Ornamental traits of Lavandula angustifolia Mill. are affected by geographical origin and cultivation substrate composition

Lavandula angustifolia Mill. (true lavender) is a small perennial shrub belonging to the Lamiaceae family, greatly appreciated for its ornamental traits and for the aromatic and medicinal properties of its essential oils. It is among the most cultivated species in Italy, together with Lavandula latifolia Medik. (spike lavender) and Lavandula × intermedia Emeric ex Loisel. (lavandin). It is the only native Lavandula species that can be found in the Western Italian Alps, located between 500 and 1800 m of elevation. Altitudinal and latitudinal variations can influence the morphological characteristic of plants within species, thus leading to potentially interesting ecotypes for ornamentals production. With the aim of identifying interesting genetic resources, we explored nine wild populations of L. angustifolia along different altitudes and latitudes of Western Italian Alps. Besides, three different cultivation substrates (70:30% peat:green compost; 70:30% peat:demolition aggregates; 40:30:30% peat: green compost:demolition aggregates) were tested to select the most suitable for lavender pot production. After two cultivation cycles the main qualitative (growth habit, density of the shrub, spike shape, leaf and flower colour) ornamental traits were recorded, according to the guidelines of the International Union for the Protection of New Varieties of Plants for lavender (UPOV, TG/194/1 Rev.). Colours of foliage and flowers were recorded using the Royal Horticultural Society (RHS) colour charts. Results highlighted that qualitative ornamental traits were mostly related with the latitude of origin of plants, affecting plant growth habit, colour of the leaves, and colour of the flowers (plain or double; RHS colour group). The substrate used for cultivation was related with plant density and both flower colour characteristics, while altitude related only with these latter two characteristics. The shape of the flowering spikes was not related to any of the considered factors.