Lavender Crop Update July 10 2015

Lavender is in bloom across the province. Although lavender does not like “wet feet”, plants without any flooding of the root system are benefiting from the recent rains with a lot of new vegetative growth. This may allow plants to recover more rapidly from the winter and frost damage. Once plants have finished blooming, pruning should be done as soon as possible to give plants as long as possible to grow back this year. Many buds are still developing in the centre of plants. It is best to wait until the majority of these have fully bloomed before a full pruning occurs, or the plant may continue to send up new buds the rest of the summer. If isolated buds continue to develop after pruning, they should be removed to encourage vegetative growth.

How far to prune back the plants will depend on their appearance at the time. Some fields are full of Septoria leaf spot that has killed back many of the leaves in the centre of the plant (Figure 1). The plants will likely start to out-pace the Septoria with new growth once they are done flowering, but pruning too heavy too early could result in poor recovery after pruning. It is best to wait for new green growth to emerge before doing any heavy pruning if Septoria is an issue. If plants are not affected by Septoria, there are two possibilities:

  1. The plants are heavily damaged and new and weak growth is coming from the base of the plant. In this case, plants should be pruned back significantly to allow light into the centre of the plant and ensure the new growth thickens and becomes stronger before winter.
  2. The plants are showing thick new growth that extends beyond last year’s wood. In this case, a normal pruning of 30-50% of the new, green growth should suffice.

Figure 1. Septoria leaf spot is appearing on many of the older leaves on this plant.

In all cases, it is a good idea to add nitrogen fertilizer at pruning to promote new growth. There are no recommendations for lavender in Ontario, but research and recommendations in other regions suggest an annual nitrogen application of 50-100 kg/ha. An application of 50 kg/ha at pruning is likely sufficient, with additional N applied if heavy rains occur. Heavy rains can leach nitrogen out of the root zone.

Some growers may have noticed a few random caterpillars feeding in the field. There are several species of caterpillar that have been found on lavender. Figure 2 shows a yellow furry caterpillar feeding on lavender at the Simcoe Research Station. One or two caterpillars were found on each of at least a dozen plants. When these were reared they developed into the black and orange woolly bear caterpillar with which most people are familiar (Figure 3). The caterpillar appears to be able to fully develop on lavender and can cause considerable damage on isolated small plants. However, the damage would be insignificant to a larger plant. Regular scouting can indicate the presence of a caterpillar, at which point it can simply be removed by hand.

Figure 2. A yellow, furry caterpillar on lavender.

Figure 3. The yellow caterpillar continued to develop into a woolly bear caterpillar. Woolly bear caterpillars are the larvae of the isabella tiger moth.

About Sean Westerveld

Ginseng and Medicinal Herbs Specialist, OMAFRA
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