1. Here's how

In Lumion 9.0 and newer versions, you can assign two types of grass to Imported Models: 

• 3D Grass Materials.
• Landscape Grass.

Landscape Grass is faster to render than 3D Grass Materials:

3D Grass Materials are very detailed and will have a significant impact on the render speed, especially if you assign these materials to large models.

Please follow the guidelines below to make sure that render speeds are as high as possible when grass is needed in your Scenes:

1.1: Use 3D Grass Materials on small surfaces and Landscape Grass on large surfaces.

1.2: If the Imported Model is double-sided, make sure that a 3D Grass Material is not assigned to both sides of the surface.

1.3: Consider using the Layer and/or Variations functionality to hide 3D Grass Materials when they are not needed, for example if they are hidden behind buildings when rendering Photos and Clips:

• Knowledge Base: How do Layers work?
• Video Tutorial: Build Mode: Add model Variations

See also:

Video Tutorial: Materials: Customized 3D Grasses

Knowledge Base: How do you work with terrains?