The Basics of Alchemy
[BONUS]
Whereas some items could be crafted with dried or preserved components, some potions or poisons required fresh-cut plants for true potency. Once gathered, plants and other organic ingredients wouldn't retain their freshness for long.
And oftentimes, it depends on their rarity like common plants would last up to 10 days. Uncommon to 7 days. Rare to 5 days and Legendary to 3 days or less.
Ren only wanted to craft the highest quality, so maintaining their freshness was a must.
In order to preserve their properties, many plants could be dried, pickled, and preserve in any form during periods of downtime and rest. Another option was to grow them in a sponge or container first and then transfer them to a home to grow for future use.
The latter was an appealing option for Ren, so he didn't have to go to certain places just to acquire these plants again. He could simply grow them in his workshop.