Music0 min ago
No best answer has yet been selected by fielakind. Once a best answer has been selected, it will be shown here.For more on marking an answer as the "Best Answer", please visit our FAQ.
If the plant is in a container you can plant it any time of the year (keeeping it well watered until established). If the plant is bare-rooted, ie not in a container, I would think late autumn is best, say November, when the soil is still workable but the plant is going dormant for the winter. But as for soil and drainage requirements, I don't grow lavender so can't confidently advise on those points.