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Prostanthera serpyllifolia subsp. serpyllifolia – Thyme-leaved Mintbush
Plants in the Prostanthera genus (also known as Mintbushes) are now commonly placed in the Lamiaceae family, but may still be held in Labiatae by some authorities. In Western Australia there are only three widely spaced records of Prostanthera serpyllifolia subsp. serpyllifolia namely Lake Grace, Zanthus and a near coastal collection on the Nullarbor, it is also recorded from South Australia's Eyre Peninsula, but none for the Esperance region. However, the photos above were of a colony growing with Eremophila serpens (see post under that name in lower right column) on Lake Halbert, east of Mt Ridley and 70 km (43 miles) NE of Esperance.
In this unusual Lake Halbert habitat there are a number of plants, but these are very localised and restricted to thin strips of sandy loam, surrounded on all sides by samphire communities growing in a clay/gypsum loam. This is the only location I have seen these plants, but they may also exist in similar habitats on other playa lakes within the region. The other subspecies Prostanthera serpyllifolia subsp. microphylla, the Small-leaf Mintbush is quite common in the northern Esperance Mallee.
The subsp. serpyllifolia differs from subsp. microphylla in several ways, the most obvious being the leaf size, being larger with subsp. serpyllifolia from 4-13 mm (almost 3/16” to over ½”), whilst subsp. microphylla are less than 4 mm in length. The leaves are also flatter and more evenly spaced along the stem; subsp. microphylla leaves are more recurved and clustered along the stem. The petiole (leaf stalk) is longer 0.4-1.5 mm or more with subsp. serpyllifolia, but sessile or to 0.5 mm with subsp. microphylla. Also the calyx with subsp. serpyllifolia is longer 6-12 mm (1/4”-1/2”), but only between 5-7.5 mm with subsp. microphylla.
The Thyme-leaved Mintbush is reasonably bushy and grows to ½ metre (18”) in height, the red flowers (to 2 cm or ¾” in length) were very eye-catching and no doubt attracted visiting honeyeaters who also checked out the Eremophila serpens flowers whilst there. Flowering time is recorded from August to December (Flora of SA), with the above in bloom during October.