The rosebay willowherb (Chamaenerion angustifolium) in the Arboretum is in flower and well worth a look.
This is a pioneer plant, among the first to colonise new ground. In America it is called fireweed because it is the first plant to take advantage of the space newly created by a forest fire. Fire sterilises the soil and breaks up its structure, turning it into an ideal seed bed for willow herb’s tiny wind-dispersed seeds.
Rosebay willow herb flowers form a spike called a raceme; each flower on the raceme will result in a long seed pod. In the header picture the fully formed pods are visible behind the flowers, and beneath each flower you can see the seed pods forming. Each pod will contain 300-400 seeds, a raceme will produce thousands of seeds, and a single plant can produce 60,000 seeds.
The pods dry when the seeds are fully formed and they open gradually, peeling back to expose the seeds. Not all the seeds blow away and the basket-like structure that the open seed pods form is a convenient landing place for foraging seed eaters.
Photographs by SMH
Southwick Country Park Nature Reserve 
Why ROSEBAY? Is it a place?
Wiki says this: “The common British name, from the passing resemblance of the flowers to (wild) roses and the leaves to those of bay, goes back in print to Gerard’s Herbal of 1597” and cites the OED as its source.
I am not going to argue with the OED – or Gerard and his Herbal.
I grew up in East London just after the war. Bomb craters and demolished building were all around us, as were fields of Rosebay willowherb. They surrounded our prefab estate and I used to pick them for my Mum on the way home from primary school.
I’ve heard it called ‘bombweed’ and I remember it growing on the bomb sites in Bristol. I have read somewhere that after the war, it was called London’s pride.