Short answer
Miracle Growth Elixir needs runes totalling exactly 1 red and 5 green. The formula unlocks late, with Fertilizers IV (150 green, 30 blue, after Zombie Vineyard). Laboratory I can brew it: Alchemic Powder + Beeswax or Coal + Pumpkin work. Mix 1 with 8 Peat at the Formulation Bench for Farming Fertilizer III, the +5 Farming bed boost.

Home and garden
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The rune formula
Target: 1 red, 5 green, no blue. Laboratory I or II works.
Two-ingredient mixes: Alchemic Powder + Beeswax, Coal + Pumpkin. Three-ingredient: Berry + Swamp mushroom + Beeswax, or Water + Swamp mushroom + Beeswax.
Each brew uses 1 Alchemy Flask.
Weak, Rich or Miracle?
The three growth elixirs differ only in green: Weak is 1 red + 2 green, Rich is 1 red + 3 green, Miracle is 1 red + 5 green.
Each feeds one tier of fertilizer: Farming Fertilizer I (2 Peat + Weak), II (4 Peat + Rich) and III (8 Peat + Miracle), for +2, +3 and +5 Farming.
If a search for 'miracle growth' brought you here but you only have Fertilizers III, you want the Rich Growth Elixir.

An official build-mode screenshot of an outdoor work area. It does not show a Formulation Bench or garden beds.
View full-size image ↗Official promotional screenshot · © Lazy Bear Games / tinyBuild · Official media kit · Material credits
The unlock
Fertilizers IV costs 150 green and 30 blue tech points and comes after Zombie Vineyard.
It unlocks both the formula and Farming Fertilizer III.
Buying it
The Alchemist sells 10 from trader level 3 at a base 15 silver, and buys it for about 11 silver 25 bronze.
Studying one pays +50 green and +20 blue tech points for 20 Science.
Sources
- communitywikily.gg — Miracle growth elixir
- communitywikily.gg — Miracle growth elixir formula
- communitywikily.gg — Alchemy formulas
Related pages
Official screenshot · © Lazy Bear Games / tinyBuildAutomationGraveyard Keeper 2 Rich Growth Elixir
Official screenshot · © Lazy Bear Games / tinyBuildAutomationGraveyard Keeper 2 Fertilizer
Official screenshot · © Lazy Bear Games / tinyBuildAutomationGraveyard Keeper 2 Zombie Gardening & Gardener Stand
Official screenshot · © Lazy Bear Games / tinyBuildAutomationGraveyard Keeper 2 Alchemy Guide
Official screenshot · © Lazy Bear Games / tinyBuildCraftingGraveyard Keeper 2 Beehive, Honey & Beeswax