This is usually the most stressful period for lavender growers. Everything else is turning green but the lavender is still looking grey in many cases, which can often lead to the conclusion that the plants have suffered severe winter kill. It is generally too early to determine the full extent of winter damage but there are a few clues that growers can use to reduce fears of total crop loss.
Before examining the plants, consider the weather conditions this past winter and early spring compared to previous years. Winter and early spring of 2021 was an anomaly. There was almost no winter that year and temperatures in March and early April were well above normal. By late April last year plants had fully greened up only to be severely damaged later by cold periods in May and frost as late as the last week of May. This year, winter was more normal and spring temperatures to this point have been slightly below normal.
Crop progress is often estimated using Growing Degree Days, which is the number of degrees above a certain base temperature each day accumulated over the season. For hardy perennial crops, a base of 5°C is often used. In Norfolk County, for example, by April 30 of last year 170 GDD base 5 had accumulated. This year, considering the forecast up to April 30, we will have accumulated 100 GDD slightly below the 20-year average of 107 GDD. At this time of year, normally 4 or 5 GDD accumulate every day, so crop progress is about 16 days behind last year at this time. For comparison, GDD in Barrie up to April 30 will be around 55 GDD, about 10-12 days behind Norfolk County. The benefit of a cooler spring is that crop progress is delayed, meaning it is less susceptible to spring freezes. Last night, temperatures were as low as -6°C in southern Ontario, but this is unlikely to have caused any damage to lavender because it has not fully greened up. The only exception is if growers waited too long to remove winter covers and plants became too advanced prior to the freeze.
So, what should lavender look like at this time of year? First, it will depend on your location. The photos below are from Port Dover along the north shore of Lake Erie in Norfolk County. Growers south of the 401 and west of Toronto will likely be near the same stage. Growers north and east will likely be a week or up to two weeks behind.
Second, it depends on whether you used row covers and when you took them off. The photos below are from my garden and no row covers were used. Plants look to be at a similar stage in my research trials that were covered, since covers were removed in mid-March before the March sun could heat up under the covers. If you removed the covers much later, plants could be more advanced.
Plants that came through the winter perfectly healthy should be showing plenty of a dull green at this time of year as shown for ‘Melissa’ plants in Figure 1. In this plant, some of the older leaves died over the winter, but there are plenty of green leaves showing. They are not vivid green yet, but that will likely come in the next two weeks. It should be noted that ‘perfectly healthy’ is not normal for Ontario lavender, so do not panic if you are not at this stage.
Figure 1. A ‘Melissa’ L. angustifolia plant greening up in Port Dover, Ontario on April 28, 2022.
There are other plants that do not look as green but still have some green showing at the tips (Fig. 2). Green at the tips means the stems are still healthy, but you lost most of the leaves from last year. This may stress the plants a bit more, but it is likely that by the end of May, plants will be fully green, and bloom be relatively normal. Some branches without green tips may have more damage, but stems are likely still healthy.
Figure 2. An L. angustifolia plant with green only showing at some of the tips.
Another plant in my garden is showing no signs of green and leaves are fully dried out and grey (Fig. 3). A field of plants looking like this will certainly stress a grower, but it is too early to declare them dead. The next step is to snap the branches. If the branches snap easily and/or are completely brown to the core, then that branch is dead. If the branch bends instead of snaps and there is green under the skin/bark, the branch is still alive and new shoots will likely grow back from the stem if it is L. angustifolia. I am not entirely sure of the cultivar in Figure 3, but it is similar to ‘Hidcote’, and I have enough experience with this plant to know exactly the extent of the damage. The upper parts of the stem are dead, but the main stems are still green underneath (Fig. 4). This plant has had this exact damage three times so far in the past 8 years. I have to cut it way back and it will sprout back from the lower stems. There will still be a bloom this year, but it will be half as much as last year. If the plant was a L. intermedia (lavandin) with this type of damage, then it may not sprout back but it is best to wait a couple more weeks to confirm that.
Figure 3. An L. angustifolia plant with no signs of green and dried out and grey leaves and branch tips.
Figure 4. Snapping a main branch of the plant in Figure 3 reveals stems are still green below the bark. This suggests new shoots will soon emerge from the lower stems.
Finally, I have a ‘Super Blue’ plant that was in full bloom in October and now looks completely dead (Fig. 5). However, in this case the upper stems are still green under the skin (Fig. 6), and it will likely sprout back from the stems within a few weeks. Plants with this type of damage will likely have a full bloom this year, but it may not be quite as vigorous as it would be without this kind of damage (e.g. 25% less bloom). This type of damage is normal for cultivars like ‘Super Blue’ and ‘Buena Vista’ that bloom in the fall.
Figure 5. A ‘Super Blue’ L. angustifolia plant with no signs of greening-up.
Figure 6. Snapping one of the upper branches of the plant if Figure 5 reveals stems are fully green below the bark (center right).
The bottom line is that is too early to be thinking about what to do with the damage that you see. Wait a few weeks for full green up and then consider what you must prune off. Lavandins are much less tolerant of winter damage than angustifolias. If in a week or two you are not seeing any sings of green-up for those, then it is unlikely they will sprout any new shoots from the base. That will be the time to consider ordering new plants to replace the dead ones. It is rare for angustifolias to be completely killed over the winter. If you end up with dead angustifolias it is more likely that poor drainage killed the plants than true winter kill, and improving drainage should be your priority.