The garden plants a tortoise must not eat
A tortoise grazing freely does not browse cautiously; it works through whatever is at beak height, and gardens are full of ornamentals that are genuinely poisonous to it. The common offenders are buttercups in the lawn itself, foxglove, ragwort, ivy in quantity, and the showy shrubs rhododendron, azalea and oleander, with yew the most dangerous planting in many gardens. Spring bulbs deserve special attention because the tortoise wakes hungry at exactly the season daffodil, tulip and crocus foliage and bulbs are at their most accessible, and the bulb itself is the most toxic part. None of these is a nibble-and-learn situation; a tortoise cannot vomit up a mistake.
The edible-looking dangers nobody suspects
The plants that catch owners out are the ones that look like food. Tomato and potato leaves belong to the nightshade family and are unsafe even though the ripe tomato fruit is not, and a tortoise helping itself in the vegetable patch will eat leaves first. Rhubarb leaves, lupins, and the foliage of runner beans all belong on the avoid list, as do fallen leaves and berries from laburnum. Set against that, a genuinely safe planting list is easy to grow deliberately: dandelion, broadleaf plantain, clover, sow thistle, hibiscus and rose petals from unsprayed bushes are all sound grazing, and a patch sown with those gives the tortoise something to choose that you have already chosen for it.
Lawn treatments: the invisible problem
A lawn can look like plain grass and be the most dangerous grazing in the garden, because treatments leave no visible trace. Weed-and-feed combinations, selective lawn weedkillers, moss killers and fertiliser granules all coat or soak into the very leaves the tortoise is there to eat, and slug pellets scattered in borders are within grazing reach too. Rain does not reliably wash a treated lawn clean for a grazing animal. The honest rule is that a tortoise should never graze a lawn that is treated at all; if the main lawn is on a treatment programme, fence a separate, permanently untreated grazing area and keep the animal to it.
Graze by design, not by supervision
Owners imagine they will watch every mouthful, and no one can. Design the risk out instead: a fenced, untreated patch with safe forage planted in it, borders of risky shrubs fenced off or removed from tortoise reach, and a habit of walking the grazing area before the tortoise comes out to lift fallen berries, suspect seedlings and any slug pellets that have strayed. Supervision still matters for predators and escapes, but the planting decisions should already have been made by the layout, not by your eyes on the day.
If you think something unsafe was eaten
Loss of appetite, unusual lethargy, a runny nose or eyes, diarrhoea or trembling in a tortoise that has been grazing are all reasons to contact a vet the same day rather than waiting to see, and to take a photograph or a sample of what you believe was eaten, because identifying the plant changes what the vet does next. Keep the tortoise warm and hydrated with a shallow lukewarm soak while you arrange to be seen, and do not wait for stronger symptoms to confirm the suspicion.
For a doubtful plant at the fence line, put a follow-up question to KinsLe's Kinny; he answers using the KinsLe Answers library.