Most herbs are native to hot, dry climates with alkaline soils, while most rhododendron species come from cool, moist places with acid soil. Herbs are usually tolerant of a wide variety of conditions as long as they get enough sun, so it is not hard to create a suitable microclimate for them in a rhododendron garden.
It is possible to extend sun exposure by making sure there are no large trees on the south and west of the herb garden. Even tall herbs, such as lovage, can shade other plants. The south side of a building creates a sun trap in which herbs can thrive and rhododendrons can die.
Since the moist Enumclaw loam is ideal for rhododendrons, it needed modification for herbs. We added four inches of sand to the beds and then tilled it in with the soil. The herbs grew well with the new soil level and composition.
Lavender is a good example of a Mediterranean plant needing a microclimate in the Pacific Northwest. Given the right conditions, it thrives within a few feet of rhododendrons.
A few herbs, such as and wintergreen, prefer shade. To create a mini-microclimate different from the rest of the herb garden, we overlapped the ellipse into a bed of large rhododendrons. The shade-loving herbs are protected by low-hanging branches of a Beauty of Littleworth and enjoy the mulched acid soil.