When a spell is cast, it is understood that there must be an obstruction-free line of effect from the point of origin of an area of effect spell for it to affect a location (PHB 204):
A spell’s effect expands in straight lines from the point of origin. If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn’t included in the spell’s area.
For a persistent area of effect spell, is this point of origin only relevant on casting of the spell, or is the spell constantly emanating from that point of origin for the entire duration?
Entangle spell description (PHB 238):
Grasping weeds and vines sprout from the ground in a 20-foot square starting from a point within range. For the duration, these plants turn the ground in the area into difficult terrain.
A creature in the area when you cast the spell must succeed on a Strength saving throw or be restrained by the entangling plants until the spell ends. A creature restrained by the plants can use its action to make a Strength check against your spell save DC. On a success, it frees itself.
When the spell ends, the conjured plants wilt away.
For example, the entangle spell (text above), creates a 20-ft square area from the point of origin. Say entangle is cast in the middle of an empty room with no obstructions, then afterwards, a wizard casts wall of stone to cut the affected area in half. Do the weeds and vines that are no longer in line-of-effect from the point of origin end immediately, or do they continue to last for the duration?
Could I throw a blanket over the point of origin to essentially block the entire spell?