Lavandin (Lavandula x intermedia) belongs to the Lamiaceae family and is a shrub cultivated in the Mediterranean region for essential oils used to produce cosmetic, aromatherapy, and pharmaceutical ingredients. Nowadays, interest in plant biostimulants is rising due to their ability to increase biomass production in a sustainable way. The scope of the present study was to assess the effects of two plant biostimulants, one based on hydrolyzed proteins (FITOSIM®) and one based on seaweed extracts (FITOSTIM ALGA®), on the cultivar Grosso for two consecutive years in three different farms, located in the Italian Tuscan-Emilian Apennine Mountains. A difference in the efficiency of treatments among farms was shown, probably due to the plant age. In 2020, FITOSTIM ALGA® increased inflorescence fresh weights (+35%), while FITOSTIM® and FITOSTIM ALGA® enhanced stem and total fresh weights (+23% and +22%, respectively) compared to the untreated control. In 2021, both treatments enhanced the fresh and dry weights of inflorescence (+47% and +38%, respectively), while FITOSTIM ALGA® also improved the total plant dry weights (+34%). The plant biostimulants did not affect the chemical composition of essential oils. Our results indicate plant biostimulants as a supplement for sustainable management practices, enhancing Lavandin’s performance in mountainous agricultural areas.