The frost on May 23 caused damage to the tips of many of the perennial herbs in the mint family in some areas of Ontario. Damage is mostly confined to the growing points and the leaves on the top 5 cm of the shoots. Affected plants include mint (Figure 1), winter savory (Figure 2), oregano and marjoram (Figure 3), bergamot (Figure 4) and sage (Figure 5). Lemon balm was the most significantly damaged with much of the new growth killed off (Figure 6). Lavender was also damaged, but is covered in another blog post. While there have been no reports from thyme, it is likely that damage occurred to thyme as well. In some cases the leaves look healthy but the growing point has been killed (Figure 7).
Figure 1. Frost damage to mint.
Figure 2. Frost damage to winter savory.
Figure 3. Frost damage to oregano. Similar damage occurred for the marjoram types.
Figure 4. Frost damage to bergamot. In this case the affected leaves turned yellow but did not die.
Figure 5. Frost damage to garden sage.
Figure 6. Severe frost damage to lemon balm.
Figure 7. An oregano shoot with mostly green leaves but a dead growing point.
It is important for growers to closely examine plants for the damage, especially if it only occurs at the tip. If this is not dealt with now, the plant will branch out and the result may be an unmarketable shoot for cut herb production. This would further delay a normal harvest.
Most of the herbs grown in Ontario are for cut herb production. In this case, if there is sufficient new growth for a harvest and only the growing point is damaged, growers have the option to harvest as normal without a significant reduction in quality. However, in all other cases it is best to prune back the amount that would normally be harvested to remove the damaged tissue. This material will need to be discarded or used for other purposes. This will allow the new growth to develop for a normal second harvest. More severely damaged plants may need to be cut back more significantly and this will delay the second harvest.
For dried herbs, only the damaged tissue would need to be removed because this tissue will degrade product quality. New shoots will form from the stems. For essential oil production (e.g. mint), plants can be allowed to grow out of the damage on their own.