Lavender bloom is nearly complete for another year. Growers with distillation equipment are likely busy distilling now and over the next few weeks. Otherwise, pruning can be done anytime between now and the end of August. Fertilization can continue to about mid-August. Planting new plants can be done in the summer as well but requires extra care. Here some notes on each of these tasks:
Oil yield and quality peaks between peak bloom and the end of bloom and then slowly declines. Some cultivars are prone to bud-drop after bloom, especially if hit by a heavy downpour of rain, and this can greatly reduce oil yield, since most of the oil glands are in the buds (calyx). For optimal oil quality, avoid harvesting flowers in the morning when flowers are wet. This will make it take longer to heat up a batch of flowers in the still which can lead to more cooking of the flowers and reduce oil quality. Drying the flowers a little in the field before distilling can improve quality but will increase labour requirements. For oil harvest, a small amount of stem is beneficial for spacing out the flowers in the still but avoid cutting leaves off at the same time. This means cutting above the highest pair of leaves, compared to bundle harvest for which those leaves are often harvested and then removed after drying.
Although some growers opt for spring pruning given the frequency of winterkill, most growers prune after flowering. Pruning will prevent plants from getting too sparse and woody over time. It can also keep plants in a rounded shape that can help block wind through the canopy in the winter and reduce winterkill and prevent the canopy from splitting open either in the winter or with a heavy flower load next spring.
Flower harvest is not an adequate pruning because none of the growing points within the canopy are affected. Dead flower stems remaining on the plant can also be an entry point for diseases. It is best to prune 33-50% of this year’s green growth, which should nip most of the growing points off the plant. Avoid removing all the green leaves from a branch or you risk killing portions of the plant. This is especially important for the lavandins.
Pruning beyond the end of August is not recommended because plants will not have sufficient time to grow back to a thick canopy before winter, which can increase winterkill. Also, tender shoots that grow back after pruning may not harden off properly and could be more prone to freezing temperatures in late fall.
For those that are splitting nitrogen applications over the season, it is best to stop fertilizing no later than mid-August. It is probably best to apply no more than 35 kg/ha of actual nitrogen in August. Fertilizing too much too late could increase vegetative growth in the fall and lead to more winterkill. If you do not have an irrigation system and plants are under moisture stress, it is probably not worth fertilizing anymore this year. Plants are likely dormant, and the fertilizer will just sit near the surface until the first good rain. If this occurs too late in the season it will be just like fertilizing the plants in the fall. Fertilizer salts sitting near the roots could also increase moisture stress.
Planting at this time of year is only possible if you can provide a constant supply of moisture. The risk of winter injury goes up if you plant too close to the fall, but I have had some success planting small rooted cuttings as late as early September (with some minor losses over winter). Based on my experience over the past week, growers should be cautious of midsummer plantings of small plants:
I took cuttings of several cultivars in May and rooted them outdoors and then seeded several cultivars that were advertised to be true to type from seed. Plants were small but established enough to be planted out into my cultivar verification trial over the past week. I planted 24 small plants scattered throughout the plot, surrounded by much larger plants, and watered them in last Friday afternoon. By Monday morning all 24 plants had been dug up, root plug and all, and had vanished from the plot. There were a few shoot tips left behind in a few holes, but otherwise no evidence of any part of the plant remaining.
I had some replacement plants that were in a tray nearby and these were not affected, so I replaced most of the plants. This time, suspecting rodents or rabbits, I put wire cages over all the new plants and pegged them down with ground staples. One of them did not have a top, so was a wire cylinder about 30 cm tall and 15 cm wide. One hour later that one plant had been removed entirely, while all others were still intact. This suggests that birds removed the plants. Yesterday, I test-planted a few spare plants in some extra holes. Most holes surrounding these plants were filled with small broadleaf weeds. A few hours later, one of these plants had also been removed and it is likely that the rest are gone today. The weeds were untouched. This shows that whatever removed the plants was specifically targeting the small lavender plants, even though the entire plot is filled with mature lavender plants with many similar shoots. I have planted hundreds of plants in the same plot over the past 4 years and this has never happened before. It is best to keep an eye out for missing plants if planting midsummer! What removed the plants and why remains a mystery, but birds are the likely culprit.
The biggest pest issues for late summer are usually Septoria leaf spot and garden fleahopper. Any round lesions on the lower leaves at this time of year is likely Septoria leaf spot unless there is still damage remaining from earlier four-lined plant bug feeding. Septoria can weaken the plants and may make them more prone to winterkill. The best method for reducing Septoria leaf spot is to promote good airflow through the field and the canopy.
Garden fleahopper populations can really build up in warm and dry conditions, especially over black plastic. Plants can be severely weakened. Look for fine speckling of the leaves within the canopy which can lead to a dusty/pale colour. Shaking the plants vigorously next to a white sheet of paper will cause the nymphs and adults to jump out of the canopy and be easily visible. Garden fleahoppers tend to be patchy in the field. If damage is severe, insecticidal soaps applied for aphid control in lavender may suppress populations and allow plants to recover. This can probably be done with a backpack sprayer targeting the most affected areas.
Some growers continue to report collapsed or discoloured portions of plants. Several samples submitted to the University of Guelph Pest Diagnostic Clinic have shown no evidence of Phytophthora root rot, but only weakly pathogenic fungi like Fusarium and Pythium. I highly suspect most of these issues are still related to winter kill of either the roots or the vascular system. Damaged plant parts can be invaded by weak pathogens over time. There is not much a grower can do with this type of damage, except hope that the plants grow back now that bloom is finished, and the plants can focus on vegetative growth.