How to get, grow and ride a Happy Ghast in Minecraft
It avoids Cactus, Sweet Berry Bushes, Wither Roses, Magma Blocks, fire and Pointed Dripstone.

The Wither Rose appears where the Wither kills a mob and withers anyone who touches it, except the Wither and wither skeletons, grants Wither in stew and gives Black Dye. Like every small flower, it gives a dye and flavors suspicious stew. All small flowers work the same; the Oxeye Daisy page explains them.
It can be composted: each one has a 65% chance to add a layer to a Composter.
This item has no crafting recipe.
It avoids Cactus, Sweet Berry Bushes, Wither Roses, Magma Blocks, fire and Pointed Dripstone.
Mining the block drops it.
It is a decorative flower that gives a dye and a suspicious stew effect. It is an ingredient in 2 recipes, made at the Crafting Table; for example Black Dye and Suspicious Stew. Effects: Wither (Suspicious Stew · 7 s). Animals that eat it: Bee.
Yes. Each Wither Rose put in a Composter has a 65% chance to add a layer (the first layer in an empty Composter always works). After 7 layers the Composter gives Bone Meal.
No: when a piston pushes it, it breaks and drops as an item.
Yes: fire can spread to it and destroy it.