Lavender bloom is wrapping up in most areas, and growers will soon focus on harvest for oil extraction followed by pruning. While growers are focused on harvest and distillation, it is easy to forget about the crop. However, this is a critical period of the year to ensure plants are not stressed, so they can put on as much vegetative growth as possible by the fall and properly harden off before winter.

Problems Associated with Wet and Dry Conditions

Rainfall has been highly variable this summer. Some growers have experienced heavy rains, which increases the risk of root diseases, while others are very dry. Both of these conditions can lead to poor health of the plants:

Heavy Rains

Lavender does not like wet feet. This is because the roots need good aeration to survive and weakened roots are most susceptible to soilborne pathogens like Fusarium, Pythium and, especially, Phytophthora. Growers should do everything possible to remove moisture and standing water from around plants, such as cutting drainage channels or adding tile drainage to the field.

Dry Soils

Lavender is adapted to a dry climate around the Mediterranean. In that climate, it survives by being sparsely populated and having a root system that goes down deep into the soil profile. In Ontario, lavender cannot have the same deep root system because the water table is often high during the winter, killing off any deep roots. There is also no incentive for the plant to have deep roots, because there is often ample moisture in the upper layers of the soil for most of the year. Dry periods in the summer can cause the plant to shut down, resulting in very little growth and poor recovery of the plants between flowering and the fall. This can reduce winter survival rates and reduce yield the following spring. Irrigation is the only way to counteract this issue.

Disease Samples Needed

As mentioned in previous posts, samples of diseased lavender plants are needed for a research study that is attempting to diagnose an unknown disease that has affected the lavender plot in Simcoe. Several growers reported similar symptoms last year, so it is likely occurring in other regions. The symptoms of the disease are random yellowing and wilting of small branches scattered over the plants. These symptoms could easily be confused with just a weak plant or winterkill. If you have any symptoms of poor growth accompanied by yellowing or death of branches and live in Ontario, please take some photos and send them to me at sean.westerveld@ontario.ca. I will be away in early August, but will respond when I return and will find a way to get some samples to Simcoe. Diagnosis of the disease will be free, except for the potential cost of shipping samples.

Insect Pest Updates

The pest with the highest potential for damage at this time of year is garden fleahopper. These tiny black insects can reach populations that are high enough to completely defoliate plants in early August (Figure 1). This pest remains hidden within the plants, and the damage it causes can be easily missed. If you have plants that are not looking very vigorous, take a white piece of paper out to the field, slip it on the ground under the side of a plant, and shake the plant vigorously. Watch for very small, hopping, green nymphs or black adults that move quickly when disturbed. One or two of these pests are likely not an issue, but 10-20 on a sheet of paper signals a problem. I have experienced populations of well over 500 insects per plant. Populations of that size require insecticide sprays to prevent plant death. Insecticidal soaps registered for aphid control in lavender can likely suppress populations. There are conventional insecticides registered for control of plant bugs in outdoor ornamental crops that can also be used, if the flowers are not used for culinary purposes.

Figure 1. A lavender plant in early August that was nearly completely defoliated by garden fleahoppers. Plants only recovered after an insecticide registered for outdoor ornamentals was applied.

Other occasional insect pests of lavender at this time of year include:

1. False chinch bug – a sucking insect that can cause significant damage in very dry conditions when they have nothing else to feed on in surrounding fields.
2. Grasshoppers – feed in the sides of stems and leaves but are unlikely to cause economic damage.
3. Woolly bear caterpillars – feed on leaves but at such low populations that they are not a major concern.
4. Tarnished plant bug – a sucking insect similar to four-lined plant bug, but with indistinct symptoms. They rarely reach economically damaging levels.
5. Japanese beetles – they feed on flower petals, which may affect marketability of cut flowers but have minimal impact on the plant.

For more information on garden fleahoppers and occasional insect pests, visit the Lavender Module of Ontario CropIPM.

Fertility

Now is a good time to make the final nitrogen application of the year. The plant will focus on vegetative growth from now until October, and adequate nitrogen application will ensure the plant can grow as much vegetation as possible. This will ensure optimal flower yield next spring. Research has shown that lavender yield is optimized by 80-100 kg/ha of nitrogen per year. This is best split into 2 or 3 applications between May and early August. Nitrogen should not be applied later than mid-August, or overwintering may be affected.

There are many options to choose from for nitrogen fertilizer sources, including organic products like compost, blood meal and bone meal, and synthetic products like calcium ammonium nitrate and urea. It is best to purchase fertilizers from agricultural suppliers rather than garden centres or retail stores. The cost per nutrient is often 10-20 times lower from agricultural suppliers. Many “snake oil” fertilizers on the market make great claims but are often just a much more expensive formulation of the same elements. Focus on standard fertilizers from ag suppliers rather than waste money on products that are definitely not proven on lavender. For any other products, conduct research on a small scale, perhaps at the expense of the company advertising the fertilizer, before adopting them on a large scale.

For more information on fertilizer sources and fertilizer calculations, consult OMAFA Publication 611, Soil Fertility Handbook.

Pruning

In my experience, the best time to prune lavender is early- to mid-August, or slightly later for lavandins that may bloom until early August. This timing gives plants a few weeks of new growth after bloom, but also plenty of time after pruning for plants to recover, develop a thick canopy, and harden off before winter. This is the time of year for an aggressive pruning that cuts plants back into a rounded shade. It is best to cut off most of the shoot tips (the actual growing point, not just the leaves) to force the plant to branch out and develop a thicker canopy. The aim is to cut off between 1/3 and ½ of the green growth. Cordless hedge trimmers are the best tools for this purpose. Some growers add customized guards to the end of the blades to prevent cutting into the plastic or drip irrigation lines when cutting off branches around the perimeter of the plant.

Newly established plants will want to bloom repeatedly in their first year. These will likely require several prunes per year to cut off the flower buds and dominant shoots. These can be pruned into early September.

Cautions around New Cultivars

It appears that many of the new cultivars on the market are designed for home gardens. These plants are often compact, with short stems, bloom more continuously all summer, and are not optimized for yield or winter hardiness. ‘Super Blue’ was the first of these types of cultivars, but there are many more now on the market. Continuous bloom may be advantageous to ensure there is something for farm visitors to look at in August and September. On the other hand, these cultivars never seem to stop developing new buds, which makes them very prone to winterkill. It also would make them nearly impossible to harvest for flower bundles and difficult to time for oil harvest. The stems are often so short that leaves are impossible to separate from flowers while harvesting for oil, making the oil lower quality.

It is important to consider exactly what purpose each cultivar has on the farm. Interspersing a few rows of these new cultivars through an agritourism field may allow for a longer season. Alternatively, you may be able to have a separate late-summer blooming field for late-season events. However, it is important to be familiar with the blooming habits of these cultivars before doing anything on a large scale. Some of them may bloom continuously (although sparsely) if left alone, but may not rebloom until late fall if they are harvested or pruned during the season.