Lavender is nearing the end of bloom which will start to wind down the busiest agritourism season for farms open to the public. The focus over the next few weeks will be harvest for oil extraction, distillation and pruning. At a time when the plants need to put on new vegetative growth it is important to continue to monitor crops for pest issues and to ensure plants are adequately fertilized.

Excess Moisture

There has been excessive rainfall in some areas of the province, but also areas close to the north shore of Lake Erie and Lake Ontario that have had insufficient rainfall. Some areas have received well over 200 mm of rain in the past 30 days. Given that this is the most significant period of excessive rainfall since fall of 2021, it may be the first in-season excess moisture event many new growers have experienced. Many growers have reported dieback of established plants over the past few weeks. Invariably the dieback is occurring in fields with heavier soil and/or poor drainage. This is the type of weather event that really underscores the truth behind the expression “lavender does not like wet feet”. What this means is that lavender cannot tolerate soil being saturated for extended periods, mainly because the roots do not get any oxygen and are weakened or die. Weakened roots are then attacked by fungi and Phytophthora (if present), which causes the plant or parts of the plant to suddenly wilt mid-season. The problem is worse in clay soils both because drainage is slow after a rain and even without the rain the air spaces in the soil are very small and do not allow for sufficient air exchange.

Saturated soil is not good thing in the winter, but it can be tolerated for much longer periods because the plant is not actively growing, and the cold temperatures lower the need for oxygen. In the summer, the plant is actively growing, and higher temperatures lead to higher needs for oxygen. Fungal pathogens are also more active in the summer and can further weaken roots in saturated soils. As a result, plants can die after a few days of saturated soil in the summer, where that may take a week or two or more in the winter.

To manage the issue, the first solution is to improve drainage. This can be done by monitoring where water pools after heavy rains and installing drains to remove the water quickly. There are various options including surface drains (e.g., French drains,  Hickenbottom drains) to remove water from low areas or tile drains to allow soil water to drain faster. For more information consult the Best Management Practices Manual “Cropland Drainage” https://bmpbooks.com/publications/cropland-drainage/.

If you see patches of plants and sections of plants suddenly wilt and the wilting spreads down the row over time, you may have Phytophthora root rot, which is the most destructive disease that affects lavender in North America (Figure 1). The pathogen does not seem to survive the average winter in Ontario but has survived mild winters. It is often introduced to the field on transplants or could potentially be airborne in some years based on observations in 2015 when multiple farms across North America were suddenly affected at the same time. If you suspect Phytophthora, it is best to send a whole plant sample to a diagnostic lab for identification. If Phytophthora is confirmed, you will have to be very vigilant to prevent spread and may have to remove all affected plants and avoid planting lavender back into the same area.

Figure 1. Symptoms of Phytophthora root rot of lavender appear as sections of a plant suddenly collapsing mid-season and then spread of symptoms through the plant and to neighbouring plants.

Other than improving drainage and preventing spread, there are no other controls available for root rots of lavender. There are no pest control products registered that will have any effect on these diseases.

Fertility

At this time of year, lavender plants are very stressed after producing a large crop of flowers. The plant will transition to vegetative growth for the remainder of the year. For rapid recovery from harvest and pruning and to ensure good flower production next spring, ensure plants are adequately fertilized, especially with nitrogen. My research has shown that lavender requires around 80-100 kg/ha of nitrogen every year. Around half of that could be applied at this stage. Do not apply nitrogen after about the middle of August to allow plants sufficient time to harden off before winter.

Garden Fleahoppers

As I found out last summer, garden fleahoppers can build to incredibly high numbers and almost defoliate plants by August. Keep an eye on populations over the next month. There is not much that can be done to control them with the products available for culinary lavender other than some potential suppression of nymphs with insecticidal soap, but there are insecticides that can provide good control when growing lavender for cut flowers. For more details on garden fleahoppers see my post from last August: https://onspecialtycrops.ca/2022/08/26/lavender-crop-update-late-summer-tasks-august-26-2022/#Garden_Fleahopper

Pruning

Pruning is a very important step in lavender production to prevent plants from getting woody and sparse. Woody and sparse plants are more prone to winter kill, are less vigorous bloomers with shorter stems, can collapse after heavy rains during bloom, and are more difficult to harvest due to tufts of flowers growing in all directions. Harvest for either oil or bundles does not count as a pruning because it does not cut off the growing points of the branches and stimulate branching. Pruning is best done between now and mid-August. Pruning after Labour Day is not recommended because it may stimulate new growth too close to winter and lead to lower winter survival. Pruning involves cutting off 33-50% of this year’s growth into a tight mound. Harder pruning is recommended for woodier plants. Some green leaves should remain after pruning on all branches. Severe pruning to rejuvenate Angustifolia cultivars, which involves cutting the branches down to near the ground, should be reserved for the spring and not done at this time of year. That type of severe pruning cannot be done with lavandins or they will likely die.