Lavender growers should recognize that this is not good weather for the crop. Lavender does not like “wet feet” and flooded and saturated soils should be avoided. If water is pooling in the fields, you need to consider finding a way to get it out of the field as soon as possible. Saturated soils can reduce the amount of oxygen that gets to the roots and they can die after prolonged periods of saturation.
Saturated soils can also increase disease development in the roots, especially Phytophthora root rot. Phytophthora nicotianae, the causal agent of the disease, is an oomycete (water mould) and produces swimming spores called zoospores. They can move through saturated soil from root to root causing rapid spread of the disease. All lavender growers should watch plants closely for the disease over the next few weeks. Affected plants will show sudden wilting and death of the new growth on parts of the plant or the whole plant. Any affected plants should be removed from the garden along with two neighbouring plants on either side of the affected plant, since these may already be infected.
There is also the potential for frost early next week. Forecast lows for Sunday and Monday night currently range from -3oC to 1oC across southern and eastern Ontario. On May 23, 2015, a severe frost caused major losses for lavender growers due to death of all of the new growth on the plants. That night, ground level temperatures were as low as -8oC. Higher dewpoints forecast for this potential frost event, due to the wetter conditions, may prevent temperatures at the ground level from getting as low. However, forecasts can change so growers should monitor weather forecasts closely over the next few days.
The other difference with this frost event versus the 2015 event is that the plants are not nearly as advanced as they were in 2015. They may be able to handle lower temperatures this time around. The exact temperature necessary to damage lavender is not known, but temperatures a couple degrees below freezing are unlikely to cause damage at this stage. Bear in mind that temperatures on the ground can be lower than what is forecast because forecasts are based on temperatures at eye level. For spring frosts, there do not appear to be any cultivars that are less or more susceptible. All cultivars of both angustifolia and lavandin types can be affected.
If a significant freeze appears likely, growers should consider putting row covers back over plants. While this can be labour intensive, it is the only practical way to protect lavender plants. Consider putting row covers over the amount of plants that you cannot afford to lose (e.g. your main display fields, plants necessary for your core products etc.). Since cold air tends to pool in low areas, these may be areas to focus on for frost protection. One or two days of labour is a small price to pay if it saves a significant portion of your crop.