Easily confused with : Wavy Bitter-cress, but, paradoxically, that can have hairy stems whereas Hairy Bitter-cress doesn't (but it does have hairy leaves). Also, the seed-pods of Hairy Bitter-cress usually over-top the flowers, whereas those in Wavy Bitter-cress are usually lower than the top-most flower. Wavy Bitter-cress has nominally six stamens whereas Hairy Bitter-cress has but four. The number of stem leaves also differs: Wavy Bitter-cress has between four and ten whereas Hairy Bitter-cress has only one to four stem leaves. Although the leaves of Wavy Bitter-cress are hairless, those of Hairy Bitter-cress only might possibly be hairy.
Not to be confused with : Hairy Rock-cress,
Hairy St John's Wort,
Hairy Garlic or Hairy Tare, etc [plants with similar names].
Hybridizes with : Wavy Bitter-cress (Cardamine flexuosa) to produce
Cardamine × zahlbruckneriana (Cardamine flexuosa × hirsuta) but luckily this is present in but very few locations, and is not present where the above photographs were taken.
Some similarities to : Hairy Rock-cress
Flowers over a longer time period than that of Wavy Bitter-cress, from February to November, and all year in mild winters.
Habitat: bare ground, usually damp, including dunes, or rocks, walls and especially garden beds. Ubiquitous almost, but not quite, throughout the UK.
But beware about using the number of stamens as the sole identifier. Hairy Bitter-cress is most likely to have only 4 stamens, but it can have 5 or 6! Likewise, Wavy Bitter-cress is most likely to have 6 stamens (2 deeper down in the flower than the rest) but it can have only 5 or only 4! Use as many other differing characteristics as possible for ID.