definitions/features/fern_double_plant_upper_feature.json

{
  "format_version": "1.13.0",
  "minecraft:single_block_feature": {
    "description": {
      "identifier": "minecraft:fern_double_plant_upper_feature"
    },
    "places_block": {
      "name": "minecraft:large_fern",
      "states": {
        "upper_block_bit": true
      }
    },
    "enforce_survivability_rules": false,
    "enforce_placement_rules": false,
    "may_replace": [
      "minecraft:air"
    ]
  }
}