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Why this healthy houseplant gets webbing in the winter

"Fielding Questions" columnist Don Kinzler also answers readers' questions about watering a poinsettia watering and storing potatoes.

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A reader wonders what's going on with this houseplant that gets dusty-looking fibers in the winter.
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Q: Can you tell me what’s on my mandevilla? The plant gets dusty-looking fibers every winter. There's no other webbing on the plant and it looks fairly healthy. I’ve sprayed it numerous times with different bug sprays, but it keeps coming back. — Beth.

A: Mandevillas are spectacular patio plants and they do very well when brought indoors for the winter in a sunny window and then put back outdoors in summer. They’re quite susceptible to insects indoors, though.

I suspect the plant has spider mites, which seem to be ever-present on mandevilla plants indoors. Light infestations of spider mites often go unnoticed, with the plants looking quite healthy, as yours does. Symptoms are more dramatic when populations balloon, causing more webbing and leaves that become grayish-green and eventually brown.

Because your plant still looks so healthy, it’s a perfect time to keep spider mite activity in check. Mites can be controlled by very wet applications of insecticidal soap. Place the plant in a large sink or tub and wet the upper and lower leaf surfaces and stems with the soap to the point of runoff.

Mandevillas are also subject to aphid infestations, especially in the tip growth. Insecticidal soap and neem oil are options for controlling both mites and aphids. Both products are available at most garden centers.

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Q: I’d like to try keeping our poinsettia growing after Christmas. How wet should the soil be kept? I know they don’t like to be overwatered, but it’s hard to know how often to water. — Dan S.

A: If you’ve ever removed a poinsettia leaf, you’ve noticed the white sap that exudes. The waxy stems and milky sap help poinsettias behave like succulents, conserving moisture within. They can go long periods between waterings.

My favorite way to determine if any houseplant needs watering is the finger-test method. Poke a finger into the potting mix up to the first joint. If you feel moisture at the fingertip, don’t water; check again in a day or two. If you don’t feel moisture, then apply enough water to thoroughly wet the soil ball.

Poinsettias will quickly go downhill if they’re kept continually too moist. If in doubt, you’re always safest to err on the dry side.

Overwatering means keeping the soil continually too soggy. Most poinsettias are sold with a decorative outer container or foil wrapping in which water can easily puddle. For long-term poinsettia growth, poinsettia pots are best removed from decorative outer coverings and placed in a saucer so excess drainage can be easily seen and immediately discarded.

Good luck with the poinsettia! Getting them to rebloom is fun, and I encourage people who enjoy plants to try it at least once or twice, so they can add it to their list of plant accomplishments.

Q: Why do the potatoes I raise in the garden start growing sprouts faster than the ones I buy at the grocery store? I like our homegrown potatoes much better, but they don’t seem to store as long. Any ideas? — Bob L.

A: When potatoes are harvested in the fall, they have a natural dormancy that prevents them from sprouting. The internal dormancy lasts anywhere from 60 to 150 days, depending on the type of potato and the temperature at which they’re stored. The higher the temperature, the quicker the dormancy is overcome, and the potato tubers begin sprouting.

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Potatoes would quickly become unsaleable when displayed at grocery store temperatures. To increase their shelf life, most commercial potatoes are treated with sprout-inhibiting compounds, either in the field or in the warehouse.

To prevent homegrown potatoes from sprouting prematurely, proper storage temperature is vital. The best temperature for long-term storage is about 40 degrees Fahrenheit, which can be found in some unheated basement rooms, storage areas, root cellars or garages.

Most refrigerators run too cold for ideal potato storage, causing the starch to convert to sugar. Refrigerated potatoes can develop sweet or off-flavors, and they tend to turn dark-colored during certain cooking methods, like frying.

If you have a gardening or lawn care question, email Don Kinzler, NDSU Extension-Cass County, at donald.kinzler@ndsu.edu. Questions with broad appeal may be published, so please include your name, city and state for appropriate advice.

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Don Kinzler, "Growing Together" and "Fielding Questions" columnist.
The Forum

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