Butterfly bush, or buddleia, attracts not only butterflies but also bees and other beneficial insects. Pairs of soft green leaves frame clusters of nectar-bearing flowers in vivid blues, reds and even yellow. As flowers fade, new ones sprout at their base, in pairs like the leaves, for a full summer's worth of blooms. Periodically, however, yellow leaves appear on lower branches. Several different causes determine the support you provide to ensure the health and growth of your butterfly bush.
Ordinarily a few yellow leaves at the bottom of a vigorous shrub are no cause for alarm. Especially with a bush which continues to bloom and grow throughout the entire summer, a few base leaves may go by the wayside. More than half a dozen leaves, however, especially if the number increases as the season progresses, signals that this sturdy grower is under stress, from watering problems, fungal infection or insect damage.
Over-watering is one of the causes of yellow leaves on butterfly bush. Your first response to yellowing should be to cut watering back by a third to a half, letting surface soil dry before watering again. Drooping flower clusters and leaves will help you determine when you have cut back too much. Often, just the decrease in water will stop the yellowing problem.
Fungal Infection and Root Rot
The most common causes of root rot in butterfly bush are pythium spp. and phytophthora spp. organisms, although other fungi can manifest in root rot and problems for all the plants inhabiting infested soil. In general, county extensions manifest low enthusiasm for chemical treatments of fungal infestation. Maryland recommends removing infected plants and aerating soil. The Oklahoma State extension service notes that phymatotrichum root rot is more common in alkaline than acidic soils and suggests that a moderately effective method of controlling the problem is to re-acidify soil balance. In butterfly bushes with mild symptoms of root rot, heavy pruning and soil aeration may ensure survival. In severe cases, bushes will continue to yellow and wither until removed.
Insect Issues and a Dilemma
Japanese beetles display a fondness for butterfly bush leaves but are unlikely to cause basal leaf yellowing. More likely the culprit is spider mites. Agriculturalists note, however, that a healthy bush can combat a siege of spider mites without permanent damage. Pesticide use presents a problem: butterfly bushes host so many beneficial insects that pesticides should be employed only as a final measure. Some gardeners use a homemade liquid soap/cooking oil spray to clear off the worst of the spider mites without harming other insects.
Supporting the Health of Your Butterfly Bush
Aside from physically improving soil drainage, most of what you will do to address yellowing leaves on your butterfly bush is aimed at strengthening your plant to combat difficulties without chemical assistance. Fungicides and pesticides appear to have their downsides; the wrong fungicide can do more damage than the fungus, and pesticides mean no butterflies. Aerating soil, decreasing watering, and adding manure to address the pH balance, as suggested by the Oklahoma State extension--this old-fashioned blooming beauty apparently responds best to old-fashioned ways.