This little alien weed is gallant soldier Galinsoga parviflora. It's a native of Peru but is currently flourishing in cracks on the pavement and along the road edge outside Durham University's Elvet Riverside building near the centre of Durham city. The plant first escaped from cultivation in Kew Gardens in 1860 and became a common weed in south-east England, but didn't turn up in Durham until it was found in a field near the city in 1940.
In his classic New Naturalist series book Weeds and Aliens (1961) Sir Edward Salisbury describes how the plant acquired its common name. Its escape into gardens around Richmond in Surrey led to a flurry of inquiries at Kew about this new invasive weed and gardeners were told that it was called Galinsoga, since at that time this Peruvian plant had no common name in Britain. The unfamiliar Latin name quickly became corrupted to the easier-to-remember gallant soldiers and has remained so ever since.
Around Kew the plant was also sometimes known locally as Joey Hooker weed, after Joseph Dalton Hooker who was director of Kew Gardens at that time.
The plant produces numerous seeds - typically about 2000 but as many as 15,000 in large plants. They are tiny and equipped with a small and not very efficient parachute of hairs, so tend to be wafted short distances and are then lodged in crevices at the base of walls where they are washed further in by rainwater. They're also easily transported on clothing.
Galinsoga has spread to many other parts of the world and showed a similar pattern of dispersal from botanical gardens in Holland and South Africa. This is a weed that's on a world tour.
Alien introductions often leave their natural pests and parasites at home when they travel, then slowly accumulate new ones in their new geographical surroundings. This one is hosting a leaf-mining larva of an insect - probably a fly or a micro-moth. You can see the silvery trail around the edge of the leaf.