- Rabbitfoot clover
- Rabbitfoot grass
- Ragged-robin
- Ragwort
- Rampion
- Ramsons
- Rattlesnake fern
- Rattlesnake master
- Rattlesnakeweed
- Red Alder
- Red baneberry
- Red Buckeye
- Red Buffalo-bur
- Red bush monkeyflower
- Red campion
- Red Clover
- Red columbine
- Red deadnettle
- Red Helleborine
- Red Huckleberry
- Red larkspur
- Red mangrove
- Red maple
- Red Odontites
- Red osier dogwood
- Red pine
- Red raspberry
- Red Sand Spurrey
- Red spruce
- Red Tasselflower
- Red trillium
- Red Valerian
- Red-berried elder
- Red-flowering Currant
- Red-stemmed Feather Moss
- Redflower ragleaf
- Redmaids
- Redroot Amaranth
- Redstem Stork's-bill
- Redwhisker clammyweed
- Redwood Sorrel
- Redwood Violet
- Reed canary grass
- Rescue Brome
- Resurrection fern
- Rhodora
- Ribwort plantain
- Richardson's geranium
- Rio Spiderwort
- River birch
- River oats
- Riverbank grape
- Rock harlequin
- Rock polypody
- Rock samphire
- Rocky Mountain Beeplant
- Rocky Mountain Juniper
- Rocky Mountain maple
- Rocky Mountains Ponderosa Pine
- Rosary pea
- Rose campion
- Rose Clover
- Rose evening primrose
- Rose Spirea
- Rose twisted-stalk
- Rose-scented geranium
- Rosemary
- Rosepink
- Roseroot
- Rosy Hyacinth Orchid
- Rosy sandcrocus
- Rosy Sundew
- Rough Bindweed
- Rough blazing star
- Rough chervil
- Rough cinquefoil
- Rough cocklebur
- Rough goose neck moss
- Rough horsetail
- Roughleaf dogwood
- Round-headed bush clover
- Round-leaved sundew
- Round-leaved wintergreen
- Round-lobed hepatica
- Roundleaf geranium
- Roundleaf greenbrier
- Rubber Rabbitbrush
- Rue anemone
- Rue-leaved Saxifrage
- Rugosa rose
- Rush Skeletonweed
- Russian Dock
- Russian olive
- Rust Weed
- Rusty Blackhaw
- Rusty-leaved Alpenrose
- Rustyback
- Ruthenian broom