Ask Mr. Smarty Plants

We will start with the wildflowers, since that is not as complicated as horse pasture. Go to this American Meadows website on Wildflowers for the Northeast. To a certain extent, wildflowers native to your area will take care of themselves, blowing in on the wind, but you can select specific mixes that appeal to you. The seed packets will have instructions on what time of year they should be planted, etc.

Since we are gardeners and not veterinarians, we don't know a whole lot about what horses eat. Here is an ASPCA article on Ten Top Nutritional Tips for Horses. And we are not quite sure what would constitute an "organic" horse pasture. You should know that the Lady Bird Johnson  Wildflower Center, home of Mr. Smarty Plants, recommends only plants native to North America as well as to the area in which those plants grow natively; in your case, Dutchess County, NY on the eastern border of New York with Massachusetts.

We have two How-To Articles that should help you start thinking about this project:

Meadow Gardening

Recreating a Prairie

To get down to the basics, not only will we recommend only plants native to your area, we will NOT recommend a seed mix. The very first thing you must understand is that this is not an effort in which you can buy an envelope or a sack of some seed mix, sprinkle it around and expect the results you need. You will probably have to buy hay and perhaps some other parts of the horse diet, but for your horses to graze, we can certainly give you links to some of our webpages on grasses native to your area.  Those webpages will have propagation information (seed? plugs?) and what environment that plant needs - sun, shade, moist soil, etc. We will also check that any plant we recommend is native in the area of Dutchess County.

Before you do anything else, make sure that the pasture is free of any poisonous plants or plants that could harm the horses. From a previous Mr. Smarty Plants answer, here are some of the plants you should either not plant or clear completely away from the pasture.

"First, do not plant any trees in the rose family.  This includes cherries, plums, apples, pears and several other common trees.  Under certain conditions, their leaves can be very poisonous to livestock.  Likewise, trees in the genus, Juglans, such as walnuts and butternuts should be avoided.  Neither yew trees (Taxus spp.) nor oaks (Quercus spp.) should be used.  Finally, some maples (Acer spp.), such as red maple (Acer rubrum) are quite toxic to horses, while others are not.  Other trees that are toxic to some livestock, but not necessarily horses include some pines (Pinus spp.), some firs (Abies spp.), hemlocks (Tsuga spp.), Douglas fir (Pseudotsuga menziesii), spruces (Picea spp.) and junipers (Cupressus spp.)"

This previous answer has a list of sources to check there are no poisonous plants for horses on the property.

Now that we have helped you determine what should not be available for the horses to graze on, here is a list of grasses native to New York.

Andropogon gerardii (Big bluestem)

Elymus canadensis (Canada wild rye)

Elymus virginicus (Virginia wildrye)

Panicum virgatum (Switchgrass)

Schizachyrium scoparium (Little bluestem)

Sorghastrum nutans (Indiangrass)