Portal Knights Wiki
Island Crops that can grow in it Merchant Location WIP
Vacant Grassland Island Emerald Tree, Spring Tree, Cotton, Alabaster Bush, Wheat, Garden Mushroom Fort Finch
Vacant Forest Island Cotton, Alabaster Bush, Wheat, Dark Fir, Garden Mushroom Fort Finch
Vacant Dry Desert Island Desert Fruit, Cactus Fort Finch
Vacant Oasis Desert Island Desert Fruit, Cactus, Sand Palm Fort Finch
Vacant Fairy Forest Island Pumpkin, Carrot, Ruby Tree, Gold Glimmer Tree, Alabaster Mushroom Fort Finch
Vacant Toxic Island Amber Bell, Big Bloomer, Swamp Shroom Mount Meridian
Vacant Swamp Island Amber Bell, Big Bloomer, Swamp Shroom, Ghost Gingko Mount Meridian
Vacant Coastal Island Aloe, Fruit Palm, Beach Palm, Palm, Sweet Potato Mount Meridian
Vacant Tropical Island Sweet Potato, Aloe, Palm, Fruit Palm Mount Meridian
Vacant Polar Island Frost Lily, Fir, Bitter Oak, Silk Cocoon Mount Meridian
Vacant Crystal Island Wonder Crystal, Flying Wonder Crystal, Magenta Berry Bush, Corundum Great Frontier
Vacant Hollow Lands Island Wondrous Geode, Magenta Berry Bush, Cluster Crystal Great Frontier
Vacant Volcano Island Blazing Palm, Peak Hip, Blazing Chili Great Frontier
Vacant Red Planet Island Jelly Belly Bush, Cobalt Leek Great Frontier
Vacant Asteroid Island Starlight Tube, Cosmic Mushroom Great Frontier
Vacant Lunar Elf Island Birch Tree Seed, Ginkgo Tree Seed, Garden Mushroom Seed Farran-Enore Main Quest
Rainbow Island probably anything DLC

There's also a "Title Deed of Vacant Lunar Furfolk Island" that is unlocked after completing all 3 portal missions and can be bought from Wildhowl for 40,000 defender coins.