Card Name: Fertile Soil

Type: World Enchant

Casting Requirements: 3 Sylvan Domain, 5 Flux points

Limitation: Lifespan (5)

Effect Description:

At the beginning of each round, casting player looses 1 Flux point. When Fertile Soil is removed, all Seedling have 1 - (remaining lifespan / 5) chance to become Sapling.

Caveats:

When Fertile Soil is removed on its own, all Seedlings become Saplings.
When Fertile Soil is removed prematurely, i.e. on turn 3/5, all Seedlings have 60% chance to become Saplings.
All Seedlings are affected, not just the caster's.

This can be seen as the Sylvan response to the Dark Legion's "All Hallows Eve" card.

For that casting cost and domain you could almost make the Seedlings all pop up as Saplings immediately. With 3 Seedlings on the map that's only a gain of 1 flux and costs a card, so it's like having another Sapling but it requires 3 domains. Seedburst can give you potentially 7 Seedlings, but a lot of the time one or more of those are in water or covered by another figure. If we assume 6 Saplings spawn, the cost is 5 vs 12 for popping them up individually, an effective gain of 7 flux...which is a lot, but that flux can only be used on 1/1/3/3/0 creatures. At 5 Saplings it's comparable to Gather Spirits, but costs 3 domain instead of 1.

I'd say make it 7 casting cost and then have all the seedlings on the map become Saplings instantly.