Winterkill

By now, lavender plants should have fully greened up. If the outer portions of the canopy are still brown, the plant will only recover if new shoots are now visible lower on the branches or near the crown. Green-up has been slower than normal this year despite a warm April. This is probably because lavender requires warmer temperatures to grow, and the April warmth was still below its requirements. Most of May has been below normal. It is unlikely that plants will catch up entirely, so bloom may be somewhat delayed this year unless temperatures go above normal again in June.

If the plants have been killed back and new shoots are emerging from the base, pruning is beneficial to get more light to these shoots. Otherwise, the shoots can become long and spindly, leading to a looser canopy.

Four-Lined Plant Bugs

Four-lined plant bugs (Figure 1) are likely emerging and causing damage to the tips of new shoots (Figure 2). The next week or so is a critical time to scout for the extent of the damage, so you can determine if any control measures are necessary. For more information on the biology of this pest and how to identify and manage it, consult the lavender module of Ontario CropIPM.

Figure 1. Early four-lined plant bug nymphs should be emerging now or within the next week. The nymphs move quickly and hide within the canopy.

Figure 2. Early feeding damage to the tips of new shoots due to feeding by four-lined plant bug nymphs.

Disease

Most of the plant damage that appears during green-up is due to winterkill if plants were healthy in the fall. Once plants fully green up, any further die-back of shoots is probably due to root or stem diseases. The highest risk disease for lavender is Phytophthora root rot, which can cause rapid and complete destruction of a field under the right conditions. Phytophthora leads to a rapid wilting and death of sections of a plant, followed by spread down the row under wet conditions. Thankfully, this disease does not appear to survive normal winters in Ontario.

I have been dealing with new symptoms over the past few years involving smaller branches turning pale green, then yellow, and then brown (Figure 2). This disease progresses much more slowly than Phytophthora, often allowing new shoots to develop at nearly the same pace as older ones collapse. This leads to a variable and thin canopy, lower yields, and increased winterkill, but does not kill the plant entirely. The cause of this disease has not been determined. This year, we have funding through OMAFA to identify the cause. If you suspect you have these symptoms in your field, please email me some photos at sean.westerveld@ontario.ca. We can collect a sample and attempt a diagnosis. The goal is to isolate fungi from these symptoms and then inoculate healthy plants to determine if the fungus is responsible.

Fertilizing

Now is a good time to fertilize the plants to ensure adequate nutrition for flower and leaf growth. Research has shown that lavender yields increase up to 80-100 kg/ha of nitrogen per year. This is best split into two or three applications between mid-May and early August. Phosphorus and potassium requirements should be based on a soil test. These are best adjusted ahead of planting and then monitored through soil testing every 2 to 3 years. For more information on determining fertilizer rates, consult this blog post.

Cultivars Performance

After 7 years, the cultivar demonstration plot in Simcoe is being removed. Despite significant damage due to the mystery symptoms described above, plants of some cultivars have become too large to manage. This past winter, I did not put row cover over the plants, so I could do one last assessment to determine how well different cultivars survived the combination of disease and winter. This is not a replicated trial, so these are just observations. Some plants are younger than others, and younger plants tend to be hardier. The plants that had the highest percentage of live shoots 2 weeks ago were:

7-Year-Old Angustifolias:

Royal Velvet (54% green)

French Fields (50% green)

Betty’s Blue (50% green)

Younger Angustifolias:

Forever Blue (70% green)

Beezee Pink (63% green)

Arctic Snow (50% green)

7-Year-Old Lavandins:

Gros Bleu (37% green)

Impress Purple (35% green)

Phenomenal (25% green)

Apparently dead or nearly dead cultivars as of May 7: Melissa Lilac, Fred Boutin

Overall, the most reliable, high-yielding cultivars with good flower and oil characteristics over the 7 years were ‘Royal Velvet’, ‘Imperial Gem’, and ‘Melissa’ for the angustifolias and ‘Gros Bleu’ for the lavandins. These results are based on rapid growth, tolerance of the mystery disease in the plot, and winter survival. Results may differ in other locations and soil types, and areas without the same disease pressures.