IBERIS
Candytuft. Spring- and summer-blooming annuals and perennials.
Description: Iberis umbellata, annual candytuft, produces 1-inch clusters of tiny flowers in shades of white, pink, red-violet, and purple through much of summer on upright plants to 15 inches.
I. sempervirens, perennial candytuft, is actually a low-growing evergreen subshrub that bears similar white flowers on 6-to 12-inch-tall, 12- to 24-inch spreading mounds of narrow, dark green, needlelike leaves. Most cultivars bloom only in spring, though ‘Autumn Snow’ reblooms in fall. Zones 3–9.
How to grow: Sow the annual type where it is to bloom in a sunny or lightly shaded spot with average, moist but well-drained soil; allow to reseed for bloom next year. Plant or divide perennial candytuft in spring or fall; give it similar conditions. Cut back partway after bloom for compact growth and possible rebloom in fall. Water during drought. Mulch in summer, and lightly cover with pine needles or straw in winter to prevent sun and wind damage.
Landscape uses: Plant at the front of borders, among rocks, or in cottage gardens. They also look lovely along paths, stone stairs, and walls. Combine them with spring-blooming bulbs and perennials.
ILEX
Holly. Evergreen or deciduous shrubs or trees.
Description: Ilex × attenuata, topal holly, is an evergreen with an upright, narrowly pyramidal form, growing 20 to 30 feet tall. The leaves are small, dark green, and glossy. The fruits, which are red and rounded, appear on the current year’s growth. Zones 6–8.
I. cornuta, Chinese holly, is twiggy, dense, and rounded, with flowers blooming on the previous year’s growth. Fruits are red and rounded; the evergreen leaves are glossy green and five-spined, with the terminal spine pointing downward. Zones 6–8.
I. crenata, Japanese holly, is a densely layered, compact evergreen with small, notched leaves and black fruit. There are many fine cultivars available; most are much smaller than the species, which grows to 15 feet tall in the landscape. Zones 6–8.
I. × meserveae, blue holly, bears spiny blue-green leaves, purplish stems, and red berries. This evergreen grows about 7 feet tall. Zones 5–7.
I. opaca, American holly, is pyramidal in youth, open and irregular with age. Growing to heights of 15 to 30 feet in the landscape, this native may reach 50 feet in the wild. The evergreen leaves are spiny, dull dark green above, and yellow green beneath. Dull red berries form on the current year’s growth. Zones 5–9.
I. verticillata is the swamp-dwelling native winterberry. It has a twiggy, rounded habit and grows 6 to 10 feet tall. Its oval leaves drop off in autumn, exposing plentiful brilliant red berries that are very effective in massed plantings. Zones 4–9.
I. vomitoria, yaupon, is a Southeastern native with an upright, irregular tree form or mounded shrub form. Yaupon has smooth gray twigs, notched evergreen leaves, and translucent red berries. The shrub form grows 3 feet tall; the tree form grows 15 to 30 feet. Zones 8–10.
How to grow: Hollies are dioecious, meaning that individual plants are either male or female. Therefore, with few exceptions, you must have a male holly in the vicinity in order to have a nice crop of berries. Purchase hollies from a reliable source to ensure that you get both male and female plants. Hollies need full sun to partial shade and evenly moist, well-drained, humusy soil. Avoid windy sites and (except for winter-berry) poor drainage.
Landscape uses: Plant hollies in groups for hedges, barriers, screens, or a mass effect. A single specimen makes a stunning focal point. Hollies are also good in natural landscapes, since their berries provide food for wildlife.
IMPATIENS
Impatiens, patient Lucy, garden balsam. Tender perennials used as annuals.
Description: Two species are commonly grown, Impatiens wallerana, impatiens, and I. balsamina, garden balsam. Both bear 1- to 3-inch flat or roselike flowers in white, pink, rose, red, orange, lavender, purple, or spotted and banded combinations. The succulent-stemmed plants can grow 8 to 36 inches tall. I. wallerana cultivars cover the entire height range on spreading, green-leaved plants, while I. balsamina types resemble 2-foot trees with longer green leaves. New Guinea hybrids have large flowers and elongated green or multicolored leaves.
How to grow: Sow seeds indoors in early spring for planting after danger of frost is past, or buy transplants, especially of New Guinea hybrids, which are often grown from cuttings. Plant in partial shade and average soil. Water during droughts. New Guinea hybrids will grow in full sun if the soil is kept constantly moist. Root cuttings of your favorites in late summer for overwintering indoors. Spider mites can cause yellow-stippled leaves, especially in too much sun or very dry weather. Control by spraying plants with a blast of water or by applying soap spray every 3 to 5 days for 2 weeks.
Landscape uses: Impatiens are superb for beds, in masses among shrubs, as edges for plantings or structures, and in pots. White- or pastel-flowered impatiens will light up the gloomy areas under trees and other shady sites with their bright blooms. In warmer areas, self-sown seeds may naturalize to form a dramatic groundcover. Impatiens, especially the New Guinea hybrids, also make lovely houseplants.
INSECTS
Gardeners don’t pay much attention to insects—until they find holes in the leaves of their favorite plants or until the mosquitoes start biting. In a rash rush to exterminate the offenders, they may assume that every insect is an enemy, but that’s far from true. As much as 90 percent of the insects in your backyard are beneficial ones like soldier bugs, lady beetles, and lacewings that naturally keep the pests in check. Honeybees and beneficial wasps also are harmed by broad-spectrum control efforts, and crop pollination can suffer as a result. Finally, a great many insects are completely benign—they may live in your garden, but do no harm to your plants—and these, too, are harmed by wide-ranging pesticide applications.
In the amazing-but-true category, consider the fact that four out of every five animals on Earth are insects. And there are more species of beetles than there are of all other animals, including insects, put together. Estimates of the total number of insect species range from 2 to 10 million. No one knows how many more species are yet to be discovered, especially in the tropics and other remote regions.
There are good reasons to learn about the behavior and biology of the insects in our gardens. The most important one is that a little knowledge about how insects function can help us prevent pest damage without using invasive and potentially toxic control methods.
Insect Anatomy and Life Cycles
All insects have six legs, and their bodies are divided into three sections: the head, thorax, and the abdomen. They have two antennae that help in sensory perception, and most insects have two pairs of wings located on the thorax. (Flies have only one pair of wings.) Insects breathe through small air tubes located along the length of their bodies called spiracles.
Some insects have strange and complicated life cycles. The best-known example is the transformation of a wormlike caterpillar into a sensationally beautiful butterfly. In general, insects follow one of the two main patterns of development.
Complete metamorphosis: In this pattern, the immature insect is transformed during a resting stage into an adult that looks like a completely different organism. The adult insect usually inhabits a different environment and eats different food than it did at the immature stage. In some cases, one stage may be a pest, while the other is harmless. For example, parsley-worms, which feed on carrots, dill, and related plants, are the larvae of black swallowtail butterflies.
As the larva grows, it periodically sheds its skin until it reaches its full size. It then forms a pupa and enters a resting stage. When the adult has finished forming inside, it splits open the pupa and emerges, expands its wings, and when its outer skeleton has hardened, it flies away to search for food and mates.
Incomplete metamorphosis: Insects that follow this pattern develop from immature to adult in gradual stages without forming a pupa. The insect starts life as an egg, which hatches into a nymph. The nymph sheds its skin several times as it grows, becoming more and more like the adult with each molt. It gets progressively larger, its body lengthens, and wing pads appear where its wings will grow. The last molt is to the adult stage, with fully formed wings and reproductive organs. The nymph and adult usually have the same type of diet.
KEY WORDS Insects
Metamorphosis. A change in form during the development of an insect.
Larva. An immature stage of an insect.
Caterpillar. The larva of a moth or butterfly.
Grub. The larva of a beetle.
Maggot. The larva of a fly.
Pupa. A hardened shell formed by a larva, within which the adult stage develops.
Cocoon. A protective cover for a pupa.
Chrysalis. The pupa of a butterfly.
Nymph. An immature stage of an insect that does not form a pupa.
Parasite. An animal or plant that lives in or on, and draws nourishment from, another organism.
Parasitoid. An insect that parasitizes another insect.
Predator. An animal that attacks and feeds on other animals.
Winter Survival
Knowing how and where both pest and beneficial insects spend winter can give you important clues about how to either control or encourage them. Most insects in the temperate zones sense the shortening day length and cooler temperatures of autumn. Their bodies prepare for winter by building up energy reserves and by undergoing chemical changes so that their blood won’t freeze. Many adult insects cannot survive winter, but at least one of their life stages, such as egg, larva, or pupa, can withstand the long cold period in a state of hibernation (called diapause in insects). The overwintering stages of many pests, including armyworms and sawflies, burrow deep into the soil or into litter on the ground. Cultivating the soil in early spring can expose these hidden hibernators to predators such as birds. Other pests, such as leafrollers and tent caterpillars, pupate or lay eggs in cracks and crevices in tree bark. Scraping egg masses off of bark and spraying trees with dormant oil can help to eliminate these pests.
One good way to encourage beneficial insects, and give benign ones garden space, is to cultivate a hedge or border of companion plants that are rich in pollen and nectar. Plants that belong to either the daisy family, Asteraceae, or the mint family, Lamiaceaea, including catmints (Nepeta spp.), mints (Mentha spp.), daisies (Leucanthemum spp.), and yarrows (Achillea spp.) are good choices.
Feeding Habits
Insects have a wide variety of feeding habits; some are vegetarians, others are carnivores. Some eat nearly anything, while others can eat only one species of plant. But only a minority of insects feed on our crop or ornamental plants.
Plant eaters: These insects chew or suck on leaves, stems, or roots for food. They are usually pests—aphids, gypsy moth caterpillars, and Colorado potato beetles are good examples. Some are considered beneficial and have been used in biological control programs to combat weeds.
Parasites: Some insects, such as mosquitoes and blackflies, suck blood from birds and mammals; others, like lice and fleas, are skin parasites on animals. Parasitoid insects are generally beneficial from the gardener’s standpoint, since they live and grow inside host insects until the hosts eventually die. The adults of many parasitoids feed on pollen and nectar.
Predators: Predators such as lady beetles and ground beetles can eat hundreds of insects during their life cycles. Many predators, such as pirate bugs, are also adapted to survive by sucking plant juices or eating pollen when prey is scarce.
Omnivores: Some insects have evolved the ultimate survival strategy: they eat almost anything. Cockroaches and earwigs are omnivores; they eat many sorts of animal or vegetable material—even soap, glue, and paper bindings.
Scavengers: Dung beetles, carrion beetles, fly larvae, and other insects live on decaying vegetable or animal material, breaking down the dead tissues or organic matter and hastening their decay.
Fungus feeders: The larvae of fungus gnats and some obscure species of lady beetles feed on fungi.
The first step in deciding whether an insect is a pest or a beneficial is to identify it. See Resources on page 672 for a list of books that will help you identify insects in both groups. For more information on insects that help your garden and how to create habitat for them, refer to the Beneficial Insects entry and also to page 6. For information on preventing and controlling pest insect problems, see the Pests entry.
IPOMOEA
Morning glory, moonflower. Warm-weather annual or tender perennial vines.
Description: Ipomoea alba, moonflower, is a twining, 8- to 10-foot vine with heart-shaped leaves. The fragrant, white, trumpet-shaped flowers open only at night. I. nil, I. purpurea, and I. tricolor, all called morning glory, are similar. All are twining vines with heart-shaped leaves. The funnel-shaped blue, white, blue-purple, or crimson blooms open in the morning and in the afternoon.
Several species bear small red flowers that are excellent for attracting hummingbirds, including I. coccinea, red morning glory, which reaches 12 feet; I. × multifida, to 6 feet; and I. quamoclit, cypress vine or star glory, growing to 20 feet.
Gardeners also grow ornamental forms of sweet potato (I. batatas) for their chartreuse, bronze, or purple-black leaves.
All are tender perennial vines grown as warm-weather annuals.
How to grow: Plant morning glory seed outdoors after all danger of frost is past. Nick the hard seed coat for faster germination. Except ornamental sweet potatoes, which are allowed to trail on the ground, all morning glories need a stake or strings to climb on, and supports should be installed from the start. To grow ornamental sweet potatoes, start with purchased transplants. You can harvest tubers and overwinter them to grow plants for next year. See the Sweet Potato entry for details. Give morning glories average, well-drained soil and full sun to partial shade.
Landscape uses: Plant vining morning glories next to fences, porch rails, trellises, and arbors. Use ornamental sweet potatoes as temporary groundcovers, in containers, or as fillers in flower beds.
IRIS
Iris, flag. Mostly spring- to early-summer-blooming perennials and bulbs.
Description: Irises offer a huge range of colors and patterns, heights, and bloom times, with variations on a common theme of flower shape and plant form. The basic flower shape consists of three inner (often erect) petals, called standards, surrounded by three other petals (usually arching out and down), called falls. Leaves are almost always flat and long, resembling swords; they grow in rather open to quite dense upright or arching clumps from bulbs or creeping rhizomes. Some have fibrous roots.
By far the most popular group is the large collection of hybrids termed the “bearded” irises, named for the hairy caterpillar-like tuft creeping out of the center of each fall. Flowers range from barely 2 inches wide to 7-inch giants in what is probably the widest color range of any plant group, lacking only pure red. They bloom in early summer, from 2 inches to nearly 5 feet above stiff, swordlike leaves. A number of cultivars rebloom from late summer into fall, to double the show; these reblooming cultivars are worth seeking out. Zones 3–8.
In place of a beard, “beardless” irises flaunt a colorful spot, called a signal, or an intricate pattern of lines. Blooms on Iris sibirica, Siberian iris, rarely exceed 3 inches wide and are more open and graceful then most bearded irises; they occur in shades of white, red-violet, blue, and purple (plus a few rare pinks and yellows, such as the yellow-and-white ‘Butter and Sugar’) on upright, grassy clumps averaging 3 feet tall. They bloom toward the end of the bearded’s season. Zones 3–9. I. ensata (formerly I. kaempferi), Japanese irises, bear 4- to 10-inch, six-petalled flattish or more double flowers in shades of white, pinkish lavender, red-violet, blue, and violet, often exotically edged, lined, and speckled. Most grow to about 2½ to 3 feet and are broader-leaved and less dense than Siberians, blooming a few weeks later. Zones 4–9. Very early-spring-blooming, bulbous I. reticulata, reticulated irises, produce narrow-petalled, sweetly scented 3-inch blooms, mostly in blue and purple shades with orange or yellow signals, among sparse, four-sided leaves that may grow to 1½ feet after bloom. Zones 5–9.
How to grow: Most bearded irises are easy to grow, but they do have specialized needs. Plant and divide every 3 to 4 years in summer or early fall, splitting them into individual “fans” with the rhizome attached, or into divisions with a few fans. Trim leaves back before planting to make up for root loss. They grow best in full sun or very light shade and average to rich, well- drained soil. Barely cover the rhizome and point the leafy end in the direction you want it to grow, ideally out from the center of a group of three to five of a kind.
Bearded irises tolerate drought very well when dormant (usually beginning about six weeks after bloom), but water them well up to the time dormancy sets in and after division. Fertilize routinely in spring and early fall, keep weeds and other plants away from the rhizomes, mulch loosely the first winter after division, and be ready to stake the tall cultivars when they bloom.
Soft rot attacks during wet seasons in poorly drained soil, entering though wounds in the rhizome made from premature leaf removal or too-close cultivation; it can also be carried on the body of the iris borer. The eggs of this pest hatch in spring, producing 1- to 1½-inch-long, fat, pinkish larvae. The larvae enter a fan at the top and tunnel down toward the rhizome, where they may eventually eat the whole interior without being noticed.
In fall, remove dead, dry leaves, which often carry borer eggs, and destroy badly infested fans in spring. You can also crush borers in the leaves by pinching toward the base of the telltale ragged-edged leaves or by running your thumb between the leaves and squashing any borers you find. They are also vulnerable when you divide the clumps; check every rhizome for this pest. If you find a few borers, try cutting them out, but destroy badly infested rhizomes.
Siberian irises enjoy similar conditions to bearded irises, tolerating wetter soil and requiring less frequent division in spring or fall. Be certain to replant as soon as possible after dividing them. Rot and borers seldom plague them.
Grow Japanese irises in much the same way, providing shade from the hottest sun. Water well before and during bloom. They need acid soil and benefit from a few inches of mulch in summer.
Plant reticulated irises in fall about 3 inches deep and a few inches apart in average to more fertile, very well-drained soil. Grow with annuals and perennials to fill the gaps left by their leaves, which wither by summer.
Landscape uses: Smaller bearded irises are perfect in rock gardens and along paths and beds. For mid- to late-spring bloom, plant taller ones in a perennial border, or in a separate bed to provide optimum conditions. They also look splendid among garden ornaments and along patios. Siberian and Japanese irises are good choices for borders and wet sites, such as along a stream or the edge of a pond, although they prefer slightly drier conditions in winter. Reticulated irises look at home among rocks, naturalized in thin grass, or at the front of borders.
IRRIGATION
See Drip Irrigation; Watering
IVY
See Hedera