This is a variant of the European native Common Dog Violet which is grown as a garden plant, but can become invasive as it self-seeds prolificly and the clumps can spread by short runners.
The heart-shaped leaves have a purple tinge, but are greener if growing in deep shade.
The unscented flowers are borne on single stems and the colours can vary from pale mauve to deep violet. The main blooming period is from March to May, but they can appear anytime between February and July.
The white, fibrous adventitious roots do not regrow, but
ensure that the growing point is removed.
There is a list of other garden plants that can become pests on the Wayward Plants page.