cage before I caught on was a royal pain. The rootstock suckers grew so quickly (think Jack-and-the-beanstalk) that they outcompeted the scion plants easily, but once they're all woven together it's hard to get rid of them. Luckily, the Maxifort has recognizably different leaves or I would have let them grow, thinking they were going to produce edible fruit.
Last year I grafted a single scion per rootstock, making the cut below the cotyledons. Far easier to manage!
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Bitterwort
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