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Martin Quigley, The Ohio State University, Horticulture and Crop Science; James A. Chatfield, Kenneth D. Cochran, |
Plant families, as you will recall from previous text, are groups of related genera. Let's look at the very important rose family (Rosaceae), which includes such genera as:
If you think about the flowers of these genera (forget for a moment the amazing diversity of some of the cultivated roses and think instead of some of the shrub roses), you will note that they are very similar.
Think of how similar each crabapple flower is to a hawthorn flower or a Callery pear flower, or, for that matter, to individual mountainash florets. In fact, if you think of each floret of a mountainash flower and the fruits of a mountainash, it is easy to see that it is far more related to a spiraea or a firethorn than it is to any of the true ashes in the genus Fraxinus, which are in the Oleaceae family. It should come as no surprise that the reproductive parts of the plants, the fruits, seeds, and flowers, provide clues to the relatives of these plants.
It is easy to fool people on plant identification quizzes with the unusual rose-salmon fruits with bright orange seeds of Euonymus europaeus, but when you ask what roadside weed it looks related to, someone in the crowd always says American bittersweet (Celastrus scandens). Both Celastrus and Euonymus, indeed, are in the bittersweet family (Celastraceae).
This familial relationship can be used in many ways in practical horticulture. For example, the disease bacterial fireblight, caused by the bacterium Erwinia amylovora, occurs only on plants in the rose family, especially on Pyracantha, Malus, and Pyrus species. Symptoms of this disease include blighted shoots that are bent at the ends in the pattern of a shepherd's crook. This symptom can also be caused by other factors, though.
It is a great diagnostic aid, therefore, to be able to rule out fireblight, even if plants have crooked blighted shoots, if the plant is a maple (Acer spp.) or an ash (Fraxinus spp.), knowing that these plants are not in the Rosaceae family. Conversely, a good horticulturist would consider fireblight as a possibility if the plant is a mountainash (Sorbus spp.) or chokeberry (Aronia spp.), which are in the Rosaceae.
Another practical benefit of knowing plant families occurs when there are cultural requirements that cover most of the plants of a family. A classic case of this is with the heath family, the Ericaceae. Although members of the Ericaceae vary in their characterization as acid-loving plants, it is not a bad generalization to be concerned about planting ericaceous plants, such as azaleas and rhododendrons (Rhododendron), Enkianthus, Pieris, mountainlaurel (Kalmia), and blueberry (Vaccinium) in alkaline soils. Other horticultural practices limited by familial relationships occur, as well, from the likelihood of being able to make intergeneric crosses (difficult at best but more possible between genera in a family), to the likelihood of being able to graft a scion onto a rootstock (if from different genera, more likely between genera in the same family).
Some families contain only one genus, or even one species, such as Cercidiphyllum japonicum (katsuratree) in the Cercidiphyllaceae. More often than not, though, it is possible to note interesting similarities, for example, of fruits, between the multiple genera in a given plant family. For example, all of the trees in the nitrogen-fixing bean family (Fabaceae), including Cercis (redbud), Cladrastis (yellowwood), Gleditsia (honeylocust), Gymnocladus (Kentucky coffeetree), and Laburnum (golden-chaintree), have bean-like fruits.