Trees create atmosphere in a churchyard more than any other plants, and the tree most of us associate with churchyards is the yew. Yews are evergreen conifers with the densest foliage of any evergreen tree, which makes them ideal for topiary. They're native in Britain only on well-drained limestone, so most churchyard yews were deliberately planted. There are two at St Paul's, a few metres apart in the south-east corner.
Yew has long been a symbol of protection and of immortality and resurrection. Some trees predate the churches built beside them: a yew at Fortingall in Perthshire may be around 9,000 years old.
A word of caution
Almost every part of the yew is deadly poisonous to humans, cattle and horses. Wear gardening gloves when handling it. The red aril around the seed is the only harmless part, but the seed inside it is poisonous.
Contrary to popular belief, churchyard yews did not supply the English longbow. The best yew was imported from Italy and Spain, and in 1472 Edward IV imposed a stave tax to ensure supply. An extract from yew bark and needles is also the basis of Taxol, a chemotherapy drug.