Determining frost tolerance in Lavandula

The genus Lavandula includes species that are well-known as ornamental garden plants. In temperate climate regions, Lavandula angustifolia is a very popular perennial plant with good winter hardiness. Other lavender species, such as L. stoechas and L. dentata are introduced on the market as annual plants, driven by globalization and an increasing demand for drought-tolerant ornamental plants adapted to climate change. These species are often characterized by a poor winter hardiness in more northern climates with frequently occurring frost periods. Winter hardiness in lavender can be enhanced by breeding and selection, but requires well-defined, standardized testing protocols to determine the frost tolerance variation among genotypes. Field observation of winter hardiness is a common evaluation method, but has also major drawbacks, including the unpredictability of winter temperatures and field conditions such as soil water status, subjective interpretation of frost injury, considerable time investment and high maintenance cost of field trials. In this paper, we evaluated three different methods to assess frost tolerance in lavender: visual assessment of frost damage after controlled freezing, electrolyte leakage and chlorophyll fluorescence. All assessment methods were able to capture the variation in frost tolerance present among different lavender species, with similar results for each method. The number of plants to be analysed is an important decision factor when considering a screening method, e.g., chlorophyll fluorescence measurements are applicable for large-scale screenings, while electrolyte leakage is more suited to determine subtle differences in frost tolerance in small plant populations. Experimental considerations, advantages and disadvantages of each assessment technique, applied to lavender, are discussed.