Hairy bittercress (Cardamine hirsuta) may be a little plant but, in the wrong place, it can be a big problem! One of the wrong places appears to be our garden where despite my wife declaring war on it the more she attacks it the stronger it seems to come back. In fact, by pulling it out one merely helps to spread its seeds as the pods burst when touched. The seeds can germinate quickly and soon increase the number of plants you have. I try to explain this but the approach remains the same, pull on sight.
It is such a shame that all forms of cultivation and wild flowers seem to be in direct conflict. Hairy bittercress is not a 'looker' as far as gardeners are concerned and so it has to go; and the same applies with other 'weeds' too, of course. Seeds often come in to gardens with plants purchased from garden centres and nurseries.
As a cress it is a member of the cabbage (crucifereae) family with four white petals but being a small flower this is not always obvious. It has the capacity to grow all year round in the south and so gardens are rarely without it!