Lavender is in full bloom across the province, and agritourism activities are in full swing. Angustifolia cultivars are nearing the end of bloom in the south, while lavandins are in the early bloom stage. This can cause lavender growers to forget about the crop itself and focus on other activities. Take some time every few days to monitor the crop. If you wait until August to start paying attention to the crop, it may be too late to solve any problems that arise. It is also important to keep in mind the ideal scenario for dealing with plants after bloom, even if labour availability prevents the perfect timing of every practice. Here are some things to consider during and immediately after bloom:

Irrigation

Rainfall has been highly variable across the province, but most areas are now under dry to very dry conditions. Dry conditions during bloom can cause reduced flower stem length, wilting of flower stems, premature corolla (petal) death, and poor recovery after bloom. The effects of blooms can degrade the appearance of the fields during the peak of agritourism. Lavender will survive dry conditions, but irrigation is highly beneficial under these conditions. You should aim for about 20-25 mm of water per week under hot conditions, which is lower than most crops but still equates to 80,000 to 100,000 L of water per acre per week. This can be reduced if you have drip irrigation that delivers the water right below the row and wide row spacing, such that the plants only occupy a fraction of the field area. While some plant stress can occur when grass is still green, a good sign that plants are under severe stress is when mowed grass starts to turn brown in your area.

Impacts of Harvest Timing on Plant Vigour

Most growers harvest plants for bundles and buds near the beginning of bloom, but the rest of the fields are left for oil and not harvested until after the agritourism season. There also may be a temptation to wait even longer as purple buds may still be attractive to visitors, even when the blooms are finished. It is important to understand the consequences of this timing.

I have mentioned that oil yield and quality peak during late bloom when all of the buds are fully formed, but before older buds start degrading and/or falling off the plants. However, for angustifolia cultivars, there is another consideration. When bloom is finished, the plants will focus on producing (lavandins do not produce viable seed). The energy required to produce all that seed stresses the plants and delays the resumption of vegetative growth. This can impact the health of the plant and potentially reduce yield potential for the following year. The sooner plants are harvested, the more recovery and regrowth the plants can make before winter. This is especially important in younger plants because it can delay the speed at which the fields reach full yield potential. As with everything else, harvest timing is an economic decision. If labour is limited or you need to delay harvest to extend agritourism activities, consider harvesting younger fields first to give them more of a chance to grow.

Fertility

Late bloom until early August is a good time to add nitrogen fertilizer to the field. Plants will focus on vegetative growth after bloom, and nitrogen is the most important nutrient to promote vegetative growth.  An application of 40-50 kg/ha (36-45 lb/ac) is optimal based on research. Do not apply nitrogen after about the middle of August, or it may affect the overwintering of plants.

Help Needed for Disease Survey

I have been dealing with a severe branch disease in my lavender plot for several years, and it appears to get worse with plant age. Based on photos sent by some growers, this may be a more widespread problem. I have not been able to identify the cause of the disease, but I am working on it. Symptoms of the disease begin with a slight yellowing of several small branches/branchlets scattered through a plant. These progress to a bright yellow colour, especially during bloom when plants are the most stressed. The yellow branches eventually wilt and die. Nearby branches begin to be affected. At any one time, a main branch may contain smaller branches that are dead, wilting, yellow, slightly discoloured, and apparently healthy simultaneously.

The progression of the disease is slow enough that new shoots emerging from lower in the canopy often replace the dead ones over time, so the plant never fully dies. However, the flower yield of some cultivars may be a quarter or less of their full potential over time.  Some cultivars are much more affected than others. The disease mostly affects angustifolia cultivars. Lavandins may have some slight discolouration but are otherwise immune.  This disease has similar symptoms to Phytophthora root rot, but Phytophthora often leads to main branches dying all at once and kills entire plants much more rapidly. See figures 1 to 4 for images of the symptoms of the current disease on various cultivars.

Figure 1. Bright yellow leaves and stems on small branches on an angustifolia lavender plant.

Figure 2. A plant exhibiting all stages of the disease, including completely grey branches that died last year, brown dead branches that died earlier this year, yellow branches that are dying now, and a few branches near the bottom of the photo that have a lighter green colour and will die soon.

Figure 3. Another plant with varying disease symptoms.

Figure 4. A recently harvested plant with less severe damage.

I am interested in knowing how extensive this problem is in Ontario. If you suspect you have the disease in your fields, please email me at sean.westerveld@ontario.ca with a photo of symptoms that you think match those above. Let me know your location, which cultivars are affected or not affected, the percentage of plants with symptoms, the total number of plants with symptoms, and the age range of affected plants.

Insects

Garden fleahoppers are promoted by hot temperatures because they lead to faster generation times. Monitor your fields regularly for this pest.

Under dry and hot conditions, obscure insect pests like false chinch bug can affect lavender. They are normally found in pastures and grasslands, but can move to the lavender when these grasslands dry down. The lavender is usually still green under these conditions, which offers them an oasis of green in an otherwise desert. These insects can lead to complete defoliation of lavender in some cases.

For more information on insect identification and management. Visit the Lavender Module of Ontario Crop IPM.