There is often talk about various plants being related to each other. There are the sunny-faced members of the daisy family, for example, and the pea family, all with pods. The mustard family includes cabbage, broccoli, and kale. What characteristics link these groups of plants as relatives?

Before you blurt out that all daisies have petal-rimmed flowers typified by sunflower and aster, picture the flowers of goldenrod, also a member of this family. And, besides waxy, bluish green leaves, just look how much cabbage and broccoli differ in form.

Family talk is not idle gossip, because plant relations are important in garden planning. Club root fungi, for example, are as fond of cabbage as its kin; starve this pest out of the soil by not growing a member of the mustard family there for a few years.

Just for the fun of it, you might want to devote a part of your garden to a single family, perhaps the pea family, with a planting of lupines and sweetpeas adding color to the dappled shade beneath a honeylocust.

Modern plant kinship is based mostly on flowers. Other classifications were previously in use. For example, third century Greek philosopher Theophrastus grouped plants as herbs, undershrubs, shrubs, or trees. Simple enough, but is a maple tree really related to a palm tree?

Credit for our modern system of classification goes to Swedish naturalist Carl von Linnaeus. Look at broccoli or cabbage or mustard flower, and you’ll see that all have four-petaled flowers — an important characteristic of the mustard family. And daisies are linked more by the intricacies of the small florets that make up their heads than by the sunny heads themselves. Linnaeus’ “classes” were determined by the number, proportion, and position of flowers’ male parts. Classes were subdivided into “orders,” each based on the number, proportion, and position of flowers’ female parts.

As genetics and evolution have become better understood in the past century, they have also been incorporated into the scheme of plant classification. Theories about plant relations continue to change with new knowledge about plant evolution and as new techniques, such as DNA fingerprinting, unravel the genetic makeup of plants.

Decisions will always have to be made as to which characteristics, such as petal number or common ancestor, are most important indicators of kinship.