The event is not cancelable.

The event does not have a result.

Importing the class

Link to importing-the-class

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.

import crafttweaker.forge.api.event.block.crop.CropGrowEvent;

Extending BlockEvent

Link to extending-blockevent

CropGrowEvent extends BlockEvent. That means all methods available in BlockEvent are also available in CropGrowEvent