The weather has been unseasonably mild all winter long. Now that both warm and sunny weather have arrived, the risks to lavender from spring freezes becomes a concern. Floating row covers work great to protect lavender in late fall and winter, but they can become a liability in late winter and early spring when thinner covers can act like greenhouses due to the higher sun angle. This can ‘wake up’ the lavender prematurely, leading to freeze damage later in the spring.

Row cover on the cultivar verification trial at the Simcoe Research Station.

Normally, I remove my covers in Simcoe at some point after about March 10, when the risk of severe cold has passed, and the forecast is for sunny and/or mild weather. Further north, that decision can be delayed a week or two, or even longer if there is deep snow cover. While it can still get very cold up until early April, the risk of -20°C weather is usually low after mid-March if the forecast is good. This year, we are experiencing late-March weather at the end of February. In fact, a strong thunderstorm just went through Norfolk County as I write this and it was 9°C at 9:00 am, the typical daily high temperature for early April in this area.

Yesterday was sunny and 9°C in the afternoon in Simcoe with a light breeze, so I decided to see how warm it was under the cover at 2 pm. The temperature under the cover was a very concerning 19°C. My lavender trial is over a solid sheet of black ground cloth, which may make it slightly warmer than if it were over grass. But, this is at a time of year that the sun is still relatively low in the sky, equivalent to about Thanksgiving in the fall (October 12). Imagine the temperature difference under the cover if the same conditions happen in early April when the sun is as high as it is on Labour Day. Given the forecast is for mild weather to continue, I will likely be removing my covers over the next week. I would probably do it earlier but will take the opportunity to get a week of temperature readings under the cover first.  

Thicker covers like Hibertex Pro do not let as much light through and the air under the cover probably does not heat up quite as much compared to the outside air temperature. If you have this type of cover, it might be worth putting a thermometer under there. The thick covers also block airflow, so warmth under those covers could lead to mould growth if it is both moist and warm for an extended period.

The decision on when to remove the covers is a very difficult one. There is always the risk that the cold returns sometime in March or even early April, especially when considering removing the covers this early. However, there is also a major risk of keeping the covers on and having the lavender start to green up before cold weather returns, especially when there is no snow cover anywhere to be seen and the ground is likely not frozen at all. The entire equation changes the further north you go, so you have to base your decision on the conditions in your field. If either snow cover or frozen ground is present in the field, then you likely do not need to remove the covers quite yet.