Choose species horses can safely browse
Plan on the basis that every leaf within reach of the fence will be eaten, because it will. The traditional hedgerow mix suits horses precisely because generations of livestock have browsed it: hawthorn, hazel, field maple, dog rose and blackthorn all tolerate nibbling and regrow well, and willow is actively enjoyed by many horses. Inside the paddock, a specimen tree for shade should come from the same safe list — willow, field maple or a fruit tree whose fallen fruit you are prepared to clear rather than let horses gorge on. Planting a mixed hedge rather than a single species spreads your risk, so one plant failing or being browsed hard never leaves a gap in the boundary.
Keep the dangerous plants out entirely
Some common planting choices have no place anywhere near horses. Yew is the worst: it is highly toxic in small amounts, and clippings thrown over a fence have killed horses, so never plant it and remove any you inherit. Laburnum is poisonous too, and privet is best avoided. Oak is a subtler case — the tree itself gives fine shade, but acorns eaten in quantity cause real harm, so either plant oaks where horses cannot gather under them or accept sweeping up acorns in a heavy year. Sycamore should not be planted by grazing at all, because its seeds and seedlings carry a toxin linked to a serious, often fatal muscle disease in horses. Ragwort is a weed rather than a planting, but a paddock planting day is the right moment to pull any you spot, since it is poisonous dried in hay as well as growing.
Protect young planting until it can look after itself
A young hedge or tree in a paddock faces rubbing, bark-stripping, trampling and determined browsing, and most new planting dies from horses rather than weather. Fence the planting line off with its own run of fencing set inside the paddock boundary, far enough out that a horse cannot reach through and strip bark or pull up whips. Individual trees need a proper guard around the trunk that stands up to a horse leaning its full weight in for a scratch. Mulch well, water through the first dry spells, and leave the protection in place for several seasons — until the hedge is a thicket and the tree's bark is mature, the fence is doing the growing for you.
Place planting where it works for the paddock
Think about what the mature planting will do. A hedge along the windward boundary becomes shelter horses will stand behind in bad weather, and a shade tree earns its keep in one corner of the field rather than the middle, where horses gathering under it will churn the ground bare. Keep planting back from water troughs so leaf fall does not foul the water, and away from gateways, where shelter planting invites the herd to congregate exactly where the ground is weakest. Leave room to mow or top around the planting line, because an unmaintainable hedge quickly becomes a ragwort nursery.
Check before you plant, then keep checking
The safe list is short enough to learn, but garden centres are full of attractive species nobody has checked against a horse. Before anything goes in the ground, confirm the species — and keep the habit going: walk the boundary as part of your normal paddock checks, looking for self-seeded sycamore, ragwort rosettes and any branch brought down within reach by a storm. When you meet a plant name you do not know, put a follow-up question to KinsLe's Kimmy before you leave the garden centre; she answers using the KinsLe Answers library.