By: Sean Westerveld, OMAF and MRA and Cathy Bakker, University of Guelph

Some lavender varieties are marginally hardy in Ontario, and most varieties can be damaged by winter conditions. Protection over the winter can reduce damage. Snow cover provides excellent protection for lavender, but is usually inconsistent in coverage and timing and cannot be relied upon for optimal protection, especially towards the end of the winter. Row covers have potential for providing protection for cultivars that are not hardy in Ontario, such as ‘Provence’, or in colder regions of the province. They could also be used to protect display gardens to ensure healthy looking plants. However, no scientific trials have been conducted in Ontario to determine if they are effective for lavender, or if they contribute to other issues such as foliar disease.

Over the past winter three row cover trials were established on lavender farms near Dundas, Freelton and Kilbride, Ontario on either angustifolia or lavandin cultivars. The cultivars used for the trial are relatively hardy in Ontario, based on the lavender variety trials currently underway across southern Ontario. A small trial was also conducted with ‘Twickle’ lavender at one site, which is known to be much less hardy in Ontario. The trials compared a thin and thick white row cover to uncovered plants (Fig. 1). The row covers were placed on the lavender on December 5, 2012 and removed on April 17, 2013 and plants were assessed on May 14 for winter survival on a 0 to 10 scale (0 = dead, 5 = half dead, 10 = no damage).

Fig. 1. Thin (back) and thick (front) row covers were compared for winter protection in lavender.

The results of the first year of the study are summarized in Table 1. Both row cover types significantly reduced winter damage of ‘Hidcote’ lavender compared to uncovered plants at both Freelton and Kilbride. The thick row cover resulted in no winter damage to ‘Twickle’ lavender at Kilbride, while uncovered plants were more than half dead at the time of assessment. At Dundas, the thick cover had the highest numerical winter survival rating, but no significant differences could be found due to variability. A freeze occurred shortly after removal of the row covers, and this may have caused additional damage to covered plants in a low area of the trial at Dundas. Row covers may need to be kept on the plants until any hard freezes have passed, or re-applied to the plants before any hard freezes. However, it is unknown if this would increase the risk of foliar disease, since there is poor air flow under the row covers.

Table 1. Winter survival ratings of lavender plants covered with thin (Agryl P-40) and thick (Hibertex Pro) row covers over the winter of 2012/13.

^z Rating scale: 0 = dead, 5 = half dead, 10 = no damage.

^y Numbers in a column followed by the same letter are not significantly different at P=0.05, Fisher’s Protected LSD Test.

Fig. 2. ‘Grosso’ plants uncovered (left) and covered with a thick row cover (right).

While conclusions cannot be made on only one year of data, the results to date are interesting and warrant further investigation. Trials will continue for the next two years and will be expanded to less hardy cultivars. The row covers may also affect the start of the bloom period, and this will be tracked over the next few weeks. One factor to consider is the cost of the row cover and its application. Thicker covers cost more. The thick cover is also very bulky and would require substantial storage space over the summer if used on a large scale.

This project would not be possible without the funding provided by a research grant from the OMAF and MRA New Directions Program through the Ontario Lavender Association. We thank the cooperating lavender farms for their assistance.