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Fielding Questions: Hoya troubles, overwintering mandevilla, potato storage

In this week's Fielding Questions, Don Kinzler offers tips for treating an ailing hoya plant, wintering mandevilla and more.

Hoya Nov. 12, 2022.JPG
A reader asks gardening columnist Don Kinzler for advice in treating her ailing hoya plant.
Contributed / Elizabeth P.
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Q: I have a large hoya that’s been magnificent for over 10 years. Lately it’s been dropping leaves that look dried up. I’m enclosing a picture that hopefully gives you a clue as to what’s happening. Normally this plant is just awesome. It’s in an east window in which it’s done so well. – Elizabeth P.

A: Hoya are great houseplants, and when they reach a certain age, reward the grower with fragrant flowers. Your plant looks like it’s been well-grown and beautiful for many years.

I notice several things that could be causing dried and dropping leaves. Over a period of years, insects such as spider mites, mealy bugs and scale can seek out older plants and build their populations, finally causing troubles. When I enlarge the photo, I might be seeing signs of insects. Treat with insecticidal soap, neem oil, or systemic granules.

When was the plant repotted last? When I see plants with problems, I consistently notice a deeper-than-advised “headspace,” the distance between the pot’s rim and the soil surface. Instead, a headspace of only about a half-inch gives a greater vertical soil depth, and gravity aids in pulling water downward, creating better drainage and root health.

I might suggest removing the plant from its pot and adding fresh potting mix below so the plant is situated with only a half-inch headspace, which will give fresh soil and improved drainage. Please keep us posted.

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Q: I’m trying to overwinter a mandevilla plant. In mid-October, I cut the stems way back and set it in our laundry room next to a north window. I thought it would "rest" until spring, but it’s sending vines out from all sides! Do I let these grow? How do I keep pruning it so that it’s full and flowering next spring? – Connie G.

A: Mandevilla can winter beautifully indoors, and my wife, Mary, and I bring our indoors each fall also. Mandevilla is a tropical flowering vine well-adapted to containers and is perfect for pots on decks, balconies and planters. A trellis inserted in the pot helps it grow vertically.

If the pot isn’t too huge, the pot-and-all can be brought indoors in fall, or repotted if the outdoor container is too large. Stems should be cut way back as you’ve done, and they quickly begin new vine growth, as you’ve found.

Mandevilla don’t really rest during winter, but instead keep growing, which is fine. They grow best in direct sunshine, making a south or east window better choices than a north window, where growth would be spindlier.

Keep the mandevilla growing as you would other houseplants. Mandevillas are prone to spider mites and mealy bugs, so treat preemptively with insecticidal soap, neem oil, or systemic granules.

In March, prune back the winter vine growth similar to the fall pruning, begin fertilizing every two weeks, repot into fresh soil if needed, and give plenty of sunshine, and the mandevilla will be ready to go outdoors later in May.

Q: The potatoes did great in our garden this year, and we’re wondering the best way to store them. We’ve been keeping them cool in our garage, but it’s unheated and we need to move them inside, because the garage usually freezes. Where’s best to store them, and what temperature should they be at? – Josh L.

A: Potatoes should be stored in conditions called “cool moist,” meaning cool temperatures and a humid atmosphere. In the old days, most homes had unfinished basements that included a root cellar, or a room where vegetables were stored, that remained cool and humid and was perfect for potatoes.

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Today’s homes can be a challenge for storing homegrown vegetables, but with a little ingenuity, it’s possible. In our own home, we’ve partitioned off a corner of our basement to make a small root cellar that excludes heat from the rest of the basement. In some homes, spaces can be found under basement stairs or in a utility room that might be cool enough.

The ideal temperatures for storing potatoes is around 40 degrees F. Most refrigerators are too cool for best potato storage, because they run around 35 to 37 degrees F. At these cooler temperatures, potato starch begins turning to sugar, affecting the flavor and cooking qualities of the tubers.

More gardening columns from Don Kinzler

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.

Don Kinzler, a lifelong gardener, is the horticulturist with North Dakota State University Extension for Cass County. Readers can reach him at donald.kinzler@ndsu.edu.
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