Due to the small area of occupancy (AOO = 36 km²) and extent of occurrence (EOO = 1,292 km²) and their continuing decline, the small number of locations (four) and the continuing decline of its habitat quality, the species is classified as Endangered as it meets the criteria B1ab(i,ii,iii)+2ab(i,ii,iii).

Thymus plasonii is endemic to parts of northern Greece; it is known from the metropolitan area of Thessaloniki and the foothills of the nearby Mount Hortiatis, a locality near Petralona, Chalkidiki, and from the foothills of Mount Lailias, near Sidirocastron, Serres (Krigas 2009, Strid 2022). Another old report from the area of north west of the village Keramos in Evros (22 May 1972; E. Stamatiadou 15330; herbarium ATH) has not been confirmed during field surveys in 2008, thus needing verification (Krigas 2009). The AOO and EOO are inferred to be continually declining as a result of urban expansion.

Most of the population has been in contact with the urban and/or suburban area of Thessaloniki, the second largest city of Greece. Subpopulation surveys conducted in 2008-2009 in the urban and suburban area of Thessaloniki showed that at least two populations previously found in the suburbs of Pylaia and Panorama (Adamović 1907, Krigas and Kokkini 2005) have gone extinct (Krigas 2009). The subpopulation of Panorama was firstly discovered in 2000 and a few years later it disappeared as the local residential development expanded over its habitat (Krigas et al. 2009); the oldest records of T. plasonii from Pylaia in Thessaloniki (herbarium LD) date back in 1904 (Adamović 1907) and should be considered extinct. A reduction of at least 50% over a decade is also observed in a nearby subpopulation located in Platanakia, Panorama. It is therefore obvious that there is an ongoing decline of the species’ population during the last centuries. Finally, the subpopulations that are far from areas of intense residential development (e.g. on Mount Hortiatis) probably remain in the localities where they were originally recorded, however without explicit field data available.

No information exists about the size and threats of the two distant subpopulations in Petralona, Chalkidiki and on Mount Lailias, Serres.

Thymus plasonii grows in open habitats, on stony (limestone) slopes and meadows with low Mediterranean scrubs. Among the co-occurring species are Quercus coccifera, Paliurus spina-christi, Pyrus spinosa, Phillyrea latifolia, Juniperus oxycedrus, Verbascum undulatum, Thymus sibthorpii, Teucrium polium, Alkanna tinctoria, Anthyllis hermanniae etc. (Krigas 2009).

Residential development over the species' habitat, and habitat fragmentation and degradation are the main threats. Disturbances due to urbanisation resulting in the degradation of its habitat is evident by the invasion of alien plants such as Solanum elaeagnifolium, Symphyotrichum squamatum and Ailanthus altissima (Krigas 2009). Reduction of mild grazing, which contributes to the maintenance of Thymus plasonii habitat (open Mediterranean shrubland), may have negative consequences for the species in the future.

Although the species is protected by the Greek legislation (Presidential Decree 67/1981), no conservation measures have been taken to date. There is no in-situ protection offered by Natura 2000 sites and therefore protection of the sites where the subpopulations are found is suggested along with low-disturbance management of its habitat to maintain the open structure of the vegetation and to prevent the invasion of alien species. Monitoring of the population size and trends as well as habitat trends is necessary.