I have had several inquiries about the potential for frost damage to lavender from the frost events in the forecast because of previous events that devastated lavender in some areas. Spring frost damage is relatively rare in lavender. I am aware of two major events. The first occurred in May of 2015 in southern areas of the province. That event occurred on May 23 and temperatures recorded in the area at eye level (which forecasted temperatures are based on) were -4C. On the ground, the temperature was several degrees lower because it was a very dry airmass with very low dewpoints. Flower buds were clearly visible at the time. All of the green growth on all plants that were not covered was killed in the event.
The second occurred in areas north and east of Toronto in late May of 2021. Temperatures in Peterborough were -1.5C at eye level on May 30. Flower buds were also clearly visible at the time and flower buds were killed by the frost, but leaves in most areas remained intact.
Both of those events occurred a week or two later in May when plants were much more advanced. Lavender should be hardy to lower temperatures now because flower buds are not yet visible. In my area near the north shore of Lake Erie, flower buds are only visible if you pull apart several layers of new leaves. Based on my experience, the newly developing flower buds are sensitive to temperatures around -2C when they are exposed and the new green growth is sensitive to around -4C or possibly colder since plants are not as advanced as in previous years.
When looking at the forecast, the air temperature does not tell the whole story. As I mentioned, air temperature forecasts are based on eye level. Cold air sinks and the surfaces near the ground also radiate heat upwards on clear nights. It can be several degrees colder at ground level than the forecasted air temperature and even colder where cold air flows and settles (e.g., valleys). This depends on the humidity level in the airmass which is used to calculate the dewpoint. The lower the dewpoint, the higher the risk because temperatures can drop much quicker. The dewpoint is a good indication of how cold it could get at ground level if it remains calm and clear at night. Any wind would be a good thing because it would mix up the air and prevent it from getting colder at ground level.
Based on the available forecasts, the risk to lavender is relatively low because it is hardy perennial and it is early in the season. However, forecasts can change and if you are concerned about it or if you live in an area prone to frosts (low areas, valleys etc.) then consider putting row cover back on your plants. Consider prioritizing covering display areas or those plants needed for your best selling products.
