Lavender die-back

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Schizolobium

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Lavender die-back
« on: March 04, 2020, 07:55:06 PM »
Although I have lived in Southern Spain for twenty years, it was not until I moved into this house in 2017 that I had the opportunity to create a garden. The site was a blank canvas, just a few geriatric fruit trees and the aftermath of building works.

Planting began in January 2018. Following Olivier Filippi’s inspirational books, I planned a dry garden, where only new plantings would be watered as required. I provided a watering basin around each plant. In the autumn of that year we had exceptionally heavy rainfall, 150mm in October and the same again in November. I mention that as it may be significant.

In early 2019, as the previous year’s planting sprung into life. I was delighted with the result; it exceeded my expectations. However, as summer progressed, I was presented with a quandary. Many lavenders started to die back but not the whole plant, just certain stems. I lifted some and potted them up, having cut out the dead sections. They recovered strongly so I reasoned that the cause was unlikely to be disease. I had considered phytophthora as the culprit but that is highly unlikely given the planting conditions – see the footnote.

Most lavender species and cultivars were susceptible, the only ones spared were L. multifida and L. dentata. Some other plants were affected, notably Perovskia atriplicifolia and Verbena bonariensis. None of the many Rosemaries were affected, which is interesting in light of a post describing a similar problem http://www.mgsforum.org/smf/index.php?topic=843.0

Examining the plants that did die, I noticed their roots showed no sign of fine feeding-root. I also saw that there was a build up, some 2 to 3 cm deep, of fine silt in the watering basins.

I conclude that root of the problem, no pun intended, is that the soil level around the collar of the plants had risen. The heavy rain washing down the slope had collected in the basins, as intended, but bringing soil with it; the plants were suffocating in silt.

Is this a reasonable and likely interpretation? If so, how can I manage the situation given that the plants are established, the profile of the watering basins too?

A corollary would be that watering basins are not appropriate for plantings on a slope.

Footnote
Hot semi-arid climate (Köppen type ‘BSh’), 330mm p.a. rainfall, SE facing slope (1 in 5), 50m altitude, 500m from sea. Shallow, calcareous, sandy soil on a bedrock of schist veined with gypsum.
Once an astrophysicist but a now down-to-earth gardener. Brought up in South Africa, spent 15 years in Sussex establishing a large woodland garden which was open to the public under the NGS. Settled in Spain, on the Costa Tropical (Granada), creating a new, medium sized, irrigation-free garden.