Plant Heritage Autumn 2005
Thymus citriodorus
Over the years I have done a considerable amount of research into the names of thymes currently available
in the nursery trade.
In consultation with the RHS Advisory Panel on Nomenclature and Taxonomy
I have gradually been sorting out the nomenclatural muddle.
As far as species are concerned, I check herbarium specimens both at The Natural History Museum and at the
Royal Botanic Gardens, Kew and I also check original descriptions as well as later papers where there has been a
change of name.
In 2003 Graham Walters of the Alpine Unit, Royal Botanic Gardens, Kew and I, discovered to our surprise that in 1811
Thymus citriodorus was first described as a species and not as a hybrid of Thymus vulgaris and T. pulegioides.
The earliest reference I found for Thymus × citriodorus was in the RHS Journal in 1955 Vol. 80.
However throughout the 1960s it was common to see this thyme recognised either as a species or as a hybrid and it was
not until the mid 1970s that the hybrid status became adopted as the norm.
It would appear that it was regarded as a hybrid by the nursery trade and gradually this status became accepted as correct.
The problem when a mistake has been made is that unless original data is consulted, everyone writing on a subject refers
to more recent data and just perpetuates that mistake.
Interestingly both the International Plant Names Index (IPNI) and Index Kewensis have retained specific rather than hybrid status.
One of the dilemmas Graham Walters and I faced concerned a gold variegated, lemon-scented, bushy thyme collected in northern Spain
and given the name T. × citriodorus 'Villa Nova'.
According to the flora of that region only one of the supposed hybrid parents is native to the area,
which would make cross pollination rather difficult!
There are also two silver variegated bushy thymes, one of which is lemon scented and the other thyme scented,
but which are otherwise practically identical.
The former is regarded as a cultivar of Thymus citriodorus and given the name 'Silver Queen',
and the latter is known as T. vulgaris 'Silver Posie' because it smells of thyme.
Quite obviously this is not a satisfactory determination of species.
In 1999 my colleague Dr. Madan Thangavelu and I carried out the Golden Thyme DNA Study, as a result of which
our native golden leaved thymes were placed within T. pulegioides rather than T. citriodorus,
in which they had mistakenly been placed because their leaves were lemon-scented.
For this study we used golden-leaved and gold variegated thymes, together with
T. vulgaris and T. pulegioides which were used as controls.
The results of the Golden Thyme DNA Study were published in Plant Heritage Vol. 9 No. 1 Spring 2002
and the new names are listed each year in the RHS Plant Finder.
Total DNA was extracted from sixteen plants comprising three species of thyme.
Random amplified polymorphic DNA (RAPD) analysis was carried out using four arbitrary oligonucleotides.
A total of 55 DNA bands were obtained, 76% of which were polymorphic.
The polymorphisms were scored and used in band-sharing analyses to identify genetic relationships between plants.
Dr. Sviatlana Trybush at Rothamsted Research analysed these data.
A tree based on the similarity matrix shows that T. citriodorus is not included in the cluster
formed by the species T. vulgaris and T. pulegioides.
Analysis of these DNA profiles, together with observations on the characteristics of these thymes,
support the decision to place the native golden-leaved thymes within T. pulegioides and demonstrate
that T. citriodorus can realistically once again be regarded as a separate species.
It is therefore proposed that it would be acceptable to revert to the original specific status of
T. citriodorus and that the gold variegated cultivars are included in the species.
Further DNA studies will be necessary to determine the status of the silver variegated thymes
and in the meanwhile they will remain in their current species.
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