Water Use and Cold Hardiness of English Lavender

English lavender (Lavandula angustifolia) is an herbaceous perennial that is utilized as a landscape plant and edible herb.  This perennial is difficult to grow in greenhouse production and is not reliably cold hardy in hardiness zones lower than 6. Our primary objective was to determine the water requirements of two English lavender cultivars (‘Hidcote’ and ‘Munstead’) in greenhouse production.  We also wished to determine whether substrate water content (Θ) of English lavender impacts cold hardiness.  English lavender ‘Hidcote’ and ‘Munstead’ were grown at four different Θ (0.1, 0.2, 0.3, 0.4 L·ŸL^-1) in a capacitance sensor automated irrigation system.  Plant height, width, and dry weight of English lavender was significantly greater as plants were grown at higher Θ.  Inflorescence number of both cultivars was greater when plants were grown in substrates with more water.  Leaf level net photosynthesis of ‘Munstead’ increased with increasing Θ; this was most likely due to an increase in stomatal conductance as plants were grown at higher Θ.   When cold hardiness was estimated by determining electrolyte leakage from leaves exposed to temperatures ranging from –2 to –42 °C, ‘Munstead’ grown at 0.10 LŸ·L^-1, had a significantly lower T50 (temperature killing 50% of leaves).