Lavender bloom is long finished, and most growers are wrapping up distillation for the year. Here are some things to consider at this time of year:
If you have not pruned already, August is a good time to prune lavender to get plants back into a rounded shape and to ensure plants are dense, which will improve winter survival. Pruning beyond Labour Day is not recommended in Ontario since it will encourage new growth that may be more tender going into the winter and may open up the canopy allowing cold air to penetrate more deeply into plants.
If plants were heavily pruned in the spring to manage winter damage, how hard to prune will depend on how much re-growth there is. If your plants are anything like mine, plants that had to be severely cut back in the spring, flowered only modestly but had even more vegetative growth than undamaged plants. These plants may need to be aggressively pruned to keep them in a tight mounded shape.
Pruning requires snipping off the growing points of most of the outer shoots to encourage them to branch out lower on the stem. This will ensure there are many tightly packed shoots that protect each other over winter, prevent snow and ice from splitting the plants open, ensure a heavy bloom next spring, and keep the blooms upright for easier harvest next spring. The growing points on the branches can be up to 3-5 cm below the tallest leaves, so it is important to ensure you are not just cutting leaves off when you prune. Most of the time, you should be pruning off between 33 and 50% of this year’s green growth; lower in that range if plants are already in a good shape and you just need to keep them rounded, higher in that range for younger plants that have vigorous new growth and any plants that have become leggy or woody in the centre. Never cut off the majority or all the green growth or plants may not come back. Pruning more aggressively should only be done in the spring if there is excessive winter kill or you need to completely rejuvenate old angustifolia plants that have lost vigour.
The only cutting that can be done after Labour Day is removal and/or harvest of flowers in newly planted or re-blooming angustifolia cultivars. For young plants, continued removal of bloom buds will prevent dominant branches from sticking out of the canopy, which are more prone to winter injury. For re-blooming cultivars, tender blooming shoots tend to die over winter anyway, so harvesting them will not substantially increase the winter kill and may help plants harden off.
Late Summer Pests
This is the time of year that populations of garden fleahopper can dramatically increase and damage can become obvious. Garden fleahoppers are tiny black jumping insects that suck juices out of the leaves within the canopy. Nymphs are green. Adults and nymphs are best seen by shaking the plant vigorously next to a white piece of paper (Figure 1 and 2). Damage is mostly to the older leaves within the canopy and results in a light-coloured mottling of the upper leaf surface. Eventually leaves turn yellow and fall off the plant.
Figure 1. Garden fleahoppers on a piece of paper after shaking the plant vigorously. A) Adult short-winged females, B) Adult long-winged males or females (females appear in two forms), and C) nymphs.
Figure 2. Close-up of a short-winged female garden fleahopper, which is the most common form.
In my research plot this year there are so many garden fleahoppers that all the leaves that were on the plant at the time of pruning are now dead, leaving only the grey new shoots emerging from the stems (Figure 3). Garden fleahoppers are normally patchy, but all 350 plants in my trial are severely affected. It appears that solid sheets of woven black plastic mulch encourage higher populations of this pest, probably by increasing temperatures around the plant and accelerating insect development. Garden fleahoppers have multiple generations per year and populations tend to build with each generation. The last generation of nymphs should be emerging soon. Black plastic may allow for more generations to be completed per year.
Figure 3. Lavender plant damaged by garden fleahopper. Most of the brown and yellow leaves were green at the time of pruning around August 1. Note the mottled appearance of the remaining yellow leaves.
Unfortunately, there are no insecticides (conventional or organic) that include garden fleahoppers on their label. Insecticidal soaps applied for aphid control may suppress garden fleahoppers, especially when the new generation of nymphs first emerge. Younger stages of insects tend to me more affected by products than older stages or adults. Insecticidal soaps are probably most effective if sprays begin early in the growing season and prevent the initial buildup of the insects. Where garden fleahoppers are severe this year, consider beginning regular applications of insecticidal soaps in mid-May next year, which may also reduce populations of four-lined plant bug.
If damage is severe and better control of the insect is required, there are other options growers should consider. When growing lavender as a culinary herb, there are very few pest control product options. However, if the crop is grown as a cut flower/ornamental there are many more options. For example, Rimon insecticide is registered for outdoor ornamentals including cut flowers for control of lygus bugs (e.g. tarnished plant bug), which are in the same insect family as garden fleahopper. When applying Rimon for control of tarnished plant bug (which is a pest of lavender), growers may see some control of garden fleahopper. Rimon is a conventional insecticide that is not approved for organic production. If a product that is only registered for cut flowers is applied to lavender, the grower cannot use that lavender for culinary use or for essential oil extraction. So, this should only be considered when no other options are available and only for sections of the field that will definitely not be used for anything but cut flowers and any of the non-culinary uses of those dried cut flowers.
Contact me if you would like to know your options for products that can be used on cut flowers. There are also products registered to control root diseases like phytophthora on cut flowers. If you are looking for products that can be applied to lavender as a cut flower, you must ensure the label specifies that it is registered for outdoor ornamentals including cut flowers and that the label does not have a defined list of cut flower species on which it can be used.
Septoria Leaf Spot
The other main pest of lavender that is common late in the season is Septoria leaf spot. Any spotting that develops on older leaves at this time of year is most likely caused by Septoria leaf spot. Septoria lavandulae, the cause of the disease, is a fungus that produces airborne spores. Spores germinate under extended periods of leaf wetness and high humidity, which occurs most frequently late in the summer when dew periods are extended. The disease is worst in areas that are shaded for part of the day and sheltered from the prevailing winds, which prevents drying of the canopy. Septoria leaf spot can weaken plants and potentially affect winter survival but does not kill the plants. Improving air flow through the canopy and ensuring the plants are healthy through proper fertility and irrigation are probably sufficient for minimizing the impact of this disease.
Fertility and Irrigation
Fertilizing beyond the middle of August is not recommended because plants need to start hardening off in the coming weeks. The only exception is if plants are affected by severe nutrient deficiencies that need to be corrected. Deficiencies could affect winter survival on their own so improving nutrition in these cases may be necessary despite the risks.
Most areas have received decent rains over the past few weeks. However, in some years there can be an extended dry period late in the summer or in early fall. Often growers forget about irrigating this late in the season. Plants under moisture stress will not grow much at a time when most of the vegetative growth of the plants normally occurs. Continue to monitor soil moisture and irrigate when necessary.