The salix genus (willow trees and shrubs -- there are thousands) are safe, yet also tasty, as has been suggested. My guys thought they had found a hanging feed bag when they visited my mother and discovered her enormous weeping willow. I would protect any young tree that you plant -- even if they don't want to eat them, they love to scratch their nether regions on them...
Like so many plant lists, the one listed above is a decent starting point, but barely scratches the surface. The best I've found so far is:
http://www.ivis.org/special_books/Knight/p...er_frm.asp?LA=1
Here's another useful list I found for plants "known" to be safe for horses (again, not an exhaustive list, and to be extra safe I would run these past your vet or extension agent)
African Violet (Saintpaulia)-
Aglaonema (Botanic name)
Ajuga Reptans (Botanic name)Â groundcover
Albizzia (Botanic name)Â tree
Alstoemeria (Botanic name)Â Â flowering perennial
Aluminum Plant (Pilea cadierei)
Amaryllis Belladonna (Botanic name)Â bulb
Anemone (Botanic name)Â flowering perennial
Angel Wing Begonia (Species) flowering annual
Artillery Plant (Pilea Microphylla)
Asparagus Fern (A. setaceus or sprengeri) fern
Aster, Giant (Townsendia sericea)Â Â flowering perennial
Baby's Blanket (Thymus serpyllum) groundcover
Baby's Breath (Gypsophyla)Â Â flowering perennial
Baby Tears (Helxine soleirolii)
Bachelor Buttons (Centaurea cyanus)Â Â flowering perennial
Bamboo, common (Phyllostachy aurea)
Bird's Nest Fern (Asplenuim nidus) fern
Bishop's Weed (Aegopodium podagaria) flowering perennial
Blue Spruce (Picea pungens)Â tree
Boston Fern (Nephrolepsis exalta) fern
Butterfly Bush (Buddleia davidii)Â flowering shrub
Camellia (Thea japonica)Â flowering shrub
Canna Lily (Canna generalis) exotic bulb
Canterbury Bells (Campanula)Â Â flowering perennial
Carob Tree (Ceratonia siliqua)Â tree
Cockscomb (Celosia argentea)
Coleus (Species) annual
Creeping Charlie (Plectranthus australis) annual groundcover
Creeping Charlie (Lysimachia nummularia) invasive groundcover
Creeping Charlie (Pilea nummulariifolia)
Crocus, Spring blooming (Species) bulb
Dahlia (Species)Â Â flowering perennial
Dandelion (Taraxacum officinale)
Douglas Fir (Pseudosuga taxifolia)Â tree
Easter Lily (Lilium longiflorum) bulb
Elephant Ear (Bergenia cordifolia) bulb
Fir (Abies or pseudotsuga)Â tree
Forget-me-nots (Myosotis)Â Â Â Â flowering annual
Forsythia (Botanic name)Â Â shrub
Fruitless Mulberry (Morus) tree
Fuchsia (Species)Â Â flowering perennial
Gardenia (Jasminoides)Â Â flowering perennial
Gloxinia (Sinningia speciosa)Â Â flowering perennial
Gourd (Cucurbitaceae) perennial
Grape Hyacinth (Muscari species) bulb
Hemlock, Tree (Tsuga species)Â tree
Hen and Chickens (Sepervivum tectorum) groundcover
Hen and Chicks (Echeveria) groundcover
Hibiscus (Botanic name)Â Â flowering perennial
Hollyhock (Althaea rosea)Â Â flowering perennial
Honey locust (Gleditsia triacanthos)Â tree
Lilac (Syringa)Â Â shrub
Locust Pods Tree (Ceratonia siliqua) tree
Manzanita (Arctostaphylos)Â tree shrub
Mock Orange (Philadelphus species)Â Â shrub
Mock Orange (Pittosporum tobira) shrub
Mountain Ash (Sorbus aucuparia)Â tree
Mulberry Tree (Morus species)Â tree
Nasturtium (Tropaeolum majus) flowering annual
Petunia (Species) flowering annual
Phlox (Botanic name) flowering perennial
Purple Passion Vine (Gynura aurantiaca)
Red Bud (Cercis Canadensis)Â tree
Rose of Sharon (Hibiscus syriacus)Â flowering perennial
Silver King (Aglaonema)
Snake Plant (Sansevieria Trifasciata)
Snapdragon (Antirrhinum species)
Snowball Bush (Viburnum)Â Â shrub
Sphagnum Moss (Botanic name) groundcover
Strawflower (Helichrysum bracteatum)
Swedish Ivy (Plectranthus australis)
Sycamore (Platanus) tree
Torch Lily (Kniphofia) bulb
Wandering Jew (Zebrina pendula)
Weigelia (Botanic name)Â Â flowering shrub
Willow (Salix)Â tree/shrub
Xylosma (Botanic name)
Yucca (Botanic name)
Zebra Plant (Calathea zebrina)
Zebra Plant (Cryptanthus zonatus)
Zinnia (Botanic name)