Despite warmer weather over the past two weeks, it still appears that lavender bloom is behind schedule. Normally, the first blooms on the earliest cultivars begin to open in Simcoe around June 15-18. This year, bud heads are just beginning to change colour, suggesting bloom will not begin until around June 20-25.

Pests and Scouting

Four-lined plant bugs are at the late nymphal stages and will soon become adults. This is the stage when they cause the most damage to leaves and stems. It is too late at this stage to control them. Take note of where levels of damage are high, because these areas are likely to have similar or higher damage next year. Insecticidal soaps can help suppress populations, but only if applied when the nymphs are still small. Damage appears as distinct brown spotting on leaves, and blocky, elongated brown or yellow lesions on stems. For symptoms and more information on this pest, visit the lavender module of Ontario CropIPM.

I was reading through old crop reports from before my time with OMAFA, when lavender was relatively new as a crop in Ontario. I found a picture of symptoms that were definitely caused by the four-lined plant bug. At the time, the insect had not yet been identified as the cause. There was speculation in the crop report that the damage may have been due to wind. This illustrates how easily the damage from four-lined plant bugs can be confused with other diseases and disorders. This is why regular and detailed scouting of fields for pest issues is essential. Key aspects of a scouting program include:

1. Walk the entire field in a W, X or zigzag pattern.
2. Examine the entire field as you walk, looking for stunted or diseased areas, or different patterns in the field.
3. Examine damaged areas closely for potential causes. This may include digging up a plant or two to examine the roots if the cause cannot be identified above ground.
4. In addition to the above, stop at 5 to 10 random locations in the field (chosen without looking at the plants first – e.g., predetermining that you will stop at the 11^th plant down the third row). Examine a group of 3 to 5 plants at each location, inspecting all parts of the plant (crown, stem, top and bottom of leaves, flower stem and flower buds/flowers) closely for insects, insect feeding, disease lesions, or anything out of the ordinary. You may need a hand lens to properly look for small insects or mites. Bring an extra sheet of white paper with you, which will be useful for tracking garden fleahopper populations. Shaking the plant vigorously can scare these insects out onto a sheet of paper placed at the base of the plant, where they can be more easily counted. This should be done on separate plants to avoid scaring away other insects that may be causing damage.
5. Compare the symptoms to the photos in CropIPM. If you are still unsure, you can take clear photos and send them to me for the next steps.
6. Flag issues, especially wilting or collapsing plants, so spread can be tracked over time.
7. Repeat this at least weekly and keep records of the number of insects or lesions found per plant or leaf. Tracking changes over time can help in decision-making, such as determining if a control measure is necessary.

The number of plants and leaves to inspect depends on the size of the plants and the planting. Ideally, the process should take about an hour to complete for an average-sized field.

Ground Cloth Cautions

This week, my summer student and I had the distinct pleasure of removing the ground cloth from my old lavender plot that had been in for seven years. This plot was a solid sheet of ground cloth held down with ground staples, rather than strips of cloth with grass between rows. This worked very well when the plot was in the ground (see below), but presented significant challenges for removal.

We flail-mowed the lavender plants down close to the plastic to gain access to the base of the plant, and then had to pull the plastic from under the remaining crowns, which were often 30 to 50 cm wide over a hole that was around 8 cm wide and not even remotely visible under the crown. This involved firmly pulling the plastic up on one side of each plant, cutting it with a knife to expand the hole, and then pulling it around the plant on each side. Most of the lateral branches had rooted through the plastic, resulting in significant resistance to this process. Ground staples had to be removed as we encountered them, even if they were left behind under the large crown.  

If that wasn’t fun enough, we then had to try to retrieve the edge of the plot, which included ground staples every 30-50 cm, from under 7 years’ worth of grass growth and accumulation of grass clippings. As the edge of the plastic was yanked, the ground staples would stay behind in the lawn, and had to be located and dug up based on where the tear in the plastic occurred. All of this was necessary because even a single ground staple could be sucked into cultivation or mowing equipment, causing significant damage at any time in the future until the staples completely rust away in about 50 to 100 years. This whole process made me question whether it would have been better to hand weed for 7 years rather than face one day of plastic removal. My plot was 9 m wide and about 30 m long and it took two of us about 8 hours to remove the plastic. Many farms are doing this on a much larger scale. Lavender growers should consider this one-time, onerous process before choosing a ground cover system! If you do have to remove a plastic sheet, it is best to cut the plastic the full length of the plot between each row with a sharp, serrated chef’s knife (not a utility knife) and then remove the plastic down one row at a time. That way, plant debris that accumulates on top of the plastic can fall to the side as you pull the plastic.

The whole process is likely much easier for strips of ground cloth with grass between rows because these can be laid with a mulch layer. These implements bury the edges of the plastic rather than relying on ground staples. This allows for mechanical means of removing the plastic, such as using a mulch lifter implement.

My next plot will compare lavender on solid sheets vs strips of ground cloth with grass between rows. While this does mean that the whole process of plastic removal will have to be done again in the future, I hope that it will be in for a long time, and I will be retired before it has to be removed. The plot will examine whether lavender growth is better without the competition of the grass between rows, or if the excess heat over the solid sheet causes more problems with disease. I will also compare plants growing with regular fertility and irrigation with plants that are only irrigated during establishment.