Tall Grass
Tall Grass is the foundation habitat — the first thing you create, the building block everything else grows from. Using Leafage (Bulbasaur's move) seeds bare ground with dense, waist-high grass that immediately changes the character of the surrounding area. Where there was dirt, there is now something alive.
Attracted Pokémon
Tall Grass attracts a wide range of Pokémon, particularly those comfortable in open, vegetated environments: Grass-types, Normal-types who favor grassland, and various others depending on time of day. Morning and afternoon tend toward one set of visitors; nighttime brings others. The diversity of Tall Grass inhabitants makes it among the most productive habitats for expanding your island's total Pokémon roster.
Tips
Early game, focus on creating contiguous patches of Tall Grass rather than scattered single tiles. Connected habitat areas have higher comfort ratings than isolated patches, which translates to more Pokémon lingering long enough to build friendship. Quantity and connectivity both matter.