- Lacy phacelia
- Ladder brake
- Lady Orchid
- Lady's Bedstraw
- Lance-leaved Coreopsis
- Lanceleaf arrowhead
- Lanceleaf liveforever
- Lanceleaf Springbeauty
- Lanceleaf Stonecrop
- Lanky Moss
- Lantana
- Large bindweed
- Large bittercress
- Large cranberry
- Large hop clover
- Large white trillium
- Large-flowered Hemp-nettle
- Large-leaved aster
- Large-leaved Avens
- Large-leaved lupine
- Largeflower bellwort
- Largeflower Mexican clover
- Largeflower pink-sorrel
- Largeleaf pennywort
- Late boneset
- Laurel sumac
- Laurustinus viburnum
- Lawn daisy
- Leadplant
- Leafy elephant's-foot
- Least Gagea
- Leatherleaf
- Leatherleaf mahonia
- Leichtlin's Mariposa Lily
- Lemon balm
- Lemon beebalm
- Lemonade berry
- Lesser Balloon Vine
- Lesser burdock
- Lesser butterfly-orchid
- Lesser celandine
- Lesser hop trefoil
- Lesser Meadow-rue
- Lesser periwinkle
- Lesser rattlesnake plantain
- Lesser stitchwort
- Lesser swine-cress
- Lesser wintergreen
- Lewis flax
- Lewis' monkeyflower
- Licorice fern
- Lilac chaste tree
- Lilac tasselflower
- Limber pine
- Lindheimer's Senna
- Lingonberry
- Lion's ear
- Little bluestem
- Little Brown Jugs
- Little bur-clover
- Little ironweed
- Little quaking-grass
- Little-leaf Linden
- Little-leaved palo verde
- Little-Robin
- Livelong Saxifrage
- Liverleaf
- Lizard Orchid
- Lizard's tail
- Loblolly pine
- Lodgepole pine
- London rocket
- Long beech fern
- Long-headed poppy
- Long-leaved Speedwell
- Long-spurred violet
- Longleaf
- Longleaf pine
- Longleaf Wattle
- Loquat
- Love-in-a-mist
- Low smartweed
- Lowbush blueberry
- Lungwort
- Lupine clover
- Lyreleaf sage