Plant of the Month: April 2022

Glaucidium palmatum Siebold & Zucc.  – Japanese wood poppy

by Peter Williams

     


Of the numerous truly beautiful Japanese native plants, Glaucidium palmatum is considered by many shade and woodland gardeners to be the ‘fairest of them all’.

Fig.1 – Glaucidium palmatum flowering in April

Despite its common name, the Japanese wood poppy is not in the poppy family and its true lineage has been the subject of controversy for decades. It has been ‘in and out’ of the Ranunculaceae, and when ‘out’ it has been considered to be a member of the Papaveraceae, the Paeoniaceae, the Podophyllaceae or the sole representative of its own family, the Glaucidiaceae. Modern genetic analysis has ended the controversy and confirmed that it is a primitive member of the Ranunculaceae and the genus Glaucidium has just one species – palmatum.

Glaucidium palmatum is endemic to central and northern Japan and is found mainly in mountainous regions of Hokkaido and Honshu where it grows in montane and sub-alpine forests. It is considered to be a vulnerable species in Japan because of historic plant collecting and habitat disturbance and is listed in the Hokkaido Red Book of endangered organisms.

Fig.2 – G. palmatum growing on Mount Higashidate, Honshu

 

Fig.3 – A pair of shoots emerging in late March with leaves protected by a sheath

G. palmatum is a rhizomatous herbaceous perennial that breaks winter dormancy in late March when fat buds appear at the soil surface

 

The shoots rapidly expand to give pairs of pale-green, maple-like leaves that in mature specimens are wrapped protectively around large, single, pale lilac flower buds that open in April and May. Like many members of the Ranunculaceae, the coloured petaloid structures are sepals and petals are totally absent.

Fig.4 – Newly opened flowers

A white form, var. leucanthemum, was first described in 1910 and is available in commerce (and is even more expensive that the standard lilac form!). It was initially grown as an alpine glasshouses subject although it is just as hardy as the lilac form and fares just as well outdoors.

Glaucidium can be a tricky plant to grow and requires at least partial shade and cool or cold growing conditions to simulate its natural montane habitat. The advice from the Japanese grower Matsuzaki published in The Gardener’s Chronicle in 1925 suggested it should be planted in soil “with plenty of humus” in a half-shaded position.

My initial plants came from a Japanese nursery (Yuzawa Engei on Hokkaido) and I grow them in a north-facing bed that receives just a little afternoon sunshine. It seems to be important to ensure that the plants are not shaded by adjacent plants. In this respect Glaucidium seems to have very similar light/shade requirements to its close relative Anemonopsis japonica and another Japanese woodland beauty, Pteridophyllum racemosum. I grow all three species in close proximity in an infertile, sandy acid soil that is largely free of slugs – a pest that is often suggested to be ‘very partial’ to this trio of lovely woodlanders.

Fig.5 Left, Anemonopsis japonica Right, Pteridophyllum racemosum

Fig.6 – Seed follicles and harvested seed

The flowers of G. palmatum are probably self-fertile but I cannot be sure because I planted three specimens in one location and they may be cross pollinating. Seed are freely produced in a very distinctive bilobed fruit that becomes woody before seed release. The seed themselves look quite like lily seed and require a cold winter period to break dormancy. Seed sown in autumn will usually germinate in spring and flowering sized plants produced in about three years.

 

Fig.7 A group of developing shoots in late March 2022

Having been fortunate enough to have grown these wonderful plants for a few years, early spring is always an anxious time because I so much want them to have successfully overwintered. I looked for evidence of life yesterday (28th March 2022) and was pleased to see that all was well as plump buds were emerging through the leaf litter for all members of my little colony of Japanese wood poppies.

 

Images courtesy of Peter Williams other than Fig 2:

Image: http://www.asianflora.com/Ranunculaceae/Glaucidium-palmatum.htm downloaded 28/03/2022

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