I am ambivalent about the removal of this species from Trillium but I accept the reasoning – both morphological and molecular data support the split. From a garden perspective there are some minor differences observable to the amateur:
- P. rivale is not known to hybridise with other trillium.
- Seed requires only one winter cold period to promote germination. Other Trillium require two winters.
- The seedling leaves (cotyledon) are rather heart-shaped like the mature leaves, rather than linear like other Trillium seedlings.
This species, the Brook WakeRobin is native to Northern California and Southern Oregon. It is easy enough to grow in the UK in a shady humus rich ‘woodlandy’ area of the garden such as under a deciduous shrub. In keeping with the natural habitat it prefers to be a little more dry in summer than most Trillium and also is not as cold-hardy as some in winter. I have friends in Aberdeen in NE Scotland who find it totally hardy there and Aberdeen is much colder than here. But I lost plants in troughs and raised beds in the cold winter of 2011 because (unlike Aberdeen) we had a month of torrential rain before the big freeze and this combination did a lot of damage. Growers in continental Europe with very cold winters find it hard to keep but the UK seems to suit it well.
The species is very variable. The first photo below is typical of the wild forms. Almost white with a few pink speckles. The petals only just meet and overlap. The subsequent pictures show the variation possible in gardens especially. Some years ago a large full-flowered form with dense colouring and speckling in the centre of the flower was named ‘Purple Heart’ by its discoverer in the US. Seedlings descended from this are often available and show similar characteristics. The final pics are of this type. I am trying to cross-pollinate the most intense forms to select even more densely coloured ones from the seedlings.
There are also forms in cultivation with the leaf veins picked out in silver, which are rather attractive. I have never seen a plant with silver-veined leaves AND big dark flowers which makes me wonder if the two features are genetically mutually-exclusive. The forms with darker speckled flowers can certainly be identified even out of flower because the leaves have some dark reddish speckles too.