This has been another challenging winter for lavender. This past winter had colder minimum temperatures than last year, but snow cover was generally more consistent in many areas. If plants were completely covered in snow in February, there may not be as much damage as last year. Last year we learned a lot about how to deal with winter damage. Here are some things to think about over the next few weeks.
For those using row covers, now is the time to consider removing the cover. As temperatures warm, there is the risk that the humid conditions under the cover could lead to decay of the plant material. Thinner covers that allow a lot of light through may also promote faster growth, which may make the plants more susceptible to late frosts once the cover is removed. Even if a thin cover is kept on the plants, they may be susceptible to damage if a deep freeze occurs later in the spring. However, thin covers could also result in early blooming, which would be a technique to extend the bloom period. This could be beneficial for farms open to the public, or for spreading out labour requirements. No research has been done to determine if this is a viable option.
The extent of winter damage does not usually become evident until later in the spring. Last year, damage ranged from loss of a few branches to complete death of the entire plants. Here are some things we learned about winter damage and recovery of these damaged plants:
- Lavandins are not fully hardy in Ontario. Last year many lavandins either had severe damage or died, whereas angustifolia plants were damaged but recovered by the end of the growing season. Row covers are recommended for lavandins to protect plants over winter.
- Young plants come back better than older plants. Many of the first year lavandin plants, even if many of the branches died, came back from the base of the plant (Figure 1). Older plants did not recover as well from the winter damage. Comparing plants of the same age, angustifolia plants recovered much better than lavandin plants.
- Substantially damaged plants grow back better if the dead material is pruned away (Figure 2). Leaving the dead parts of the plant on the plant shaded out any new growth from the base and resulted in elongated and weak new growth. If the plants were pruned or mowed close the crown, this new growth was much stronger and resulted in a much more rounded plant, although smaller than the original plant. This was especially true of angustifolia cultivars that tended to sprout more from the base of the plant.
Figure 2. A Lavandula angustifolia cultivar with winter damage in spring of 2014. The dead material shaded any new material coming up from the crown and resulted in weaker new growth by the end of the growing season compared with plants that were mowed down to open up the centre of the plant.
- A plant that looks dead by mid-May probably is dead. Last year we expected that plants with most or all of the branches killed could sprout back from the base of the branch or from the crown. However, plants with no green growth did not sprout back, and plants with this level of damage should be replaced. Many lavandins had some life in a couple of small branches near the ground (Figure 3), and by the end of the year, these branches formed tufts of new growth well away from the centre of the plant. These plants probably need to be pulled out simply due to their uneven appearance. Since leaving those branches on the plant is not a viable option, the only way to deal with the plants is to prune them back and see if new growth will emerge closer to the crown.
If plants were not substantially damaged over the winter, spring pruning should be kept to a minimum. Any pruning that occurs shouldn’t be done until early May when new growth appears. A hard pruning of plants prior to bloom can cut off many of the developing flower buds. While new buds will still emerge, bloom can be delayed and staggered over a longer period. Spring pruning should include removal of any dead material and minimal rounding of the plants to remove any branches sticking out of the canopy. Hard pruning is best done after flowering in mid-summer.