Irrigation

Much of the summer has been hot and dry, a climate that lavender is used to. However, lavender in Ontario may have struggled with moisture stress because root systems are not as extensive as they are in the plant’s native Mediterranean region. This is partly due to root pruning that occurs over winter when water tables rise and flood the lower root system, and because the roots do not need a deep root system for most of the year. Moisture-stressed lavender will not put on much vegetative growth after bloom and is more susceptible to some insect pests as well as winterkill in the coming winter. To help plants recover, continue to irrigate if rainfall has been limited in your area.

Insects and Diseases

I probably mention this too frequently, but the worst insect pest at this time of year is the garden fleahopper. These fleahoppers are very small and hidden within the canopy, and their damage often goes unnoticed or undiagnosed by growers. Damage appears as a silvery mottling of the older leaves in the canopy (Figure 1).  Affected plants can look like they are moisture-stressed or just not very vigorous. Close inspection, along with vigorously shaking the plant, will reveal tiny black, hopping adults and even smaller green nymphs. Garden fleahoppers have several generations per year and generally build up over the summer, with peak damage occurring in August when plants are stressed after bloom and putting on new vegetative growth. They appear to reach the highest populations on plastic mulch, potentially due to the monoculture and a lack of natural enemies that may be more numerous in mowed grass areas. Populations of garden fleahopper can reach hundreds, if not thousands, per plant and can cause complete defoliation (Figure 2). Insecticidal soaps registered for aphid control in lavender may suppress populations of garden fleahopper, especially if applied when the green nymphs are more numerous.

Figure 1. Mottled leaves due to garden fleahopper feeding.

Figure 2. A plant with severe defoliation, mostly due to garden fleahopper feeding (Image taken Aug. 29 in the Simcoe Research Plot).

Dry conditions in the area of a lavender field can cause other insect pests to move into the lavender, leading to damage, since the lavender is often still green long after neighbouring pastures have turned brown. This includes false chinch bugs, leafhoppers, and other sucking insects. Feeding by aphids and some other insects can also lead to the transmission of alfalfa mosaic virus, which causes bright yellow sections on stems and leaves. If this is found in your fields, pay special attention to insect management and regularly sanitize pruning and harvesting equipment.

Lingering issues with unknown root diseases are still being reported from many farms. The most common symptom is random yellowing followed by wilting and death of small branchlets throughout the canopy. These symptoms progress much more slowly than Phytophthora root rot, and often do not kill the canopy entirely. Their symptoms often peak in August after the stress of bloom. The cause of this disease has not been identified, but I am working on it.

For more information on these insect and disease pests of lavender, visit the lavender module of Ontario CropIPM.

Pruning

If you have not yet pruned your lavender, it is best to do it within the next week. This will give the plants enough time to recover and put on new growth before winter. If pruning occurs too late, the plant will send out new shoots too close to winter, and these may be more sensitive to winterkill. First-year plants are growing more rapidly and may need to be pruned more than once over the summer. The last pruning on these can be a little later into September. Also, plants with a second bloom will be susceptible to winterkill regardless of whether you prune or not, so some light pruning or harvest to remove these flowers can occur into the fall.

Pruning involves cutting the plants back to a round shape. Harvest of flowers is not considered a pruning. For harvest, you are only cutting off the flower stem. Pruning involves cutting deeper into the canopy to keep plants rounded and encourage branching, which keeps the canopy dense. A compact, dense plant has several advantages, including:

• Keeping plants more insulated over winter, less woody, and reducing winterkill
• Helping plants withstand the weight and billowing of winter row covers, especially when they are covered in snow and/or ice
• Ensuring next year’s flowers remain erect and untangled, even when weighted down by a misty rain

Generally, it is good to remove about 50% of the green portion of the stems. This ensures that most of the dominant growing points are cut off to encourage branching, and that there are leaves left behind to photosynthesize and ensure there is enough energy for new shoot development.

Pruning can occur right after bloom, but if plants are highly stressed, such as if they are suffering from disease issues, it is best to wait a few weeks for them to start regrowing before pruning.

Fertilization

It is getting too late in the summer to fertilize, especially with nitrogen. Similar to late pruning, late nitrogen application can promote too much vegetative growth ahead of winter, making plants more susceptible to winterkill. If you suspect a nutrient deficiency, ensure the crop is fertilized before green-up next spring.

Weed Control

There is the temptation at this time of year to focus on postharvest activities and less on the field. However, the fall provides ideal conditions for winter annual and perennial weeds to grow and store energy for winter. What may look like small weeds now can become major problems by the spring that are difficult to remove. If the planting holes in the plastic are exposed, continue to weed until the end of October. Even beyond that, some weeds such as mouse-eared chickweed and annual bluegrass will continue to develop in periods with no snow cover.

Hand weeding will likely be necessary, but there are a few herbicide options for lavender. Centurion/Select is registered for control of grassy weeds in lavender and can be applied right over the row. It is most effective on younger grassy weeds and can kill both the tops and the roots of these weeds. Axxe and Biolink Herbicides provide burn-down of above-ground growth of broadleaf and grassy weeds that they contact directly. They can only be applied with a hooded/shielded sprayer to ensure the lavender foliage is not contacted by the herbicide. Because weed roots are not affected, multiple re-applications will be necessary to weaken perennial weeds, ensuring that they do not have much time between applications to rebuild the root system. The burn-down herbicides may be more practical as a method to kill back clovers and grass that creep over the plastic and grow into the lavender over time. If you are unfamiliar with these products, try them on a small scale first to see how much control they provide and whether the cost is worthwhile compared to the labour required for hand weeding methods.