No not St Johns wort .
It's a very very common Welsh weed round like a green daisy in leaf shape … a singular small plant max height about 6 inches high max by 4 inches dia ,has white flowers and when the seed sets if you touch them they explode throwing out tiny bean pod shaped seeds up to about 18 inches away, nothing like shepherds purse but spreads the seeds in the same manner.
Can't find a picture of it at present but our newly seeded lawn has produced dozens of the dam thing , the raised beds also have more than their fair share at present . So long as I weed them out before they seed they're done in .
If you can , use a long pole and drape old dust sheets over the plum tree, whenever a frost is forecast ….till the end of April or better still first week of May . I had dwarf plums & apples so it was easy for me .
Worcester fruit growers used to burn green wood on a forecast frost night to produce a blanket of warm smoke in the still air to protect the trees .