Lavender bloom continues to be well ahead of normal. In Simcoe the lavandins are coming into full bloom, whereas angustifolias are mostly at or past peak bloom.

Row Spacing Considerations

The lavender plot at Simcoe is on two side-by-side 4.5 m (15 ft) wide solid sheets of woven black plastic (Figure 1). There are 7 rows within that area resulting in a between row spacing of 1.2 m (4 ft). The plot was established in 2019 and without any significant winter kill over the past few years, the plants are now very large. Plants of many of the angustifolia cultivars are about 1.2 m wide in full bloom, meaning there is no room to walk between the rows (Figure 2). Many of the lavandins are about 1.5 m wide in full bloom, resulting in significant merging of flowers between the rows (Figure 3). It is nearly impossible to walk down the rows without causing major damage to the flowers.

Figure 1. The cultivar demonstration plot at Simcoe in its second year in fall 2020 showing the planting system on solid sheets of woven black plastic.

Figure 2. Two rows of angustifolia plants on 1.2 m row spacings with flowers just meeting in the middle of the row after 5 years of growth.

Figure 3. Two rows of Edelweiss plants on 1.2 m row spacings with flowers significantly overlapping leading to an impenetrable wall that is impossible to walk through.

Based on this, it appears the absolute minimum between row spacing would be around 1.2-1.5 m for angustifolias and 1.5-1.8 m for lavandins to accommodate 5-year-old plants. Plants may continue to grow, so minimum spacing may be even wider unless plant size is kept smaller through aggressive pruning. For agritourism, consider a minimum of an additional 1.2 metre of row spacing to allow for two people to pass each other or walk side-by-side down the row.

Black Plastic and Heat Stress

My demo plot in Simcoe has been plagued with major plant health problems ever since the plot was established. In the second year, some plants died, and many plants lost several branches to an unexplained wilt issue (Figure 4). Peak bloom that year coincided with a heat wave. In the following years there were also significant issues that were in patches and affecting certain cultivars. Over the last two years many flowers aborted before reaching bloom resulting in sparse flowers in some cultivars (Figure 5). This year, there are still a few lingering issues, but most plants are very healthy and flowers are fully formed (Figure 6). If I had to graph the severity of the damage, it would be worst in 2020 (the year after establishment) and then decline every year since. I was never able to identify any specific cause of the damage in the plot, just weak pathogens like Fusarium and Pythium which are always present even in healthy soils.

Figure 4. Collapse of plants in 2020, a year after establishment, due to an unknown root issue that may relate to heating of the soil.

Figure 5. Poorly developed flower heads on some cultivars due to an unknown root issue that may relate to heating of the soil.

Figure 6. The lavender plot at Simcoe in June 2024.

I don’t think it is a coincidence that the plants at the same time were getting larger and larger and are now covering most of the ground cloth. This would result in much less heating of the plastic, reducing air temperatures around the plants and soil temperatures. I think it is highly likely that my problems were entirely due to overheating of the soil resulting in poor root growth, weak soil pathogens attacking the weakened roots, and major problems in hot weather in June and July when plants are in bloom and have the largest water demands of the year. Higher air temperatures above the plastic would also result in increased water demands which the weakened roots could not supply.

Having strips of plastic with grass between rows probably reduces this issue significantly. However, the potential for overheating of the roots is something that all growers should consider. The worst issues are likely to be on solid sheets of plastic and in the first few years after establishment. It is probably more of an issue if there is a combination of the heating and another issue in the soil. This could be the wrong soil type (heavier soil), excess moisture in the soil, or weak pathogens.

If you are having problems with plant health issues consider overheating of the roots as a potential issue. It may be difficult to determine the exact cause of the issue, but here are some things to look for:

• Plants look fine coming out of winter, but issues start to emerge as the flower buds develop.
• Flowers buds abort and some never fully develop.
• Some branches suddenly wilt and collapse while others remain healthy.
• Plants recover somewhat after bloom and no further damage is noted for the rest of the year.

If you think this may be a problem in your fields, consider methods to cool the soil or prevent overheating until the plants are large enough to shade the plastic. While these have not been tested and some may not be practical, here are some potential options:

• Organic mulch (e.g., bark mulch) over the plastic. You may want to remove the mulch after a few years or it will break down and eventually support weed growth on top of the plastic.
• Using irrigation to wet and cool the soil and reduce heat stress during dry and sunny periods.
• Using a greenhouse whitewash spray to essentially paint the plastic white, but with an environmentally safe solution.
• Using a strip of white plastic over the black plastic in the row.

Pests

Four lined plant bugs are done feeding in most regions and will not emerge again until next May. Note where damage was the worst and consider those areas for an insecticide application next spring.

Garden fleahoppers have finished one generation and the second generation of nymphs will be emerging soon. This could cause an explosion of populations. Monitor for them regularly. Garden fleahoppers can completely defoliate lavender by August under ideal conditions. They appear to be worst on solid black plastic. Growers should monitor plants regularly in random plants throughout the field and consider applying an insecticide once damage to the leaves becomes obvious and populations reach hundreds per mature plant.

To learn more about identification and management of these pests, visit the lavender section of Ontario CropIPM: https://cropipm.omafra.gov.on.ca/

Fertilizer

With all of the heavy rains so far this spring, nitrogen in the soil may have been leached out of the root zone. This is probably a good time to do a second application of nitrogen fertilizer since plants will soon be shifting back to vegetative growth. Another application could occur at the beginning of August.