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Plant of the Month: April 2013

Shrubby Gromwell
Glandora prostrata (Loisel.) D.C. Thomas 2008
= Lithodora prostrata (Loisel.) Griseb.
= Lithospermum prostratum Loisel.
= Lithodora diffusa hort., non (Lag.) I.M. Johnst.
= Lithospermum diffusum hort., non Loisel
BORAGINACEÆ; Borage Family

A beloved, commonly cultivated ornamental plant prized for its petite size, darkest evergreen foliage, and profuse indigo-blue flowers . . . is nearly altogether named incorrectly. Hence this brief article. It is frustrating when scientific names of familiar garden plants are changed. Alas! --here is another example of just such a thing. But the confusion is in this case due to sloppy garden writers, rather than nit-picking scientists.
    Shrubby Gromwell is a widespread heathland plant throughout the Atlantic side of SW Europe, France to Portugal, and N Morocco. It has been grown in England since at least 1825. As long as it has sunshine, adequate drainage, and non-alkaline soil, it is easily grown. It blooms mostly in April through June. Other names include: Purple Gromwell, Gentian-blue Gromwell, and Acid-soil Lithodora.
    The original Gromwell, Lithospermum officinale, is a perennial medicinal herb of Europe, devoid of ornamental beauty. In contrast, Shrubby Gromwell is indeed a miniature woody shrublet, that, to gardeners, is categorized variously as a rockery plant, a dwarf shrub, a groundcover, or (not aptly) as a perennial. Pretty, in any event, as can be seen by the photos below.
    Glandora prostrata cultivars include (some likely synonyms):

'Alba'
'Baby Barbara'
'Blue Star' --may = 'Star'
'Cambridge Blue'
'Compacta'
'Grace Farwell'
'Grace Ward'
'Heavenly Blue'
'Inverleith'
'Minima'
'Pete's Favorite'
'Picos'
'Star'
'White Star'

    I noted that Sunset's Western Garden Book, Mabberley's Plant Book, the European Garden Flora, the RHS Plant Finder and all other horticultural sources checked, uniformly contradicted recent taxonomic studies. Therefore, I contacted two botanical experts (Daniel C. Thomas, David J. Mabberley) who kindly confirmed that the names cited in this article, are correct. In a nutshell, the common plant sold as Lithodora diffusa, is really Glandora prostrata, while true Lithodora diffusa is not cultivated, and is properly called Glandora diffusa.

    Regardless whether one accepts Glandora as a genus distinct from Lithodora or not, certainly the two species prostrata and diffusa are differentiated adequately. They grow in different soils (acid vs. alkaline); their flower stamens vary; their seed coats vary (rough vs. smooth); and they differ at the molecular level.

Glandora diffusa (Lag.) D.C. Thomas 2008
= Lithodora diffusa (Lag.) I.M. Johnst.
= Lithospermum diffusum Lag.
Endemic to the Cantabrian mountains of N Spain; grows in limestone. Viewed through magnification, its seed coats are smooth rather than rough. Not cultivated, but its name is generally misapplied to Lithodora prostrata.
I wonder if true G. diffusa can be cultivated easily in a lime soil. Likely, some members of Rock Gardening societies will know. It is as attractive as the other species. I expect the main difference on a practical level, is the soil pH requirement.

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Shrubby Gromwell

Shrubby Gromwell; photo by ALJ

Shrubby Gromwell

Shrubby Gromwell; photo by ALJ

Shrubby Gromwell

Shrubby Gromwell; photo by ALJ

Shrubby Gromwell

Shrubby Gromwell; photo by ALJ




Arthur Lee Jacobson plant expert
Arthur Lee Jacobson plant expert
Arthur Lee Jacobson plant expert
Arthur Lee Jacobson plant expert
Arthur Lee Jacobson plant expert
Arthur Lee Jacobson plant expert
Arthur Lee Jacobson plant expert
Arthur Lee Jacobson plant expert
Arthur Lee Jacobson plant expert
Arthur Lee Jacobson plant expert
Arthur Lee Jacobson plant expert
Arthur Lee Jacobson plant expert
Arthur Lee Jacobson plant expert
Arthur Lee Jacobson plant expert

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