As with Humans and Animals you need the recipe Life to make a Plant.
Fire + Earth = Lava
Lava + Earth = Volcano
Earth + Ocean or Sea = Primordial Soup
Volcano + Primordial Soup = Life
The following are the recipes you will need to use to make a Plant.
Nest = Bird + House
Container = Safe + Alchemist
Plant = Nest + Container
Listed below are the recipes using Plants.
Algae = Plant + Water
Ash = Plant + Fire
Cactus = Plant + Sand
Cotton = Plant + Cloud
Dew = Plant + Fog
Farmer = Plant + Human
Flower = Plant + Rainbow
Forest = Plant + Tree
Garden = Plant + Grass or House
Grass = Plant + Earth
Greenhouse = Plant + Glass or Aquarium
Hedge = Plant + Wall
Ivy = Plant + Wall
Moss = Plant + Stone, Rock or Boulder
Oxygen = Plant + Sun
Pollen = Plant + Dust, Air or Wind
Reed = Plant + Pond or Puddle
Rose = Plant + Love or Blade
Seaweed = Plant + Ocean or Sea
Seed = Plant + Pollen
Smoke = Plant + Fire
Sunflower = Plant + Sun
Tobacco = Plant + Smoke or Fire
Tree = Plant + Big or Nest + Container
Vase = Plant + Pottery
Vegetable = Plant + Field or Domestication