Row Covers for Winter Protection in Lavender

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 row covers lavender

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.

Treatment

Winter Survival Ratingz

Dundas

Freelton

Kilbride

‘Grosso’

‘Hidcote’

‘Hidcote’

‘Twickle’

No cover

5.2 ay 5.0 a 6.1 a 4.8 a

Thin cover

5.4 a 8.6 b 8.7 b

Thick cover

7.7 a 8.9 b 9.1 b 10.0 b

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 covered and uncovered

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.

About Sean Westerveld

Ginseng and Medicinal Herbs Specialist, OMAFRA
This entry was posted in Herbs, Lavender and tagged , , . Bookmark the permalink.

9 Responses to Row Covers for Winter Protection in Lavender

  1. Kristin says:

    When is the best time to apply the covers? Is mid fall too early?

    • Sean Westerveld says:

      We normally put the row covers on when the plant is no longer growing and temperatures are consistently cold. In Southern Ontario that is usually in early to mid-November when freezes begin to occur frequently. If you do it too early it may be too warm under the row cover and you may encourage disease.

  2. Edgar Anderson says:

    Sean. I plan to use agribon-70 floating row covers (2 oz weight) to overwinter my lavender plants. All plants are trimmed and growth seems to have stopped already. Would it be a good idea to install covers right now even if temperatures still get to mid 40’s during the day and low 30’s at night?….

    • Sean Westerveld says:

      Edgar. It depends on the weather forecast in your area. The lavender will not need protection at the temperatures you mentioned, but if it is forecast to get a lot colder, then I would put the row cover on. If the temperatures are forecast to increase again, you might want to hold off. Even if the plants are dormant, diseases could progress under the cover if it warms up. In most areas it is probably too early to put the cover on yet.

  3. Seamus Quinn says:

    When should new greenery be expected in the spring? ( we live in central BC) So far our plants show no signs of life after this winter.

    • Sean Westerveld says:

      Seamus. We usually see greenery by early to mid-May in Ontario. I suspect you should see it earlier in central BC. Sometimes the old leaves and tender shoots die off and new growth grows from the woody tissue below. In that case, it can be a couple weeks longer to green up

  4. Rob says:

    Hi Sean, my wife and I are starting our lavender farm in Muskoka with mostly munstead planted. We were thinking of putting the thick hibertex pro row covers over our first year plants. Do we need to put supports to keeps the material off the plants? And do you see the weight of the snow (we get quite a bit!) causing any issues? Thanks for your help!

    • Sean Westerveld says:

      Hi Rob. We have not used any supports for the row covers. If the plants are young and very green, the row cover can cause the branches to bend over. In this case, a support may be beneficial, especially in your area. If they are a more rounded shape and more woody, we see no major damage from the cover. In fact, in periodic heavy wet snows the row cover tends to prevent larger plants from splitting open in the middle.

  5. Christina Hary says:

    Hi,
    Thank you so much for all your hard work and experiments and sharing the results. It makes growing easier to know where to find valuable information.
    I am a small grower of lavender (600 plants) and garlic (approx. 18,000 plants) in the Uxbridge area. I am trying to find a source to purchase winter covering for my lavender, can you point me in the right direction?

    Respectfully
    Christina

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