We need to unmask an impostor.

Before we do, I'd like to explain why I save things. In trying the minimalism trend, I reduced the pile of sawed-off two-by-four ends I was saving in case I needed them for something someday. And we shortened our stack of old Cool Whip containers that could have refrigerated a century's worth of leftovers.

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But I'm happy I never threw out the big black three-ring binder filled with yellowing typewritten pages of gardening articles written 40 years ago while I was with the North Dakota State University Extension Service covering topics from petunias to potatoes. Most gardening basics are timeless, and I sometimes look back to review a topic.

A friend recently asked why their Christmas cactus refused to bloom, and I knew I'd written about a similar case in the big, black book. But before we get to the whys and why-nots of flowering, we should reveal the impostor.

The Christmas cactus that refused to bloom wasn't a Christmas cactus at all - it was a Thanksgiving cactus, masquerading behind the snazzier Christmas name. In fact, most of the holiday cactuses sold today as a Christmas cactus aren't really so.

What's the difference? The two types are distinct botanical species, and the difference is noticeable. Christmas cactus is Schlumbergera bridgesii (synonym buckleyi) and Thanksgiving cactus is Schlumbergera truncata.

Look closely at the stem sections of the cactuses. They don't really have leaves, but rather flattened pad-like stems joined in segments. The pads of a Christmas cactus have lobes that are smooth-rounded. The pads of a Thanksgiving cactus have very visible tooth-like pointed prongs that project outward along the stems.

The flowers are shaped only slightly different. Thanksgiving cactus flowers are held more horizontal, while Christmas cactus flowers tend to hang downward. Thanksgiving cactuses usually bloom earlier, but not always, and both types can rebloom again in late winter or early spring, with bloom times often overlapping.

What's wrong when these holiday cactuses refuse to bloom? The answer lies in understanding that short days and cool temperatures trigger their flowering, and when day length and nighttime temperature reach the right combination, flower buds form. If the right formula isn't met, no flowers form.

Several combinations can happen. If the nighttime temperature drops to between 50 and 55 degrees for a few weeks, flower buds will form regardless of day length. That's why many of our grandmothers' Christmas cactuses bloomed easily in old, chilly houses. Sometimes locating a cactus near the windows of today's homes can provide a microclimate with the necessary low temperature.

But if nighttime temperatures drop only to between 60 and 68 degrees, then the holiday cactus needs the added stimulus of 14 hours of total darkness each night for six weeks. That's why some people cover their cactus each evening with a dark cloth or black garbage bag to prevent interference from indoor lights.

If nighttime temperatures remain 70 degrees or higher, holiday cactuses often will not bloom, regardless of what you try. If your home remains warm at night, locate the plant near the chilliest window you can find.

An easy and reliable method to trigger a holiday cactus into bloom is to give it a summer vacation outdoors in a protected, filtered-light location until late summer, when night temperatures fall consistently to between 45 and 50 degrees for several weeks. Bring indoors before temperatures dip below 45 degrees, and the cactus should be blooming by November or early December.