You won't get the discount for Vine Tree/Samara Tree casting a creature or conjuration or something with the Plant and Vine traits for instance, as the discount only applies to those types of spells when they are cast.
The exception is enchantments. If Fellella or some other source cast a Plant enchantment, you could use the discount from the ring when you revealed the enchantment. The discount for enchantments doesn't care about where the enchantment came from, only that you control it and it has the necessary subtype.
Take a look at Death Ring and compare the wording. Death Ring specifically gives you a discount when your mage or a spawnpoint casts a spell of a certain type. The Druid's Leaf Ring specifies that the discount on casting spells is only when the Druid casts the spell. Having a spawnpoint cast a spell is not the same thing.