"Difficult color, green," Maggie Smith sniffs in "Gosford Park." While the British actress is making a smart-aleck comment about another character's dress in the Robert Altman movie, much the same could be said about many people's yards this time of year.

The spring flowers that had covered the magnolias, dogwoods, azaleas, weigelas and other trees and shrubs are long gone, leaving nothing behind but dull green. In my own yard in upstate New York, hydrangeas and roses are the only bushes in bloom.

But there's a way to break out of this midsummer monotone. I'm talking about plants whose leaves are decorated with streaks, stripes or spots of white, silver or yellow. These so-called variegated-leaf plants are a welcome relief in an otherwise solid-green row of shrubs or trees.

While not commonly known, many green-leafed plants also come in a variegated-leaf version. There are ginkgo trees whose leaves are streaked with yellow, redbud trees whose leaves are a mottled, marbleized white and green, winterberries with gold-blotched leaves and miniature hemlocks whose branches look like they are covered with frost.

This honeysuckle vine has leaves outlined in white, making it stand out even after the flowers are gone. Bart Ziegler

From a distance these plants can resemble the work of an impressionistic artist or a stipple-paint home decorating job. They give you something interesting to look at all season long, flowers or no.

Variegated-leaf plants once were rejected by many serious gardeners and plant professionals, who considered them too strange or showy. But they have gained popularity as the garden industry searches for new things it can promote each year to what seems to be a declining base of avid gardeners.

In most cases these plants are naturally occurring oddities of nature, not some product of plant breeding or bioengineering. A plant expert or astute home gardener might notice, on an otherwise ordinary plant, a branch that has extraordinary leaves. This branch is cut off, rooted and grown into a variegated-leaf tree or shrub, from which many new shrubs can be created. In other cases the unusual leaves arise by chance on the seedling of a green-leafed plant.

You're not likely to find many of these plants at a big box store like Home Depot and even a well-stocked garden center may sell only a handful. You may need to seek them out at the more creative plant catalogs such as ForestFarm, Klehm's Song Sparrow Farm or Bluestone Perennials or at specialized nurseries.

While many variegated plants are trees or shrubs, even the perennial flower world has its variations. I have a clump of tall garden phlox that looks interesting all summer long because its green leaves are streaked in white, compared to my other all-green phlox that you barely notice except for the three or four weeks they are in bloom.

My variegated mock orange plant, whose leaves are so showy they overshadow the fragrant flowers. Bart Ziegler

You do have to be a bit careful in using variegated-leaf plants or you could end up creating an unintended clash day in your yard. A few years ago I planted a weigela with yellow-striped leaves a few feet from a honeysuckle vine with white-and-green variegation. That was fine until the vine grew and ended up slithering alongside the nearby fence and into the shrub, creating a messy jumble of green, yellow and white.

Wolf Eyes dogwood Bart Ziegler

Another thing to be aware of is that not all variegated plants are "stable," as they say in the plant world, meaning that they suddenly can develop some solid-grean leaves. One of my weigelas has this bad habit, while my other one never loses its variegation. It's good practice to cut off the branch on which the all-green leaves appear. Otherwise you risk eventually having much of the tree lose the very leaves for which you bought it.

There's another downside of sorts to these plants: The leaves can be so dominant that they overshadow any flowers the tree or shrub develops.

Variegated Kerriea Japonica Bart Ziegler

That's the case with a mock orange tree (Philadelphus) I planted near my house. It covers itself with white flowers each spring but they're hard to see among the cream-and-green leaves--though you still know they're there by their sweet fragrance. Some variegated plants also can be less hardy than their green-leaf counterparts and may grow more slowly. Still, none of these are reasons not to try variegated plants. I've successfully interspersed them in lots places where they stand out amid all the green.

That's especially helpful in shady spots. I have two Kousa dogwood trees of a variety called Wolf Eyes. Their white-and-green leaves pop out near the edge of the woods in my yard like white highway lines at dusk.

And in my hillside shade garden I have clusters of a plant called Brunnera 'Jack Frost.' Its heart-shaped leaves are covered with silvery-white veins that glow in the shadows long after its blue springtime flowers have disappeared.

Write to Bart Ziegler at bart.ziegler@wsj.com