Most cultivars are now finished blooming and oil harvest will be nearing completion soon. This is an important time of year to ensure good growth before winter. There are three key things that should be done at this time of year:
Pruning should be done soon after harvest to reshape plants and allow time for regrowth before winter. The only exception would be if plants are highly stressed and pruning would result in the loss of the majority of the green leaves. In this case it is best to allow some new growth for a couple of weeks prior to pruning to ensure the plant can recover. This is the time of year to do the most aggressive pruning. Approximately one third of this year’s growth, not including the flower stems should be removed. This will get plants back into a rounded shape and ensure branches are stiff enough to not collapse open over winter or during next year’s bloom. A second lighter pruning is possible after a few weeks if plants are vigorously growing.
More vigorous pruning may be necessary for old plants or plants that you are trying to rejuvenate. However, it is important to avoid pruning more than half of the new growth or the plant could die. Instead, consider pruning less aggressively a few times in the year rather than all at once.
After the stress of blooming, this is a good time of year to fertilize plants with nitrogen to encourage new vegetative growth. Our fertigation trial is starting to show differences among treatments with anything over 80 kg/ha of nitrogen (based on a 60 cm row width and ignoring the grass between the rows) showing improved growth and yield. This can be split among a few applications over the season, with at least one of those applications right after bloom, or applied entirely after bloom. In our trial, we are applying the nitrogen in three applications at mid-bloom, after bloom, and in mid-August. To get that nitrogen down to the root zone, it either needs to be applied by fertigation through drip irrigation lines, water soluble fertilizer applied in liquid form, or applied as a granular fertilizer followed by a rainfall or overhead irrigation. Organic producers will still require the same amount of nitrogen, but applied in a form approved by your certifying body.
Phosphorus and potassium applications should be based on a soil test and are best applied prior to planting followed by small supplemental applications made once per year either in the spring or after bloom. Because these nutrients are immobile in the soil it is difficult to get them down to the root zone during the growing season. A liquid fertilizer is the best option for applying these nutrients, which would have to be accompanied or immediately followed by sufficient irrigation to wash them into the root zone. Click here for a guide to fertilization of specialty crops like lavender that don’t have specific Ontario guidelines.
Regardless of the amount of fertilizer you apply to the lavender, there will be minimal regrowth if there isn’t adequate soil moisture. When the plant is moisture stressed the stomata, or small pores on the underside of the leaf that allow for air exchange, close and photosynthesis and uptake of nutrients virtually stops. Ensure plants are adequately irrigated by constantly monitoring soil moisture levels either by hand or using soil moisture monitoring equipment. Click here for more information on soil moisture monitoring.
All three of these practices should be done before the end of August at the latest. Beyond this point, it is important to allow the plant to harden off before winter. Irrigation is necessary throughout the year whenever soil moisture is low, but fertilizing and pruning too late will encourage succulent new growth that is more susceptible to winter injury.