Take a look at 5 Unity packs with asset collections and tools that would help improve and speed up your vegetation workflow. 

Oak Forest Environment

Grab a collection of assets for oak forest scene creation. The pack also includes HQ models vegetation assets.

HQ Models Vegetation:

• 5 Oak Trees
• 1 Bush
• 5 Plant Bush
• 1 OakForestStump
• 3 Grass Flowers Textures
• 3 Ground Textures
• 1 Demo Scene Game Ready 1X1km
• 1 Post-processing profile: OakForestHQ

Vegetation:

• 12 OakForestTrees
• 16 Bushes
• 3 GrassDetail
• 6 Ground Textures
• 4 Grass Flowers Textures
• 2 Demo Scenes Game Ready 2X2km
• 1 Post-processing profile

European Vegetation Pack Two

Check out an updated version of the European environment pack.

This package contains:

• 9 BirchTrees
• 13 OakForestTrees
• 4 AspenTrees
• 2 CurvedTrees
• 2 OakTreeDryBig
• 2 DryTreeSwampGroup
• 16 Bushes
• 2 SwampLily Pink&White
• 3 Swamp Sedge Green
• 3 marsh bulrush Green&Dry
• 3 Ferns
• 2 BurdockMarsh
• 25 Grass&FlowersDetail
• 4 RockForest
• 9 GroundTextures
• 5 GrassTextures
• 2 RoadTextures
• 2 SkyBox

Enviro - Sky and Weather

Get a system for easy sky production and weather simulation.

System's features:

• Profile system for tweaking skies 
• Day-night cycle 
• Skybox 
• Lightning 
• Seasons
• Clouds
• Fog 
• Weather 
• Volumetric lighting 
• Aurora Effect
• Extended Reflection Probe
• Scene View Effects
• Export to HDR Cubemap
• Networking 
• VR

Winter Forest Environment

Check out a cool pack for creating a realistic snowy scene.

• Vegetation Studio Pro support (for Unity 2018.2.11 or newer)
• 29 tree models: 7 birches, 8 spruces, 9 pines, 2 generic deciduous trees, and 3 large bushes with snow and no snow variants, with multiple LOD levels and billboards for the final stage
• Separate tree prefabs optimized for Vegetation Studio Pro with support for indirect instancing and dynamic snow coverage
• 27 small ground coverage plants
• 17 decor models such as branches, fallen logs sticks, tree stumps with snowy variants
• 4 rocks, 7 soil banks, 2 snow mounds
• A rustic village set including 4 houses, 2 shacks, barn, smithy, firewood pile, fences
• Particle effects for snow, smoke, and fog
• 8 terrain textures with 6 heightmaps for advanced blending and tessellation

Nature Renderer: Personal License

Have a look at Nature Renderer that will help boost your vegetation rendering workflow. 

Features: 

• Physically Based Rendering
• LOD
• Shadows
• Wind
• Custom Shaders
• Align to terrain
• Familiar Tools
• Live Editing
• Fully multi-threaded for best performance
• Fade out vegetation in the distance
• Efficient culling of vegetation
• Works with multiple terrains
• Minimal GC Allocations and stable consistent performance
• Works with multiple cameras
• Floating Origin support for large worlds
• All data is compressed for optimal memory usage
• Easily disable rendering when a player is indoors or can't see the terrain