Hi. I’m located in zone 5b and I’m growing Munstead lavender in a container in full sun. I received the plant from a friend, and only a few branches around the outside of the plant have new growth. I assume the center rotted out during the winter. What would you recommend to help the plant achieve a better shape?
What you are describing is consistent with root rot of Lavendula angustifolia, or English lavender, of which ‘Munstead’ is one of the hardiest choices for our area. Root rot can happen when soil is poorly drained. Lavenders prefer full sunlight, which you are providing, as well as dry-to-average moisture levels and very well-drained soil. If drainage in your container is not ideal, your plant will not thrive, so if this is the case it is worth re-potting with fresh soil and good drainage in the bottom of the pot.
Hardy to zone 5 in Canada, this lavender is vulnerable to extremes in winter temperatures when it is grown in a container (known as the freeze/thaw cycle), and this might also be the cause of die-back of the roots.
If the roots are still viable, as they seem to be, pruning for shape is an option. “Munstead” responds well to pruning, and although many lavender-growing sources say that pruning should be done following flowering (i.e., following the harvest of the flowers), in your case you could prune now into a rounded shape, removing any flowers, and taking off no more than 1/3 of this year’s new green growth. The hope is that you will see new growth into the middle of the plant. Note that cutting into the older woody stems can damage, or even kill, this lavender.
Sometimes root damage is serious enough that a plant cannot be brought back to health. You will likely be able to tell by the end of the summer whether this is the case.