The Hawthorn is also known as the May Tree and I've just realised that the saying 'ne'er cast a clout till May be out' doesn't mean don't take off any clothes ('clout'- old word for clothes) like your woollen vest, until the end of May. It means that Spring hasn't properly arrived until the May tree flowers are out.
My dad used to love Hawthorn trees. I remember him talking about eating the 'bread and cheese' leaves, but it didn't make much sense. I looked it up and found the tender leaves of Hawthorn used to be a 'famine food', eaten by hungry young children.
Humans aren't the only ones to benefit from hawthorn as a food source. Its flowers offer pollen to bees and are eaten by dormice. Around 300 insects enjoy the hawthorn as a food plant, and the the berries or haws feed many birds in Winter.
It's supposed to be bad luck to bring any hawthorn blossom inside, perhaps because people used to think the blossom smelt like decaying bodies or the Great Plague!
The blossoms were used outside though, especially as part of crowns for the May Queen. I can't quite figure out how they'd do that without it being a very thorny crown?