Culpeper places hawthorn under Mars, and the ground is the thorns. This is the doctrine of signatures at its most literal: an armed plant belongs to the planet of weapons and wounds. It is the same reasoning that puts nettle under Mars for its sting, and it has the same weakness, it reads a defensive structure as a statement of pharmacological character, and the two have no relation.
The folk associations run somewhere else entirely. The fairy tree, the May blossom, the guardian of the boundary hedge: none of this is martial in any sense Culpeper's scheme would recognise, and it constitutes a body of tradition that simply does not intersect with the planetary one. They coexist without engaging.
The modern use is different again and is the most substantial of the three. Hawthorn leaf, flower and fruit extracts are among the better-investigated herbal preparations for cardiac indications, particularly chronic heart failure, and are a licensed phytomedicine in Germany. The constituents of interest are oligomeric procyanidins and flavonoids, and the proposed mechanisms include mild positive inotropy and vasodilation. The trial evidence includes some reasonable-quality work with modest positive findings on symptom measures, and a large mortality trial that did not show benefit on its primary endpoint. This is described here as the state of a research literature, not as a recommendation, and hawthorn interacts with cardiac medication.
That a plant assigned to Mars for its thorns turns out to act on the heart is a coincidence, and it is worth resisting the temptation to make it more than one.