What Causes Purple Leaf Color on Hydrangeas?
Purple leaf color on hydrangeas isn’t normal and may indicate fungal disease or simple environmental problems.
Fungal Disease
Purple spots on hydrangea leaves are a good indicator of cercospora leaf spot, a common leaf fungus in these plants. Plants are rarely killed, but spotted leaves may shed prematurely, weakening the plant and reducing viable buds. The small purple to brown spots usually start near the base of the plant, spreading outward and upward as water splashes the spores to other leaves. Spotting patterns vary based on the type of hydrangea involved. Slow the spread of cercospora by cleaning up fallen leaves and watering your hydrangea at the base. Opening the canopy by thinning up to a third of the branches inside a tightly packed hydrangea bush will increase air circulation, making it difficult for the spores to germinate. If cercospora is severe and widespread, azoxystrobin, chlorothalonil, mancozeb, myclobutanil, or thiophanate-methyl should be applied at 14 day intervals.
Phosphorus Deficiency
Hydrangea leaves that turn purple may be trying to tell you that there simply isn’t enough phosphorus nearby to keep the plant healthy. Sometimes, in their rush to change their hydrangea’s flower colors, gardeners may accidentally cause the pH to drop so low that other chemical compounds bind phosphorus. Bound phosphorus can’t be used by plants, leaving them short vital nutrients. Check your soil pH— acidic soils with pH below 6.0 often allow aluminum to tie up phosphorus, those alkaline soils with pH above 7.0 may be binding it with calcium or magnesium. Adjusting your soil’s pH is the first step to freeing phosphorus, but if this doesn’t show a noticeable difference in a couple of weeks, you will need to apply a phosphorus fertilizer to the hydrangea’s root zone.
Weather’s Influence
Weather can also influence the color of hydrangea leaves, causing large areas of purple discoloration. Cool weather near the end of the growing season may trigger the plant’s dormancy early, allowing the purple leaf color to show through as the green chlorophyll factories shut down for the season. Frost damage can also cause a purple discoloration. Pluck off the badly damaged leaves when they dry out but leave those only partially injured behind until new leaves form.
