Ontario lavender growers should consider putting on row covers over lavender in the next two weeks. Row covers should be put on when lavender plants are mostly dormant, which usually occurs once night temperatures average below freezing. They definitely need to be put on before any substantial snowfall occurs that is likely to remain on the field into winter. If ground staples are being used, they should also be put on before the ground freezes or it will be very difficult to get them into the ground.

Following the extremely cold winters of 2013/2014 and 2014/2015, there was major damage to most lavender cultivars. Damage was particularly extreme in the lavandin (Lavandula x intermedia) cultivars.  Many growers lost most or all of their lavandin production for the following year and some had to replant their whole fields. Research showed a significant improvement in winter survival with white row covers (moderate thickness of around 40 g/m²) over that same period. In a harsh winter, row covers could be the difference between no crop next year and a normal healthy crop.

Row cover experiment in progress in 2014.

The extremely mild winter of 2015/2016 may give the false impression that you can get away without the covers, especially for lavandin cultivars. New growers may not have experienced the damage from those two previous winters. Last year’s mild winter was primarily due to the effects of a strong El Nino, and that is not occurring this year. Most forecasts suggest that we will have average to below average winter temperatures this year. An average winter usually results in some winter kill, depending on the amount of snow cover at the time of the coldest temperatures. Even a warmer than normal winter can include a cold period where plants can be killed.

When deciding whether or not to put row covers on your plants consider the following:

1. Hardiness of the cultivar. Most lavandin cultivars will benefit from protection over the winter. Most Lavandula angustifolia cultivars are hardier, but may have higher yield if they are protected over the winter.
2. Age of the plants. Young plants (1 or 2 years old) are more vegetative near the crown and can survive winter better than older plants. If damaged, they can easily grow back from the crown. The exception would be plants transplanted late this year that may not have fully established root systems.
3. What are you willing to lose? The potential damage to plants this coming winter is unknown. Historically, 2 of the last 20 winters were severe enough to cause major damage to lavender, but those two winters happen to be 2 of the last 3 winters as well. Other winters have been highly variable in temperature and have also resulted in damage that would be reduced by row covers. If you consider the worst case scenario, without the use of row covers you could lose 100% of your lavandin production for next year and may have to replant all of it, and 60-80% of your Lavandula angustifolia production next year, with somewhat full recovery the following year. At a minimum, you should cover the amount of plants that would get you through a bad year if the worst was to happen. This may be the display areas that are open to the public, a few plants of each cultivar for taking cuttings, and/or the amount of each cultivar that you would need for your core products.
4. How sheltered the plants are from winter winds. With the exception of damage from “wet feet” over the winter, most of the damage from severe winters is due to drying winter winds when there is low snow cover. In highly sheltered areas with evergreen hedgerows or fields surrounded by forests, damage can be less severe without a cover. These are also areas where snow tends to stay around the plants.