The event is not cancelable.

The event has a result.

If result is set to default, the default vanilla growth mechanics will run.

If result is set to allow, it will force the plant to advance a growth stage.

If result is set to deny, it will prevent the plant from advancing a growth stage

It might be required for you to import the package if you encounter any issues (like casting an Array), so better be safe than sorry and add the import at the very top of the file.

ZenScript
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import crafttweaker.api.event.block.MCCropGrowPreEvent;

MCCropGrowPreEvent extends MCCropGrowEvent. That means all methods available in MCCropGrowEvent are also available in MCCropGrowPreEvent