Keep in mind I never made any working seed balls my self. This is how I would approach the problem:
It is very dependent on the seeds used but for germination some factors need to be just right.
One of them is moisture that gets provided by coating them with wet clay. A second factor is temperature.
If the balls get dried in a cool/cold place with added windchill it should be enough to stop them from germinating.
Let us know if you got it working and how.
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