Family values
Summary
The next highest level of the Linnaean taxonomy after genus is the family. Plants in the same family often share similar characteristics. Knowledge of these characteristics can help in identifying unknown plants, in selecting plants as part of a design, in successfully growing plants in a garden, and in managing potential problems with the health of plants.
So far on Getting Dirty, we have looked at a variety of ways in which plants can be classified. At the highest level (article here), this is into the four main groups: bryophytes (mosses), pteridophytes (ferns), gymnosperms (conifers), and angiosperms (flowering plants). Angiosperms can then be further grouped into monocots and eudicots (article here). Plants can also be classified according to their lifecycle (article here): based on their lifespan (ephemerals, annuals, biennials and perennials); whether or not they lose their leaves in winter (deciduous and evergreen); whether or not their tissues become rigid over time (woody and herbaceous); and whether or not they flower repeatedly (monocarpic and polycarpic). Woody perennials are also further divided into trees, shrubs and sub-shrubs.
Some of these classifications are quite loosely defined. A small multi-stemmed tree and a large shrub can often be difficult to tell apart (with many plants being described as either). Some plants are evergreen to a point, losing their leaves when conditions become too harsh (and are then sometimes called “semi-evergreen”). Others are technical scientific groupings; many people would be unfamiliar with terms such as bryophyte.
A more rigorous approach to classification is the Linnaean taxonomy (article here), with plants (and other forms of life) classified according to an increasing detailed hierarchy, down to the species, written in binomial nomenclature with a generic (indicating the genus) name and a specific (indicating the species) epithet, such as Asphodelus albus. A still more rigorous approach is cladistics, which seeks to trace the point at which the common ancestor of species diverged in the evolutionary record, but this is beyond the scope of Getting Dirty.
The level in the hierarchy of the Linnaean taxonomy just above the genus is the “family”, which groups together different genera (plural of genus) that share common characteristics. Plant families were historically determined mainly in terms of their reproductive structures (flowers for angiosperms), and are given names in Botanical Latin that end with the suffix “-aceae”. More recently, plant families have been determined using genetic analysis, resulting in some plants being moved from one family to another (for example, the horse chestnut tree, Aesculum hippocastanum was originally placed in the Hippocastanaceae family, but genetic analysis showed it belonged in the Sapindaceae family).
The largest plant family, the Asteraceae, has around 32,000 species divided into approximately 1,900 genera. There are several plant families that contain only a single species (“monotypic” families), such as the Gingkoaceae, which contains only Gingko biloba (other species in the family went extinct millions of years ago).
Why learn about plant families?
In many cases, particularly when the family is small or monotypic, knowing which family a particular species belongs to is neither necessary nor useful. However, when the family is larger, its common characteristics can prove very useful: for identifying plants, for selecting plants in a design, for growing plants, and for mitigating issues with plant health.
Plant identification
There are many occasions when, as gardeners, we need to identify a plant. For example, we may wish to determine if a plant is a desirable border plant or a noxious weed. We may like the appearance of a plant, and want to understand how we can best propagate it; or to seek out plants with similar looks. Or we may wish to eat a plant, and need to know whether it is edible or toxic (you should always be certain of your identification and your knowledge of a plant before attempting to eat it).
Identifying a specific plant directly from amongst the 400,000 or so named species would be an extremely challenging task, and likely beyond the capabilities of anyone, no matter their expertise. However, with practice, many plants can easily be placed into the correct family based on observation of their characteristics. This significantly reduces the challenge of identification, even if we only manage to narrow down to the largest family, the Asteraceae. Further learning within key families can help to identify the particular genus of a plant (for example, by studying the shape of leaves and how they attach to the stem), and knowledge of the growing environment can then help to pin down the species. The process is by no means infallible, but a knowledge of the plant family is the easiest place to start.
Plant selection
Selecting a plant for a given location is the opposite problem to identifying a growing plant. Rather than starting with the plant and using our knowledge of its family, its particular characteristics and its growing environment to identify it, we start with the known growing environment and desired characteristics to select a plant that will both thrive in that location and meet the requirements of our design, whether aesthetic or practical.
Knowing the main characteristics of plants within particular families can help us to first narrow down the family of plants that is likely to meet our requirements. This reduces the challenge of selection to a more manageable problem, since we can use our knowledge of the growing environment to select genera that are likely to thrive in that location, then research the species that are likely to meet our requirements.
Growing plants
Often, but not always, plants within a particular family have similar requirements in terms of light, moisture, soil acidity/alkalinity, temperature, etc. Rather than needing to learn how to care for every plant we are likely to encounter in our gardens, we can learn how to grow plants within the most common families, then focus our efforts on knowing and understanding only the exceptions. These may be those plants within a family that have different requirements, or plants that are the only members of a family that we are likely to grow.
Knowing the family of a particular plant can also be useful when we want to propagate that plant. Plants within the same family typically bear seeds in a similar way; so if we know how to collect seeds from one member of that family, we can most likely guess how to do so from other family members. Those seeds are also likely to have similar requirements for successful germination. Similarly, members of a particular plant family are likely to respond in the same way to cuttings being taken. Knowing the family can help us to determine what time of year a cutting should be taken and from which part of the plant, and how best to get that cutting to root.
Plant health
Plants within the same family may be susceptible to attack from the same, or similar, pests and pathogens. Furthermore, other plants from a particular family may act as hosts for these, undermining our efforts to manage the health of our plants if growing nearby. An example here is the Rosaceae family. This family is important both agriculturally and horticulturally, as it includes a great many important genera such as Prunus (plums, peaches, cherries, apricots, almonds), Malus (apples), Pyrus (pears), Rubus (blackberries, raspberries), Fragaria (strawberries), Rosa (roses), Cotoneaster, Pyracantha, Potentilla, Alchemilla and many others. It is also notoriously susceptible to diseases, including black spot, fireblight and replant disease. Growing hawthorn (Crataegus monogyna, a member of the Rosaceae family, and a common hedgerow plant) near to a fruit orchard might present a risk of diseases finding safe harbour on the edges of the growing area.
Some common plant families
There are over 400 plant families (the exact number is itself sometimes disputed, as some classifications are still based on observed differences, whereas others are based on genetic analyses). A detailed review of the characteristics of all of these families, or even just the most common ones, would make this a very long article, so we will restrict ourselves to highlighting just a few. For those wanting to learn more, there are excellent resources online (including Wikipedia), and the RHS has published a book, “Genealogy for Gardeners” that provides a lot of detail on many plant families.
Some commonly seen plant families include:
Asteraceae (the daisy family): Composite flowers made up of tiny blooms fused together, surrounded by rays of petals
Fabaceae (the legume family): Butterfly-like blooms, seeds in pods
Rosaceae (the rose family): Five petals, surrounding a dense cluster of stamens
Geraniaceae (the geranium family): Colourful, radially symmetric flowers within five petals and five stames
Liliaceae (the lily family): Large showy flowers with parts arranged in threes
Apiaceae (the carrot family): Tiny five-petalled flowers arranged in compound umbels (umbrella shapes)
Hydrangeaceae (the hydrangea family): Small flowers clustered into inflorescences, sometimes “mophead”, sometimes “lacecap”
Note that plant breeders have selected for particular mutations, such as double flowers, which may conceal the basic structure of a flower in some cultivars. This is commonly the case in roses, for example, where only the species roses found growing wild in hedgerows are now commonly seen to have the simple flowers typical of the Rosaceae family.
A final word…
This is by no means the last word in plant classification. Plants can also be classified by their need for water (hydrophytes - aquatic plants; hygrophytes - bog plants; mesophytes - plants requiring moderate amounts of water; xerophytes - drought loving plants; and halophytes - salt water plants). They can be classified by their habitat (alpine plants, desert plants, jungle plants etc). They can be classified by how they grow (geophytes - bulbs, corms and tubers; chamaephytes - herbaceous perennials that periodically die back to resting buds at ground level; phanerophytes - plants, usually woody perennials, whose stems project into the air with resting buds well above the ground; epiphytes - plants that grow on other plants; and aerophytes - plants that obtain moisture and nutrients from the air and rain). They can be classified by their tolerance of alkalinity (calcifuge and calcicole).
Many other classifications are possible, and may be of interest to botanists, or to other scientists conducting detailed analyses (possibly for academic interest, possibly for practical or commercial interest, such as pharmaceutical or agricultural scientists). However, as gardeners, we would rarely have need of these groupings; and when we do, it is for a highly specific context, such as aquatic plants to stock a pond; and in most cases, we would rely on a very limited selection rather than seeking to grow a wide variety. For our purposes, it is often more useful to think in terms of the adaptations that plants make to survive different environments, and to seek out plants with these adaptations. This is a topic to which we will return…