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Crystals on sweet potato vine not insects or disease

The white substance on this sweet potato vine is crystallized sappy material exuding through the pores or stomata of the leaves. Special to The Forum

Q: This fall, for the second year in a row, I've brought my sweet potato vines indoors, and just like last winter they now have some very strange white stuff growing on several leaves. Do you have any idea what this is? - David Jondahl, Fargo.

A: I've never experienced it on either the purple or lime-green sweet potato vine that we've grown, but your photos show an interesting phenomenon that has been reported by others. The white crystal-like substance with the sugary appearance isn't insects or disease, but is sappy material exuding out through the pores or stomata of the leaves. The material crystallizes into a white residue.

The exact cause doesn't seem to be known, but it's believed to be caused by an interaction between soil moisture and temperature. Something environmentally, like moisture change or heat, triggers the process. If it happens on many leaves, you might examine the watering, and change it a bit.

For example, if you're watering regularly, maybe let it dry out more in between. It's possible that the same watering routine that was used outdoors is too much indoors during winter's lower light and short days. The plant might be responding to too much water by pushing excess material out through the leaf pores.

Q: I have been struggling with some little things on my houseplants I've heard referred to as scale. Any suggestions for control? - Ione H., Moorhead

A: Scale insects are oval-bodied, barely moving insects that cluster along plant stems and leaf undersides. Their bodies are tan or brown and are often covered with a waxy, tan-white coating. They suck sap from houseplants, and if left unchecked can cause yellowing of leaves and general plant decline. A shiny, sticky honeydew can often be seen on surrounding surfaces.

Scales' waxy body protects them from many insecticides. Horticultural oil and insecticidal soap can suffocate them. Systemic houseplant insecticide granules applied to the soil are very effective, as the product is taken internally by the plant, killing insects as they suck the sap.

Q: Don, at your urging, I'm attempting to overwinter geraniums for the first time. They keep setting blooms, and I'm wondering if I should pinch those back? - Donna Sauvageau, Detroit Lakes, Minn.

A: If the plant itself is healthy, well-filled with foliage and well-branched, then it's perfectly fine to let geraniums flower, and enjoy the blossoms. In fact, they will frequently flower nicely all winter if light is sufficient. But if the plant itself is small and struggling, then it's best to pinch off the flower buds as they form, so the plant's energy can be devoted to a healthy, well-branched structure, and then allow blossoms to form later.

In spring, cut geraniums back (about March 1) which eliminates weak winter growth, and stimulates new, healthy shoots branching from the base. Begin fertilizing every one-to-two weeks also. By May, the refreshed, healthy plants will be blooming again and ready to go outdoors.

If you have a gardening or lawn care question, email Don Kinzler at All questions will be answered, and those with broad appeal may be published, so please include your name, city and state for appropriate advice.