It is so easy to pass by a flower, jump to name and keep walking and end up misidentifying a species because you did not stop to think. I say 'you' but I really mean 'me'! It is one of my many bad habits and it's why I never made a good botanist. With the hoary willowherb (Epilobium parviflorum) it is so easy to just think "great willowherb" and yet there is really no reason why you should, there are several obvious differences.
Although the flowers of hoary and great willowherbs have the same form, the greater is, indeed, greater than the flower of its hoary cousin and it is a deeper pink. Great willowherb also grows much taller than hoary, reaching well over a metre in height where the hoary would never get anywhere near a metre tall. The third and really the most obvious difference when you see it is that the hoary willowherb is very hairy whereas the greater is smooth. The hairs on the hoary willowherb are white and give the plant a grey appearance; hoary means greyish white. It is almost as if the plant is covered in hoar frost but as it flowers from July until September it obviously isn't! This is not as common as greater willowherb but they share a liking for damp habitats and they can both be found in ditches and alongside fresh water. Great willowherb is ore tolerant of dry conditions.
In traditional medicine hoary willowherb was thought to be a remedy for various problems with the kidneys and urinary tractHoary Willowherb: a touch of frost