I've tied my sunflowers too tight!

Hello, I tied my sunflowers into bamboo canes as baby sunflowers. They are now over 6ft tall and as the trunk grew the string has cut into them. In some cases it has gone right through the chunky stem, almost cutting it in two. Is there anything I can do? Plant tape? Masking tape? Have I lost them? I'm so annoyed with myself!
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