Hopefully winter is finally over. Thankfully the cold weather in April and early May kept plants from getting too advanced and likely prevented significant damage to the lavender over the past week. The only plants that may have damage are plants that were too advanced due to the winter row covers being kept on too long. Most plants with this kind of damage will recover from new buds.
This has been a very unusual year, with most of the “winter” damage occurring in a hit of winter in November and potentially again in late April and early May, and no damage over the relatively mild winter.
As plants begin to green up significantly over the next week, it will be a good time to conduct winter damage ratings of the lavender. Winter damage ratings can help you in comparing cultivars for their performance on your farm, and to determine if there are any areas of the farm that result in more damage. This can help you correct problem areas through improved drainage, windbreaks, or better winter protection.
The best way to assess winter damage is to estimate the percentage of the canopy of the plant that is visibly green. I use a scale of 0 to 10, with 0 representing a dead plant, 1 representing 1-10% green, 2 representing 11-20% green all the way up to 10 representing 91-100% green (Figure 1). Conduct the assessment on at least 10 plants of each cultivar in each distinct area of the farm (e.g. low vs high areas, windward vs leeward areas, sandy vs loamy areas) and then take the average. The more plants you do, the more accurate the assessment will be. Map the results to look for patterns.
There are different types of damage that can occur:
- Dead leaves (Figure 1). In a good year, the leaves from last year should green up and remain active in the spring. However, winter damage and/or Septoria leaf blight in the fall can result in those leaves dying while the stem and tips remain alive and sprout in the spring. This can result in the plant looking completely dead early in the spring and recover later than normal. This type of damage is mostly confined to angustifolia cultivars. Plants with this type of damage would bloom later than normal and bloom may not be as vigorous as it should be.
- Dead branches (Figure 2). Both types of lavender can have whole branches die over winter, while other branches nearby are healthy. Often the dead branches are near the top of the plant where they may have been more exposed to the wind. If only a few branches were killed, shoots from nearby branches will quickly fill the hole. If many branches were killed, the plants may have a hole in bloom or a very patchy bloom when looking at the plant close-up. For angustifolia cultivars, new shoots can emerge from lower on the stem and fill the hole by the end of the season. For lavandins, the whole branch will likely have died. Removing the branch can help new shoots emerge from the base of nearby healthy branches.
- Dead roots/crown. Severe winter kill, wet conditions over winter, and/or freezing rain/ice can cause the roots or the crown of the plant to be damaged or die. This would result in either a completely dead plant in the spring, or a plant that looks okay initially and then quickly dies as new growth tries to emerge and the weather warms and dries. These plants will not recover and will have to be replaced.
Figure 1. Two ‘Grosso’ lavandin plants with the right one covered with row cover over the winter. The left one lost many of its leaves over the winter but the stems and tips remained alive.
Figure 2. An angustifolia cultivar with dead branches in the centre. This plant fully recovered by mid-summer with new shoots emerging from lower on the stem.
Figure 3. ‘Folgate’ lavender killed completely over a wet and cold winter.
The three types of damage can affect your winter damage ratings in different ways. In case #1, the rating might be poor early on because of the visibly dead leaves and small developing shoots, but the plant could be perfectly healthy by mid-summer. In case #2, the rating will likely be accurate throughout the spring. In case #3, the plants may get decent ratings early on and then the plant dies as it warms up. As a result, it is a good idea to conduct the winter damage ratings both in May and again in June to get a better idea of the type of damage that occurred.