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).