Right now there's no benefit to reaching the maximum level in gardening. The highest spells you can train unlock at rank 15. I feel that the status of Verdant Gardener deserves a little recognition.
Enter the Magic Reagent Spawner castle magic item.
Purchasable for gold but from a new gardener NPC (Carly?) located in Novus. (Insert hand-wavy backstory about asking Dasein for the ability to grow reagents directly.) These items are outside only, limit 3 per house, maybe 9 per character to prevent abuse.
Carly also sells recipes for castle magic treasure cards such as "Spawn Frost Flower", "Spawn Black Lotus", etc. One recipe for each of the basic reagents we find growing around the Spiral.
These recipes take 100 of the related reagent to craft, and are only purchasable by rank 20 gardeners.
The Magic Reagent Spawner accepts one of these cards and spawns the reagent in that location however often reagents normally spawn. (I had assumed it was something like 12 hours, just read it's only 30 mins)
For house visits, these reagents act like any other plant, decorative and only harvestable by the owner.
This idea could be expanded to include other base reagents with an NPC jeweler/miner for Stone Block, Sandstone, Fulgurite, etc.
Enter the 