Summer is winding down, but vegetative growth of the lavender will continue for another month or more. This is a critical time for lavender when it has recovered from flowering in July and is now putting on new growth that will hopefully increase bloom for next year. Here are some things to think about during this period.
Some areas have had a dry second half to summer resulting in moisture stress to the lavender. Even at this time of year when temperatures are decreasing the plants can still suffer moisture stress and growth can be stunted. Monitor the soil moisture levels closely and ensure plants are irrigated when necessary. Control of weeds is also important to avoid competition with crop for moisture and nutrients.
While vegetative growth is important at this time of year, earlier fertilization should have supplied the needs of the crop. Fertilizing too late in the year can promote too much vegetative growth later in the fall and this could reduce the hardiness of the plant over winter. Only fertilize now if the plants are showing deficiency symptoms (e.g. yellowing leaves) and keep rates low (e.g. <20 kg/ha N).
Generally, pruning late in the year is not recommended because it can stimulate young vegetative growth that will be more prone to winterkill, and it can create wounds that may not heal in time for winter. Some light pruning of plants can be done to remove flowering shoots, which can reduce new growth and will die over the winter anyway. Light pruning can also be used to keep the plants in a rounded shape, since shoots that stick out from the main plant are likely to be more exposed to wind and die over the winter as a result. Experience in Ontario suggests that the bulk of the pruning for lavender should be done after bloom in August.
Septoria Leaf Spot
Septoria leaf spot is the main pest of lavender at this time of year (Figure 1). It shows up first on older leaves, especially in areas with poor airflow. It is especially severe if the plants are stressed by either poor nutrition or low soil moisture. The impact of this disease on the growth of lavender is poorly understood. By reducing the healthy leaf area, and therefore photosynthesis, it would have an impact on plant growth. The added stress of the disease could also impact winter survival. There are no fungicides (organic or conventional) registered for control of this disease on lavender. Reduction of this disease depends on reducing stress on the plant through irrigation and fertilization and reducing humidity around plants by increasing airflow (e.g. weeding, pruning nearby vegetation).
Figure 1. Septoria leaf spot within the canopy of a lavender plant.
Preparing for Winter
Based on last winter, it now appears that row covers are essential for winter protection of lavender in Ontario (Figure 2). This includes both lavandins (Lavandula x intermedia) and angustifolias (Lavandula angustifolia). While both types may go through some winters fine without protection, weather is becoming more variable and the risk is too high not to cover the plants. Even if the winter is not particularly harsh, warm and wet falls and prolonged cold and wet springs seem to be increasing, and these contribute to winterkill. At the very least, it is important to cover enough lavender to keep your operation going if nothing else survives the winter. Now is the time to purchase the covers and plan for how you will manage to cover the plants and weigh them down. Most growers used 30 ft (9 m) wide sheets of row cover and cover many rows at once. This reduces the labour for weighing or pegging down the cover down every row, but still requires substantial weights (e.g. rocks, sand bags, wooden posts) to prevent the row cover from blowing off in winter winds. Availability of labour in November or December when the covers need to be put on seems to be the biggest barrier to the use of rows covers. There are crews that can be hired for a day to come and do manual labour tasks if labour availability is an issue in your operation.
Figure 2. A covered lavandin plant (left) beside an uncovered plant (right) showing the benefits of row covers in Ontario.