Ask Mr. Smarty Plants

A tree is a very thoughtful gift; one that keeps on giving.

You didn't mention the environment where this tree is to be planted which makes it hard to suggest a particular one.  So this is going to be a two step process; first, find the name of a suitable tree, and then determine if it is toxic to horses.

To find a tree, go to  to our Recommended Species page and select Pennsylvania on the map; you will get a list of 135 commercially available native plant species suitable for planned landscapes in Pennsylvania. Go to the "Narrow Your Search" column where you can become more selective.

You see that you have several parameters from which to choose. Choose Pennsylvania  for the state, Tree under "Habit", Perennial under "Duration",  Sun under "Light requirement", the appropriate moisture for your site under "Soil moisture" (I chose moist), and you can ignore "bloom characteristics" for now.  Click the "Narrow your search" button and you will get a list of 36 plants that meet these criteria. Clicking on each plant name will bring up its NPIN page that describes the plant's characteristics.

You can modify your listing by changing the choices  for "soil moisture" and "light requirement" to match the location where the tree will be planted.i

Once you have found some plant names, go to our Suppliers Directory to locate businesses in Pennsylvania that have these plants for sale.

 As to the question of toxicity, I've listed several toxic plant databases below.  You can check for the tree you have selected on these lists. Please note that the absence of a name  from these lists does not guarantee that it is non-toxic to horses, but not finding it on the lists makes it less likely to be toxic.  To search the lists, I recommend using the scientific name  since those names are generally standard, whereas the common names often vary in spelling and usage.

Universtiy of Pennsylvania's Poisonous Plants Database

Poisonous Plants of North Carolina

Cornell University's Plants Poisonous to Livestock and other Animals

Texas Toxic Plant Database

Additionally, here are databases that are specific for plants poisonous to horses.

Equisearch.com

Trail Blazer magazine

ASPCA

Ohio State University