Although the winter got off to a sudden harsh start in early November, all indications so far suggest this has been a pretty good winter for lavender survival. Some new growth at the tips of the shoots was killed off by the sudden freeze in November, but very little damage occurred since then, at least from the cold. In areas that received heavy snowfall, rodent feeding damage can be a localized issue. It was a relatively wet winter again, so plants that were in heavier or poorly drained soil could also have significant dieback. In most places the soil never froze, so any moisture could drain away.
Row covers should be off by now in all areas, although the timing of row cover removal is not an exact science. Keeping them on too long can cause the plants to break dormancy too early and be more susceptible to cold. Research has shown that temperatures under a thinner cover can be over 10oC warmer than the ambient air due to the more intense sun at this time of year and the greenhouse effect under the cover. The covers were removed from the cultivar research plot in Simcoe in early March, and plants are still looking good despite a few cold periods since then.
There is still the risk that abnormal cold weather can come as late as late May or early June and damage plants. A severe freeze on May 23, 2015 killed all the new growth on most lavender in the province (Figure 1), which devastated agritourism and harvests for that year. If you have the option to keep the covers rolled up next to the field, it may be helpful for re-covering plants for a night or two if the weather forecast warrants it. Typically, the cold that damages plants in the spring occurs on calm, clear nights. Redeploying the covers on these nights may not require as many weights or ground staples to secure the covers as are required for securing them for the winter.
So, what temperatures can damage plants at this time of year? There has been no research on the sensitivity of lavender to cold at different development stages. A rough estimate based on experience with other plants and lavender suggest that plants can tolerate as low as -10oC at a normal stage of dormancy up to mid-April. We are a little ahead of normal this year, and plants may be a little more sensitive at this time. Once the new green growth starts to emerge, plants may be sensitive to air temperatures of -5oC. By the time new buds are visible, as was the case on May 23, 2015, plants are likely sensitive to -2oC or colder. Different cultivars are sensitive to different temperatures. Wind can add an additional variable, although the coldest temperatures after about April 10, usually occur on clear and calm nights.
For growers that had row covers, plants can look really healthy and green when you first take the cover off, especially for thick covers (e.g. Hybertex) (Figure 2). However, it is common for them to go a little greyish green once exposed to the wind and sunlight again for at least a few weeks. Even uncovered plants often look worst in early April a few weeks after snow melts. New bright green growth will likely become apparent in late April or early May. It is best to wait until the new growth is apparent to determine if any winter damage occurred and whether that damage warrants any pruning or plant removal.
Figure 2. Uncovered ‘Grosso’ plant (left) next to a ‘Grosso’ plant covered by Hybertex Pro row cover (right) on the day of cover removal. The bright green of the plant on the right faded a little before blooming slightly earlier than the uncovered plant.