Since lavender is greening up across most of Ontario, now is a good time to assess lavender plants and determine the extent of winterkill. Depending on the severity of the damage, further action may be required in some cases such as pruning out the dead growth.
The winter of 2020/21 was generally mild across Ontario with ideal conditions for lavender survival. Generally, it was drier than normal with the only really cold period coming in February at a time when plants were dormant with little chance of being damaged. Spring has been a bit more of a rollercoaster in temperatures but nothing that should have caused damage to the plants.
A plant with no winter damage should have some of the leaves from last year showing a greyish green, with new growth continuing at the tips that is bright green (Figure 1). This is true of both angustifolia and lavandin cultivars.
In some cases where there was vigorous growth last fall or a late bloom (e.g. ‘Super Blue’), the tops of all the branches are dead with new growth coming from the stem (Figure 2). It is likely that these plants will still bloom vigorously this summer despite this damage. Pruning off the dead tips will help to get more light to the new growth and may keep the plant slightly sturdier during bloom. However, pruning is not necessary and may not be worth it for large acreage when considering the cost of labour for pruning.
Some angustifolia cultivars tend to be less hardy as they age. It may be the woody growth in these plants is damaged over winter. In these cases, some branches may be dead and others showing some greening but new bright green growth is slow to develop (Figure 3). It is worth watching these plants closely over the next few weeks. If new growth develops, the plant may be fine. If the plant remains a grey-green, the branches may eventually die. New growth usually comes from the centre of the plant (Figure 4) and will be more vigorous if the branches die, since all the energy of the plant is focused on the remaining growth. All the branches may need to be pruned off close to the crown (5-10 cm height) to get more light to the centre of the plant. The angustifolia plant in Figure 3 is about 7 years old in my garden (cultivar unknown) and has gone through two cycles where the old growth has died off and had to be pruned off, letting the new growth coming from the base re-develop into a healthy bush. Plants pruned off 5-10 cm from the ground now will bloom this year, but yield will likely be reduced until next year.
Lavandins that are not damaged over the winter usually appear healthier than angustifolias initially. Some branches often die, especially in the centre of the plant, if there is some winterkill. These can be selectively pruned out to improve the look of the plant, but it is not essential. If there is no green now on any of the branches, you can check the centre of the plant for new green growth. However, most lavandins do not grow back from the base of the plant. These plants are likely dead and will have to be removed and replaced. Some winterkill may not be due to winter at all, but soils that are too saturated during winter, resulting in root death. Winter survival will always be worse in wet areas. Despite a mild winter, many lavandin cultivars are not hardy in Ontario (e.g. ‘Provence’, ‘Tuscan Blue’, ‘Seal’, ‘Fred Boutin’, ‘Alba’) and will likely have considerable damage or die in any winter.