Herb Robert (Geranium robertianum) and Little Robin (Geranium purpureum)

Herb Robert (Geranium robertianum) and Little Robin (Geranium purpureum)

My Geranium ID Key Blog of last August highlighted the main differences between these two similar species; i.e. the smaller flowers of Little Robin and its yellow anthers. The blog did lack detail on the hairs that Little Robin displays. Here are a few additional photos to show Little Robin against the much more common Herb Robert.

Herb Robert left and Little Robin right, showing the size difference in the flower. 11th May 2020, Cambridge.
White petal lines more prominent in Herb Robert.

Little Robin showing the yellow anthers, Cambridge

Herb Robert showing orange anthers 28th June 2019 Cork

Herb Robert with dark purple anthers 17 June 2019

Herb Robert normally has dark purple anthers but can have orange anthers.
The pollen of both species is yellow so when comparing the anther colour pick a flower that has anthers which have not opened to avoid confusion.

Little Robin, sepal hairs 11th May 2020. Cambridge

Neglecting the stray hairs picked up from a nearby composite, the Little Robin has sepal hairs which are different lengths and have red glandular tips. These are similar to the hairs on the upper flower stem.

Herb Robert 28th June 2019, Cork showing the very long sepal hairs.

Herb Robert sepal hairs, 11th May 2020, Rampton

Herb Robert also has tiny red glandular tips to the sepal and flower stem hairs. The length is much longer for many of the hairs compared with Little Robin. The sepal hairs are longer than the flower stem hairs but the flower stem hairs also have some long hairs. Some non glandular hairs are also present on the flower stems and the main stems have curved simple hairs pointing downwards.

Little Robin, flower stem hairs similar to sepals but less dense. Ignore the stray composite hairs.

In summary Little Robin has sepal hairs that look quite normal whereas Herb Robert seem excessive.
This is a generalisation and sepal and flower stem hairs will vary depending on the population. The amount of hairs is a variable feature, however this is an interesting difference.

Leaves.

Little Robin has pale green leaves that look smooth and hairless to the naked eye. A close look shows they do have some hairs. Herb Robert can have slightly darker green leaves and again it is easy to miss the sparse hairs. Leaf shape is similar between these species. Stems are often red in both species. It is usually regarded as impossible to identify these two species apart, using vegetative features.

Pale green leaf of Little Robin, with some sparse hairs just visible. 11th May 2020 Cambridge

Some leaves have a thiner shape.

Little Robin leaf, 11th may 2020 Cambridge.

Stem Hairs

Stem hairs are mainly short downward curved hairs without glandular tips. G. purpureum

Stem hairs are mainly the short curved hairs but occasional simple long hairs are present or some stems may be hairless.

Hybrids between these two species have been found and the situation in Europe is apparently more difficult due to variation.  Herb Robert has chromosome count 2n=64, Little Robin 2n=32.
Some suggest that Little Robin is a parent of Herb Robert. Cross fertilisation experiments have been done but the situation in the wild is not clear. It is suggested that some hybrids have no flowers but I can't see how you would prove that this is a hybrid. Some plants have been found with intermediate features. It is not known if any recent research has been done on these two species to confirm their relationship.

Little Robin records from Cambridgeshire have been from sites associated with the railways. Recent expansion of Little Robin may be related to winters with less hard frosts.

Peter Leonard
Rampton, Cambridgeshire.
11th May 2020


All comments welcome.