I have an existing English lavender plant in full glorious purple bloom. It’s been 3 years since planting. It’s been pruned twice a year every year in my care. Last fall I inherited a second mature English lavender plant that had not received any routine pruning and had a lot of woody elements. It overwintered well, but now that it’s blooming, half the plant has white-ish flowers, the other half matches my original lavender shrub. I verified that there were no edelweiss or alba varieties that may have accidentally come home with me. So, why are the blooms losing colour?
North York, clay soil, full sun, south east exposure, occasionally watered only during prolonged dry spells, no fertilizer or amendments added aside from some cedar mulch.
From your photo, we can see bright purple flowers as well as a few lighter purple blooms and some whitish flower heads that look ready to erupt into light purple blooms. English lavender (Lavandula angustifolia) has the vibrant purple flowers that you describe on your existing plant. The plant loves full sun, warm but not hot/humid weather, sandy/dry soils with a slightly alkaline pH and once established, does not like excessive watering. Fertilizing is usually not needed.
There are about 24 species and numerous varieties of lavender, including several varieties of English lavender. English lavender is available in different colours, for example, L. angustifolia ‘Jean Davis’ has light pink flowers, while L. angustifolia ‘Munstead’ has flowers that come in blue, lavender, pink and purple.
You mention that you are certain that other varieties of lavender (e.g., alba, edelweiss) are not in the mix, but could your inherited plant be another variety of English lavender than your existing plant? However, even if this were the case, this would not explain what seems to be a single lavender plant sporting 2 colours of bloom. I found nothing in the literature to suggest that this occurs. I wonder if your inherited plant might actually be 2 separate plants that have been grown together – the different coloured blooms would appear to be from a single plant.
Nor did I locate information concerning lavender bloom colours fading. Even if colours did fade, it would be very odd if only half the plant’s blooms remained dark purple, while the other blooms were lighter. If something were affecting the bloom colour, this would be expected to affect the whole plant.
You seem to be doing everything right in regards to your lavender plants, and they sound healthy. I suggest that you continue to treat your inherited plant as you do your existing one, and see what happens!