If you've ever grown a passionflower vine (Passiflora spp.), you're familiar with its exotic, fringed flowers that have an odd-looking, triangular-shaped central crown. Several species are cultivated, including the common passionflower (Passiflora incarnata) and the purple passion fruit, (Passiflora edulis) or maypop, which bears small edible fruits. They grow in U.S. Department of Agriculture plant hardiness zones 5 through 9 and USDA zones 10 through 12, respectively. These plants are vigorous, considered invasive in some warm-weather climates and usually easy to grow, although any of several problems could cause leaves on the plants to turn yellow.
Passionflower plants do well planted in the ground, but can also be container-grown -- in cold-winter areas, they can overwinter successfully indoors. They're quick growing and need a constant, even supply of water during the growing season from spring through fall. If the soil dries out completely, older leaves may start turning yellow, eventually drying up and falling off. A container-grown plant is especially susceptible to drying out, because heating of its pot in the sun speeds water loss. Check the plant's soil every day or two with your finger and water it well whenever soil is dry to the touch. Adding a layer of organic mulch to the area under the plant also helps conserve soil moisture.
Because passionflower plants are quick-growers, they need regular fertilizing to sustain growth and promote flowering. If soil nutrients are low, leaves could begin turning yellow, starting with small young leaves and spreading to larger ones, and flowering might be poor. To keep a plant's foliage healthy and green, feed it with a 10-5-20 granular fertilizer, giving it a total of 3 pounds of fertilizer per year -- divide it into four equivalent treatments, spaced equally from spring through fall. Scratch the fertilizer into the soil under the plant, being careful not to disturb roots, and water well after feeding. Withhold fertilizer during winter, when the plant slows its growth.
Exposure of a tender maypop plant to light frost could cause yellowing of its leaves, and if prolonged, they might dry up and the entire plant could die. If your area has cold winters, bring a maypop plant indoors in early fall and over-winter it in a bright, sunny window. In areas with milder winters, protect a maypop from occasional cool weather by growing it against a warm south- or west-facing wall or in a container on a covered patio or porch. The common passionflower withstands some frost but can develop yellow, dying leaves in extra-cold weather. It also benefits from planting in a warm spot, or you could cover it loosely with burlap to protect it from an especially cold stretch of weather.
Pests and Diseases
Passionflower plants are susceptible to the cucumber mosaic virus, which can cause yellow patches on leaves that eventually curl and die. There's no cure for this problem, but it's spread by aphids, which can also cause leaf damage. If you see tiny, soft-bodied aphids on a plant, destroy them with insecticidal soap, diluted at a rate of 6 tablespoons per gallon of water -- spray the plant until it's dripping wet, and repeat every two weeks or as needed. Wear gloves and protective clothing when spraying. The plant might also develop a fungal disorder, leading to wilting and yellowing of leaves. Fungus thrives in moist, cool soils and humid air, so choosing a warm, sunny site with good air circulation can help prevent this problem.