Far more than any other group discussed in this guide, climber plants (plants that use other plants or supports to grow upward naturally) and trailer plants (those that grow along the ground or have a lax, graceful habit) can be thought of primarily in terms of one design element: line. Many provide colorful flowers and foliage and others offer intriguing textures, yet it is their ability to be trained into lines and linear forms (climber plants) or their naturally linear growth habit (trailer plants) that offers endless design potential for container gardeners. If you want a container planting to direct a viewer's eye vertically (either upward or downward), horizontally, or in an arching or circular manner, consider employing climbers and trailers first.
In more concrete terms, if you would like to raise the height of a container planting without placing the pot on an inverted pot, flight of steps, or plinth, construct a simple teepee from bamboo poles and raffia directly in the pot, or insert an elegant wooden trellis or wrought-iron frame. Then allow a climber to twist its way up the support, or periodically tie a suitably flexible trailer to the frame; in time, you will have added as much height as you wish. Maybe you would rather create a cascade of color flowing from a window box. If so, include climbers and trailers with other plants (if any). Would you like to make a container planting appear to cover more square footage than just the surface area of the pot? Include some trailer plants, which stretch out and increase the width of a planting without needing to root along the ground. A row of hanging baskets overflowing with climber plants and trailer plants will create a visually irresistible line of sight; when placed closely together, the baskets create a suspended screen. Finally, if you truly want to impress your gardening friends, train a climber onto a wire hoop, sphere, or other form to create a memorable piece of living sculpture.
 Morning glories (shown here is Ipomoea acuminata) climb rapidly and produce abundant blooms. Go easy on the nitrogen fertilizer, though: Too much will result in few blooms and rampant foliage.
 Trailing petunias, such as this Petunia integrifolia, and the related Calibrachoa are both available in a huge color range and can grow quite large in relatively small pots.
The adaptability of climber plants and trailer plants does not end there. You can carefully train, tie, and clip a climber into a buttoned-down, very formal-looking shape. Alternatively, you can let the plant go where it will along a frame or perhaps into other plants in the pot or near it. All climbers can be trained with some effort on the part of the gardener, but by the same token, any climber lacking some sort of support will grow downward from a pot or hanging basket (and those in the list of climbers provided all look good doing so). Why not let a morning glory tumble out of an earthbound pot or aerial hanging basket?
Climber plants can present a few challenges, especially to the anal-retentive and time-challenged among us. If you want a climber to cover a support neatly and precisely, then be prepared to periodically wind, tie, and/or clip it to make it bend to your will. Failure to do so will result in a neat-freak's nightmare, with shoots jumping willy-nilly from one part of a support to another or even twisting around their companions.
As with every plant, try to determine the growth rate and potential size of your climber or trailer. Some of these - especially among the tropicals - can cover many linear or square feet in a season or two, while others may extend only a couple of feet, if that. Don't try to grow a morning glory or passionflower in a six-inch pot, and be prepared to include several smaller growers, such as dichondra or lobelia, in a monumental hanging basket.
A SELECTION OF CLIMBER PLANTS FOR CONTAINERS
CissusFamiliar grape ivy (C. rhombifolia) quickly and easily provides a great many green diamonds (look at the individual leaflets to see what I mean), while C. discolor, sometimes called rex begonia vine, offers the stuff of a designer's dream: fantastic silver- marked, rich green, elongate leaves with red-purple undersides on gracefully hanging and curving stems. Don't let it grow too densely, or you will lose the space between the leaves and stems and diminish the foliage impact.
ClematisDon't be afraid to try a clematis in a pot. Give it a suitable support, such as a trellis or even a hoop, and keep the potting mix moist and out of hot, drying sun; then enjoy the colorful stars, pinwheels, and bells that appear among usually medium-textured foliage on slender stems. Clematis are often brittle, so shelter them from strong wind.
Ficus pumilaYounger plants of creeping fig project an intricate, fine texture when clinging to a wall, and they can do the same when hanging out of a container or trained onto a support. Train a few plants onto a wire hoop, globe, or other three-dimensional form to produce a specimen plant loaded with visual interest provided by all five design elements. Variegated and miniature-leaved selections offer even more to look at but tend to grow much more slowly than the plain green species.
Hedera helixI consider English ivy my first candidate for best all-around climber for containers. Hundreds of selections offer a range of colors, leaf forms, and textures, and almost all hang gracefully from a container or lend themselves to training on a form. A few are very cold hardy, but most benefit from overwintering indoors out of the worst cold weather. Try bright yellow 'Amber Waves', white-painted 'Calico', and fine-textured, busy-looking 'Needlepoint'.
IpomoeaLate-blooming moonflowers (I. alba) open their giant white, morning-glory-like trumpets as night approaches, bringing spots of light to the night and attracting people and moths with their heavenly scent. Think of the now widely available selections of I. batatas as English ivies on steroids: big, boisterous, wildly colorful, and rather coarse, but endlessly useful in larger containers. 'Ace of Spades' and 'Blackie' bring a strong suggestion of shadows to sunny spots, 'Margarita' offers dazzling chartreuse and gold, and 'Sweet Caroline' combines variable decorator shades of brown and bronze and purple. Cardinal climber, I. xmultifida, produces little red "morning glories" on delicate, finetextured but vigorous vines.
MandevillaAlthough they may take a year or two to reach a statement-making size, they are worth the wait. Mandevilla xamoena 'Alice du Pont' bears animated morning-glory-like pink blooms throughout warm and hot weather on twining, fairly coarse vines. Allow plenty of room in the container for one of these. Others, sometimes considered members of the genus Dipladenia, tend to be less rambunctious and make suitable additions for smaller containers. Look for these with red, pink, or white flowers and red-toned new growth.
MuehlenbeckiaFast, fine-textured, and fascinating with its tiny, crown-shaped, Gummi-bear-like white fruits, an angel vine (M. complexa) can be trained on virtually anything or can be left to dangle from its container. Impatient gardeners take note: it provides seemingly instant gratification.
PassifloraPassionflowers should be featured prominently in any depiction of the Garden of Eden: they are lush, sensuous, and so tempting to touch. Vigorous plants produce abundant leaves in a variety of shapes, but the flowers are built mostly on the same exquisite plan of fringed stars with a central cluster of exclamation points. P. vitifolia produces vibrant red blooms over a long season. Unlike most of its relatives, blue-lavender P. caerulea tolerates quite a bit of cold.
Rhodochiton atrosanguineusYou will not find another plant that looks quite like this: a mass of dark green, heart-shaped leaves festooned with dusky purple "bells" with almost black "clappers." Its medium to fine texture stands in bold contrast to the brooding flowers. Blooms the first year from seed where happy, which in my experience is in cool summer areas, such as the Pacific Northwest and coastal Maine.
Solanum jasminoidesGrow potato vine for its profusion of medium-textured foliage, especially the golden-variegated selection, and for the pretty, blue-white or white starry flowers. Expect a small, well-tended plant to put on at least three feet of growth in one season.
Thunbergia alataBlack-eyed Susan vines are easy to grow and always seem to have at least a few cheerful, five-parted blooms decorating the pointy foliage. Most selections bear flowers in shades of orange, yellow, and white with central black "eyes." Those that do not have black eyes miss the whole point of their charm. There are many other species of Thunbergia in many flower colors, but most would prove too much to manage in all but very large containers.
 Most ivies grow lustily, so keep them separate or be prepared to restrain them. Some variegated selections, such as Hedera canariensis 'Gloire de Marengo', can take on pinkish tints with the onset of cooler weather.
 Give Ipomoea batatas 'Margarita' and other ornamental sweet potatoes plenty of water, fertilizer, and root room to produce cape-like masses of trailing foliage, or restrain them for less abundant but still showy chains.
 The surreal-looking flowers captivate anyone who takes the time to examine a Passiflora 'Incense'.
 "Superplant" is not an undeserved moniker for Scaevola aemula, which blooms abundantly and quickly bounces back from severe wilting.
A SELECTION OF TRAILERS FOR CONTAINERS
Bacopa (Sutera) 'Snowflake'Written about and commercially available under two genus names, this small but mighty workhorse delivers all season by producing hundreds of starry little white flowers above rather fine-textured foliage. Other selections have recently become available with bigger flowers and are worth a try.
Dichondra argenteaWithout question one of the top peacemakers for separating two or more potentially conflicting colors. Started from a few small plants, a fine-textured curtain of silver - yes, it actually looks silver, particularly when sunlight bounces off the leaves - can hide a pot or dangle gracefully from a hanging basket in a few months' time. Look for 'Silver Falls'.
Helichrysum petiolareFormerly my favorite finer-textured gray-silver trailer until I discovered Dichondra argentea, licorice plant still deserves its role as a harmonizer for many other colors. Actually a shrub, it grows quickly and gently curves out from its container, contributing a pleasing line and spreading form. Chartreuse 'Limelight' deserves your consideration too.
LantanaOne of the very first plants I remember growing was a hot-colored selection of L. camara. Most of the kinds available today are still in that color range; for white and lavender, choose selections of L. montevidensis. In contrast to the gently outward-arching L. camara, L. montevidensis is a weeper, hanging straight down from its container or nearly so from the trunk of a standard topiary. All have sharply scented foliage, which some noses enjoy and others detest.
Lobelia erinusFine-textured, low-growing, trailing selections bear masses of flowers in jewel tones of blue, pink, lilac, and white over a long season if you can keep them moist and cool. In warmer areas, enjoy their elfin beauty until heat and drought claim them.
Lotus berthelotiiCertainly one of the finest-textured of all plants suitable for containers, parrot's beak provides a mass of gray-green threadlike foliage to cover the "ground" or cascade from a container. Overfertilization may lessen the silvery gray impact of the leaves. Gardeners in favorable locations enjoy the bright red, curved flowers.
Lysimachia nummularia 'Aurea'Without question, golden creeping Jenny holds a place on my top-ten list of plants for containers. A hardy perennial, it seems to quickly and effortlessly produce long chains of little, rounded leaves that are golden in sun and chartreuse to almost plain green in shade. Little golden flowers appear briefly in summer but do not bring much to the party, I believe. A knockout with deep-toned selections of foliage plants such as Carex, Euphorbia cotinifolia, Hemigraphis, Solenostemon (Coleus), Strobilanthes, and Tradescantia (to name a few).
Pelargonium peltatumIvyleaved geraniums are not quite the challenge they once were in regions with hot summers, now that breeders are releasing more heat-tolerant selections. Of course, gardeners in regions with cooler summers can enjoy these all season. Starlike foliage (gold-netted in 'The Crocodile', but the color fades with the onset of heat) produces the backdrop for clusters of flowers that look just like those on bedding geraniums. The advantage here is that they appear on billowing, cascading plants instead of on sticks.
PetuniaTalk about a makeover: once dowdy and martyrs to heavy rain, petunias now take their place among the boldest and sturdiest plants for containers. Even the pastel-colored, large-flowered, frilly selections have more oomph these days. Look for the deeply saturated colors of the 'Wave' series as well as stridently magenta P. integrifolia, constantly adorned with little trumpets with black "eyes" even if you forget to water it occasionally. Some selections have a gentle fragrance, and all but the smallest flowers clamp down on your nose when you take a good whiff. Try it.
PlectranthusLots of useful, vigorous trailers here, including boldtextured, slightly stiff-looking, silvery gray P. argentatus; precisely white-edged P. forsteri 'Marginatus'; and several others that travel under a welter of confused and conflicting names. Almost all of the latter group luxuriate in warm and hot weather and bear stickyfuzzy, pungently scented foliage. Perhaps the most useful is a dark green selection of P. coleoides with dark red undersides. It combines with everything.
Salvia discolorAnother silver trailer (I really like them, can you tell?), but this one has gray upper and almost white, fuzzy lower leaf surfaces, and it looks quite open. The flowers come close to being black, but don't grow S. discolor solely for that reason: the flowers do not pack nearly as much punch as the foliage.
Scaevola aemulaLet's all thank Australia for producing this champion. 'Blue Wonder' and similar selections bear fanlike clusters of similarly fanlike flowers for months. By season's end, a planting of this at the top of a wall at a friend's nursery had produced a traffic-stopping, three-foot-long, violet-blue shower of constant color. Don't garden without it.
Tradescantia pallida 'Purpurea'Unlike anything else on this list, 'Purpurea' (also known as 'Purple Heart') creates an energetic, sprawling mass of bladelike, dark purple foliage, especially when grown "hard" (in plenty of sun and in smallish containers that are allowed to go a bit dry before watering). Pinkish violet flowers add a whimsical touch to the drama.
VerbenaMost verbena hybrids bear flower clusters that look like brooches in shades of red, purple, white, and peach. The blooms appear all season on vigorous, spreading plants, and some are lightly fragrant. While most verbenas have low-key, medium-textured foliage, the selections of V. tenuisecta (aptly called moss verbena) offer very fine leaves that give the effect of a wispy green cloud.
 Whether brilliantly or subtly colored, the flower clusters of Lantana attract another purveyor of color: butterflies.
 A little bit of baby's tears (Soleirolia soleirolii) grows rapidly among other plants. Once it reaches the edge of the pot it will cascade down in a fine green rush.
 A few stems of Tradescantia pallida 'Purpurea' provide linear interest when trailing out of a pot, but a mass of it causes a scene.