At this time of year, plants are still in the process of entering dormancy and are not as hardy as they would be in mid-winter. The recent, sudden cold snap has been very hard on many perennials including lavender because of the quick transition from mild to cold. This quick transition can result in injury at a time of year we normally do not see it. At the Simcoe Research Station, for example, the first freeze of the year occurred on November 6. Seven days later the low for the day was -13.2oC. Other areas of southern Ontario were even colder that day: Delhi -15.5oC, Hamilton -14.1oC, Kitchener -16.5oC, Mount Forest, -13.9oC, Toronto -13.9oC, Peterborough -22.9oC. However, lavender plants in colder areas of Ontario such as Peterborough may have adapted to colder temperatures earlier and may be less affected, even if the temperatures were colder.
I have established a cultivar verification and demonstration trial at the Simcoe Research Station this year with 44 cultivars in the plot. Plants are in their first year of growth. Normally it is too early in this area to cover the plants with row cover, and there was no row cover in place at the time of the cold snap. Despite a covering of 15 cm of snow on top of the plants for the coldest nights, considerable damage occurred on some cultivars (Figures 1). The cultivars that are most damaged have unknown or poor hardiness in Ontario. It appears that at least one lavandin cultivar ‘Seal’ is already almost completely dead (Figure 2).
The cultivars that are known to have hardiness in Ontario have much less damage, with some showing damage only on the tips and others with no visible damage. Except for ‘Seal’, most of the lavandin cultivars have no visible damage. This is surprising considering that angustifolia cultivars tend to much hardier than lavandins. The cultivars that bloom in the fall tend to have more damage than other cultivars. It is likely that the younger plants with their vigorous vegetative growth will have more damage from the early cold snap than older plants would.
At this time of year there is nothing that can be done to help the plants recover from the damage. It is best to leave them until the spring to determine what parts of the plant need to be pruned off. Given that the ground has now cooled, and the weather appears to remain cool in the extended forecast, row covers can be put over the lavender at any time if they haven’t been already. Row covers would likely have prevented much of the damage. However, it is risky in some years to put row covers on too early because it can result in more damage to the plants if the weather is warm and wet. Growers may need remain flexible in the future to put on row covers based on the forecast rather than a specific time of year, which may mean putting them on the day before the onset of a cold snap. Knowing when to take them off again in the spring is also equally challenging and is highly dependent on the weather.