Just like in May 2015, frost damage to new growth has occurred in some areas of Ontario, mainly north and east of Toronto. The following article is adapted from 2015 with new insights based on what we learned during that event. New insights are highlighted in bold.
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.
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.
The buds on the very tip of the branches all sprouted and emerged this spring. There are very few if any bud initials that can emerge from the outer 2-5 cm of the stem if all of the new growth was killed by the frost. There appears to be no further harm in pruning off this dead growth to allow more light to get at the new growth emerging lower down. If there is little growth in the centre of the plant now, more buds will likely emerge over the next week. Be careful not to cut off flower buds on new growth emerging from the centre of the plant. For lavandin cultivars, do not prune them back too severely. New growth will not emerge from woody tissues.
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.
Our experience in 2015 suggests that the outer 2-5 cm of the stems may also be dead. Opening up the canopy by pruning away the dead growth may result in more vigorous new growth in the centre of the plant, but is not essential. Eventually the new growth will emerge past the dead growth, but unpruned dead growth will impact the appearance of the plants at bloom.
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. In 2015, this forecast was verified. Some growers partially made up for a delayed and weakened bloom with sporadic re-blooming for the remainder of the year.
What we learned in 2015 is that all is not lost. Agri-tourism activities still occurred with a less vigorous bloom than anticipated. For those with new plants, they may be more severely impacted, but the stem could still be alive. It is advisable to wait a week or two before replacing any plants to give time for any new buds to develop.
Lavandins may be less affected from the frost because flower buds are less developed. However, even if they were affected, our experience from 2015 suggests that new growth will emerge from the stems. It is best not to cut these back severely, because new buds are less likely to emerge from the woody stems. Ensure some green stems and older green(ish) leaves remain after pruning.