Frost damage to new growth has occurred in many areas of Ontario. In some cases the damage is severe and caused all of the new growth to be killed, and in other cases the plant looks healthy but damage has occurred to the developing buds. In both cases, new shoots coming from the lower in the plant are still intact and can be encouraged to replace the dead tissue, but bloom this year will be severely impacted. How to deal with the damage from this frost will depend on the level of damage and the health of the plant going into the frost. This is the first time we have seen this kind of damage to lavender in the spring in Ontario. How to deal with the damage from this frost will depend on the level of damage and the health of the plant going into the frost. Since we have not dealt with this issue before, there are many unknowns on how to respond.
In the more severe cases, all of the new growth on the outside of the plant has been frosted off (Figures 1 and 2). Younger plants also had major damage (Figure 3). The stems may still be alive, but it is unknown how they will recover from the damage. If the plants were already thin from winterkill the last two years, then it is likely best to prune off all of the outer tissue to get light down to the new growth developing from the base (Figure 4). This will allow the new growth to develop stronger stems that will form a denser canopy and will be better able to survive next winter. However, if the plants were very healthy and formed a dense canopy before the frost, it might be best to wait to see if new buds form from the stem. The bloom will still be impacted, but the plants will be larger going into next year. It is a good idea to try it both ways on a few plants to see which plants recover best, so the decision will be easier in the future.
Figure 1. New growth on lavender killed off by frost on May 23, 2015.
Figure 2. Several days later the new growth has dried up. The stems still show some green that may develop healthy new shoots.
Figure 3. Young ‘Phenomenal’ plant with severe frost damage.
Figure 4. Plant pruned back to allow more light to get into the crown of the plant. Care should be taken not to cut off the flower bud on these shoots.
Some plants look healthy from a distance, but a closer inspection reveals damage at the very tip of each of the shoots (Figure 5). This damage, which may not be apparent, would result in death of the developing flower bud. More mature buds, such as from plants under row covers earlier this spring or from stems closer to the ground tended to have less damage.
Figure 5. A healthy looking plant from a distance. On closer inspection the tips of each of the stems including the flower buds are frosted off (Photo courtesy Cathy Bakker, Univ. of Guelph).
It is very difficult at this time to determine how to best deal with this situation, and it will depend on how the plants looked going into the frost. For really thin plants, it is probably best to prune out the dead growth to allow new growth at the base of the plant to develop. However, if the plants were relatively thick and healthy going into the frost, it is probably best to leave them intact and wait for new secondary buds to develop from the stems, which should still be alive.
In either situation, if pruning back to the base of the plant, care should be taken not to cut off the developing flower buds. These buds are probably the healthiest blooms that will occur this year. Hand pruning is probably the best option because it allows for careful removal of only the dead tissue. Mowing off the new growth is a labour-saving option, but with potential to cut off developing buds and to cause damage to the stems. Powered hedge trimmers would provide some labour savings while still providing better accuracy.
After the initial bloom in July, most damaged plants will try to send up new flowers sporadically throughout the summer. Some growers may rely on these flowers for sales and development of products, due to a shortage during the main bloom period. However, for the health of the plant it would be best to prune off developing buds to encourage healthy new growth for next year. This will be an economic decision that each grower will have to make.
Hi! Thank you so much for this post and all the research on lavender being done in Ontario. I refer to you often. I’m in western Montana. The plants are green at my farm but not showing any bud yet. We’re expecting 20° with 7-10mph wind all night the next couple nights. I have a blanket for every row. You think I should throw them on? I don’t see that you said what month or temperature extreme in this post.
If the plants are already actively growing, then they are sensitive to that kind of cold and you should attempt to cover them. If they are just beginning to green up, they are probably hardier and should be okay. If you have the option to cover them, then it wouldn’t hurt. The sensitivity of lavender to cold decreases as they green up, so there is no exact temperature at which they will be killed. Temperatures of -2C (28F) are OK up to early bloom bud development.