With spring finally arriving, it will soon be time to assess how well lavender has survived the winter. This was a harsh winter for lavender, with extreme cold in December and January, a quick melt and flooding in February, and a prolonged cold period that included a freezing rain event in April. Unless the lavender was protected by consistent snow cover and/or row covers, there will likely be some damage from this winter. If row covers were kept on during the sunny but cold weather in March, they also could have some damage from warming up prematurely under the covers and then freezing again in April. Any lavender in poorly drained areas may also have been killed by flooding.
It may be too early to fully assess the damage to lavender plants, but undamaged tissue should be greening up within the next two weeks. At the Simcoe Research Station, Grosso plants are already showing some green. In these plants, the tissues at the top of the plant that were likely exposed during the cold period in January were killed (Figure 1). If they had been covered with row cover, the plants would probably have little to no damage in this location. Once the majority of plants green up, conduct an assessment of the plants to determine if any pruning is necessary. There are four types of damage that can occur: 1) The plants can be mostly healthy with a few dead branches mixed in the canopy, 2) The upper parts of most of the branches can be killed, with buds developing along the stems near the base of the plant, 3) in older plants, most of the plant can be killed with a few low side branches that were under the snow surviving and no new buds developing at the base of the plant, and 4) the plants can be killed entirely.
Figure 1. Grosso plants on April 24, 2018 showing green tissues on the outside of the plant and a dead patch at the top of the plant. This damage would probably not need to be pruned out because there is sufficient green material.
In scenario 1 above, the plants can be left alone. It may be useful to selectively prune out the dead branches to allow more light to the new shoots developing lower in the canopy, but this can be labour intensive. Whether or not you prune out the dead branches, the plant should fill in during bloom and the plant can be pruned back into a rounded shape after bloom.
In scenario 2, it is useful to prune off the dead tissues to allow light to get to the new developing shoots along the stem. How aggressively you prune should depend on how high along the stems new shoots are developing. If new shoots are only developing near the base, the tops could be mowed off close to the ground. Following the late frost in May 2015, mowing off the plants resulted in a faster plant recovery. This should not be done with a rotary mower, as this will cause too much damage to the remaining stems.
In scenario 3 and 4 above, the plant will likely need to be replaced. In older plants, even if a few side shoots are alive at the ground level, the plants don’t have much capacity to grow back into a rounded shape. You may need to wait a few weeks to ensure that buds do not develop along the stem, but if nothing develops by the end of May, the plants will not recover.