Not really an environmental thing but Hunger of Hadar(1/2 Movement & DOT) + Plant Growth (1/4 Movement) is basically a death trap covering a massive area for anything without Freedom of Movement.
Just be careful of candles otherwise your Plant Growth will turn into a massive fire ground hazard rather than Difficult Terrain.
I haven't added Spike Growth to the mix, but it would be good for the Druid's concentration. For the most part the strategy above has been one of the opening turns for my party's Warlock. Pops Haste potion, drops both spells, and then uses Repelling Blast to knock enemies in/back in on subsequent rounds.
Plant growth is insane area denial for things that can't teleport. Definitely have to be careful with fire spells cause the burning ground is so much less useful than restricting movement.