Lavandula angustifolia cultivars are past peak in most areas and L. x intermedia (lavandin) cultivars are beginning to bloom. Harvest for essential oil can begin at any time for angustifolia cultivars and harvest for bundles and buds has begun in warmer regions for lavandins.
The biggest pest issues at this time of year are garden fleahoppers and Japanese beetles. Garden fleahoppers usually hide within the canopy and scrape the leaves causing them to turn silvery. Shaking the plant vigorously above a white sheet of paper will make them visible. Look for tiny green nymphs and black adults that jump easily when disturbed. If populations are high, they can stunt the plants and prevent regrowth after bloom. This appears to be more likely over black plastic. Insecticidal soaps registered for aphid control may suppress garden fleahopper populations.
Japanese beetles can often be found sitting on a flower cluster. They chew through the sides of the corolla (petals) to get at the nectar. This causes the flower petals to die. This would not affect postharvest uses of the plant, since they do not affect the calyx (buds), but can cause a reduction of flowers for agritourism if populations are high enough. There are no products registered for control of Japanese beetles in lavender, but control is not normally warranted.
Dealing with Dry Conditions
It is becoming extremely dry in most areas. While lavender is tolerant of dry conditions, it does not thrive under these conditions. The plant will often stop growing until moisture returns, which can limit yield in subsequent years. Lavender plants in cold regions are also not the same as lavender plants growing in the Mediterranean region. Die-back of plants and roots over winter due to cold temperatures and high water-tables can limit root growth in Ontario. Instead of a root system that can extend several metres in the Mediterranean, the root system in Ontario is likely much less extensive. This makes plants more vulnerable to dry conditions in Ontario. Irrigation will have major benefits under these conditions, but it is extremely important to understand how much the crop needs so you don’t make things worse.
Most of the moisture loss from a crop during the growing season is lost through the leaves, a process called evapotranspiration (ET). Farmwest (farmwest.com) provides recent and historical average evapotranspiration rates for various locations in Ontario. These are based on the moisture loss a grass cover crop would lose based on temperature, humidity, and wind speed. Warmer days with less humidity and more wind result in more ET. If we look at Guelph, somewhat central in the Ontario lavender industry, the historical average ET is about 4.5 mm/day. During a heat wave, that number can increase to 6 or more mm/day. That results in an average moisture loss of around 30 mm per week and occasionally up to 40 or more mm/week for a grass crop. Many broadleaf crops have even higher ET rates than grasses. The average rule of thumb during the entire growing season, is that crops require about 1 inch (25 mm) of water per week, but you can see that is an underestimate during the peak of the summer.
ET Rates for Lavender
Lavender is not an average crop. The leaves are covered in dense hairs designed to inhibit moisture loss. ET values for lavender will be much less than that of a grass crop. While there are no studies to examine ET values for lavender that I could find, a reasonable estimate is that lavender would lose half as much of a grass crop. For the average ET at Guelph, that equates to 15 mm per week. However, it may come as a surprise to most growers, but the primary crop in the field for most growers is not lavender. Most growers have widely-spaced lavender rows with grass growing between rows. The grass may cover up to 75% of the field. The roots of the grass cover will easily extend under the lavender rows and well over a metre deep. This means that ET from the field is more driven by the grass than the lavender. This brings ET values for the average lavender grower back up to around 30 mm/week.
Determining Water Requirements
Due to the grass cover between rows, lavender growers do not need to water the entire area of the field to keep the lavender happy. However, for agritourism purposes there is considerable value to keeping the grass green for both visual appearance and walkability for guests. To keep just the lavender irrigated depends on the size of the plants. New plants will require much less water, but irrigation is critical for young plants to prevent complete plant death. ET is driven by the area of the canopy. Since we are at full bloom, the canopy of a 2- to 3-year-old plant will range from 50 to 100 cm in diameter depending on the type of lavender. With an average in-row spacing of 50 cm (20 in.), that means that each plant occupies about 0.25 to 0.5 m2 in area (based on a rectangle rather than a circle since the plants are often merged within the row). Unless you are on solid plastic mulch between rows, most growers will be irrigating some grass at the same time as the lavender even if the water is directed at the base of the plant. If we assume an ET rate of 30 mm/week based on the grass, that means 30 mm/week of irrigation is required to replace what is lost if there has been no rainfall.
30 mm depth over 0.5 m2 equates to 15 L of water per plant per week (0.03 m depth x 0.5 m2 area = 0.015 m3; there are 1,000 L per m3, so 0.015 m3 x 1,000 L/m3 = 15 L), so plants will require between 7.5 and 15 L per week depending on size. The average watering can is 6 L, so each plant would require 1.25 to 2.5 full watering cans per week if watering by hand. With good pressure in town, my garden hose delivers about 18 L of water per minute. The average lavender grower has about 4,000 plants per acre. That means that watering an acre of lavender at 7.5 to 15 L of water per plant per week would take 28 to 55 hours per week of non-stop irrigating with a garden hose for 2- or 3-year old plants, and that is only if the water can be directed at the plant so the bulk of the grass does not have to be watered. Older/larger plants may require more. Even a young plant with a canopy that may only be 20 cm in diameter still requires around 1.2 L/plant per week. All these numbers can be divided in half if there is no grass between rows or if the grass has just been established and there are no grass roots in the lavender rooting zone. If you only have overhead irrigation and will be irrigating the grass and lavender together, then around 120,000 L is required per acre per week when the ET value is 30 mm/week.
Note: Anyone collecting more than 50,000 L of water in a single day from any water source needs a Permit to Take Water. Click here for more information.
Is Watering by Hand Worthwhile?
It should be clear from these calculations that an irrigation system is the only way to economically water a crop of lavender unless you only have a few plants. If you do not have an irrigation system, consider whether irrigating by hand is worthwhile. For plants just established this year, you must water no matter what the economics of getting water to the plant. Young plants will die without some supplemental water under these conditions. For older plants, consider the trade-off between the costs of getting water to the plants and the lost yield from not irrigating. Not irrigating will probably not kill the plants unless there is severe drought, it will just slow or stop growth until moisture returns. However, if the dry conditions persist all summer, yields could be significantly impacted next year, and plants may be weaker heading into the winter. Irrigating too little might harm plants more. If you water too little and only wet the top layer of the soil consistently, the roots may all grow towards the surface and be much more susceptible to drying out completely later. It is much better to water deeply and infrequently than repeatedly applying too little. In fact, it may be better not too apply irrigation at all than to water only the surface, given that surface watering can only hurt the crop but still takes labour to apply.
Also consider the method of applying the water to the plants. It takes about an hour for 30 mm of water to infiltrate a sandy soil and over 3 hours to infiltrate a clay. If you dump the full amount of water all at once at the base of the plant, where does the water go while waiting to infiltrate? If it runs off right away, very little will reach the roots. It is often best to apply a little, wait for it to infiltrate, and then apply more and so on until the water is completely absorbed. This would add to the labour of hand-watering and the reason most horticulture growers opt for an irrigation system that delivers water more slowly over an extended period.
Symptoms of Moisture Stress
The symptoms of moisture stress in lavender can be subtle. When flowers are still developing, drooping of the stem in the heat of the afternoon is the first sign that the plant is under stress. Once flowers are fully elongated, they stiffen up and will no longer droop. The flower petals (corolla) can prematurely shrivel under moisture stress. If the flowers have no fully expanded corollas while still in bloom, then it is likely the plants are stressed. A symptom that moisture is desperately needed is drooping of leaves. If leaves begin to droop, you will likely need to irrigate, especially for plants that have just been established. Moisture stress can also make other issues more apparent such as root diseases and winter injury. These issues lead to a less extensive root system and/or may block flow of water through the vascular system. This can cause sections of the plant to wilt and/or die completely. This type of injury can resemble phytophthora root rot, but phytophthora is not likely to be active under dry conditions.