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  Tucson Cactus and Succulent Society

Growing in the Desert Column

(List of Growing in the Desert columns)

Putting Adeniums and other Tropicals to Sleep for the Winter

by Mark Dimmitt

Photos by Mark Dimmitt unless noted

(This article may only be reprinted with the author's permission.)


Adeniums are succulents related to pachypodiums, plumerias, and oleanders (Figure 1). They are the newest ornamental plant to be domesticated (the process began only 20 years ago), and their popularity is growing rapidly. They are easy to grow if their cultural needs are understood and provided for. Spring is one of the critical times during which many adeniums and other winter-dormant tropical plants are lost (see article April 2012). The other critical skill is to recognize when to let them go dormant for the winter. Remember this crucial fact: Adeniums hate cold, wet roots. The plants are nearly indestructible during the hot summer. But only a few days of cold nights can kill the most sensitive clones if the medium is wet. The critical night temperature is about 50 degrees, but it is influenced by the daytime highs, as will be discussed later in this article.

What is dormancy?
Dormancy in plants means a cessation or great slowdown of active growth. The main evidence is that the plant produces few or no new leaves (Figure 2). Below ground, root growth and water absorption greatly diminish, so the potting medium dries out more slowly than it did in summer. A dormant plant may retain leaves, or either shed them all at once or slowly over the winter; don't be fooled by that. A dormant plant may also flower, either with or without leaves. What’s important is that it has slowed down and is using much less water than during its growing season. Therefore you must give it much less water, or even none for weeks or months at a time.

This discussion recognizes two horticultural groups of adeniums:

1. Most adeniums in cultivation are A. obesum and its hybrids. Almost all of the plants in this group are potentially evergreen if they are kept in tropical conditions (sunny, hot days, warm nights) through the winter (Figure 3). A sunny greenhouse is best; a sunny window in the house is good. If you can't provide such conditions, the plants will go dormant during the cool season. Depending on the temperatures, light, and the particular plant, they may retain most or all of their leaves.

2. All of the other species, except some A. arabicum, have an obligate winter dormancy, even if the conditions are sunny and hot. Most species will shed their leaves in fall or early winter, often suddenly (Figures 4 and 5). Adenium swazicum may sleep for only a month or two; A. multiflorum and A. crispum sleep for three or four months. Adenium boehmianum and A. socotranum often remain dormant for six months. Adenium arabicum is extremely variable over its large natural range; you need to learn the invidividual plant's habits. Some become spontaneously dormant in fall and shed their leaves (Figure 6). Most become dormant but retain most of their leaves well into winter if kept warm (Figure 7). Some clones are potentially evergreen and even grow through winter if kept under hot conditions (Figure 8).


Overwintering Tips:

If most of your plant’s leaves suddenly turn yellow and fall off , this is a clear signal to greatly reduce or cease watering (Figure 9). This may happen as early as the fall equinox, or more likely after the first cool nights later in the fall. A more common response is that the plant’s water consumption greatlydiminishes. It often happens quite suddenly. One week you need to water three times, and the next week only once or not at all. Monitor the moisture in the potting mix of each plant carefully in autumn in order to catch this change and respond accordingly. If your adeniums are overwintered in a hot greenhouse, almost all of them will still use much less water even if they keep growing. My Adenium house gets up to 100 degrees F on sunny days, and is heated to 45 at night. Most of my plants stay in leaf and often flower well into winter. Those with obligate dormancy enter it at different times; some keep growing slowly until January, when the short days finally stop them. Adenium obesum and its hybrids keep growing under these conditions, taking only a brief rest about March, just before they begin growing again. As long as my plants have leaves, I water and lightly feed them every week or two. Gene Joseph has an unheated greenhouse that also gets to 100 F or more on sunny days, but drops to near freezing on the coldest nights. The hot days seem to average out the cold nights, and his adeniums stay active well into winter.

 

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