American Holly (Ilex opaca)

American Holly, Holly, Winterberry

Ilex opaca, commonly known as american holly, is a medium-sized evergreen tree, commonly found in southeastern areas of North America as an understory tree in humid forests. Its bright red fruits are poisonous to humans but are a very important source of food for numerous species of birds.

I. Appearance and Characteristics 

Ilex opaca, the American holly, is a species of holly, native to the eastern and south-central United States, from coastal Massachusetts south to central Florida, and west to southeastern Missouri and eastern Texas.

Ilex opaca is a medium-sized broadleaved evergreen tree growing on average to 10–20 m (33–66 ft) wide, and up to 30 m (98 ft) tall. Typically, its trunk diameter reaches 50 cm (20 in), sometimes up to 120 cm (47 in). The bark is light gray, roughened by small warty lumps. The branchlets are stout, green at first and covered with rusty down, later smooth and brown. The winter buds are brown, short, obtuse or acute. The branches are short and slender. The roots are thick and fleshy.

The leaves are alternate, 5–7.5 cm (2.0–3.0 in) long and 2–4 cm (0.79–1.57 in) wide, stiff, yellow green and dull matte to sub-shiny above (distinctly less glossy than the otherwise fairly similar European holly, Ilex aquifolium), often pale yellow beneath; the edges are curved into several sharp, spike-like points, and a wedge-shaped base and acute apex; the midrib is prominent and depressed, the primary veins conspicuous; the petiole is short, stout, grooved, thickened at base, with a pair of minute stipules. The leaves remain on the branches for two to three years, finally falling in the spring when pushed off by growing buds.

The flowers are greenish white, small, borne in late spring in short pedunculate cymes from the axils of young leaves or scattered along the base of young branches. The calyx is small, four-lobed, imbricate in the bud, acute, margins ciliate, persistent. The corolla is white, with four petal-like lobes united at the base, obtuse, spreading, hypogynous, imbricate in bud. The flower stem is hairy with a minute bract at base. Like all hollies, it is dioecious, with separate male and female plants; only female plants produce the characteristic red berries. This fruit (drupes) appear late in the season, and whether due to the need to ripen or being a food of last resort, often last until midwinter. They are poisonous to dogs, cats, and humans, often causing diarrhea, vomiting, dehydration, and drowsiness if ingested. Cedar waxwings (Bombycilla cedrorum) will strip the trees of fruit if they are not already bare during their northward migration. One male can pollenize several females. Male flowers have four stamens, inserted on the base of the corolla, alternate with its lobes; filaments awl-shaped, exserted in the sterile, much shorter in the sterile flower; anthers attached at the back, oblong, introrse, two-celled, cells opening longitudinally. The pistil on female flowers has a superior ovary, four-celled, rudimentary in staminate flowers; style wanting, stigma sessile, four-lobed; ovules one or two in each cell.

The fruit is a small red drupe 6–12 mm diameter containing four seeds; it is often persistent into winter.

A ratio of three female plants to one male plant is required for ideal fruit production.

Due to its shade tolerance, Ilex opaca typically grows as an understory tree in moist forests of the east-central, southeastern, and south-central United States. It is found in sparse numbers in the northern part of its range from Cape Cod, Massachusetts, south to northern New Jersey (including southern Connecticut and southeastern New York, on Long Island). It is abundant further south on the Gulf and Atlantic lowlands. It will grow in both dry and swampy soil, but grows slowly. Ilex opaca var. arenicola, or scrub holly, is found as a shrub component in xeric scrub habitats of the Florida peninsula. The ideal yearly precipitation average for the species ranges from 102 cm to 165 cm.

The flowers are pollinated by insects, including bees, wasps, ants, and night-flying moths. It is a larval host plant for Callophrys henrici. The tree also forms a thick canopy which offers protection for birds from predators and storms. Songbirds including thrushes, mockingbirds, catbirds, bluebirds and thrashers, as well as some game birds and mammals frequently feed on the berries.

II. How to Grow and Care

Sunlight

American holly requires an environment with bright and soft light to grow. It grows well in shady places, sheltered on one side, and even in direct sunlight. Mature plants have higher fruit growth rates in places with good light. Young plants, on the other hand, need a shady place (shade cloth can be used) in summer to avoid direct sunlight and prevent sunburn.

American holly kept indoors should be placed in sunny rooms, about 50 to 100 cm from the window. Do not place the plant in an environment where light is completely blocked. After extended periods of insufficient light, the plant’s photosynthetic rate will decrease. This will slow down the growth rate possibly to zero, cause thin and weak leaves, and decrease the flowering and fruiting rate.

Temperature and Humidity

American holly generally does well throughout the temperature range of its hardiness zone, 5 to 9.

Watering

American holly grows in humid environments and is not resistant to drought. Keep the soil moist during spring and summer. If rainfall is less than 2.5 cm per week when cultivated outdoors, manual watering is required. Water once a day and ensure that the soil around the root system is completely wet. Surplus water should drain away smoothly without accumulating. Spray mist during the day to increase air humidity, which is preferably kept at 70%-90%. After the fruiting period in winter is over, gradually reduce the watering frequency to once a week, and completely drench the soil only if it is completely dry.

It is best to water american holly in the morning and evening because the temperature around the root system tends to rise if it is done at noon, especially in summer. This will cause damage to the plant. Use rainwater and distilled water rather than tap water, which contains a large amount of calcium and magnesium mineral salts. Long-term use of tap water will cause salinization, hardening, and poor permeability of the soil as well as affect plant growth and ornamental effect. Inadequate or excessive water will both cause the leaves to turn yellow and shed.

Soil

American holly likes acidic soil with a pH value of 5-6. In neutral or alkaline soil, its leaves tend to yellow and the plant may even die. It requires humid culture media that retain water and are rich in organic matter. Humus soil with an appropriate amount of coarse sand or perlite added will increase soil permeability. A formula for the culture medium, for example, could be 1/2 leaf mold + 1/4 coarse sand or perlite + 1/4 decomposed bark.

Before putting it into the pot, the medium should be soaked in a carbendazim solution to kill any bacteria and insect eggs. Some ceramistes can be put at the bottom of the pot to improve drainage and avoid water accumulation caused by excessive daily watering. Change the soil every two years.

Fertilizing

American holly does not require much fertilizer. Apply some fertilizer during the growing season to promote luxuriant branches and glossy leaves. Apply thin fertilizer once in early spring and once in early autumn. Compound fertilizer mainly composed of nitrogen fertilizer can make the leaves brighter and bigger.

Stop fertilizing when the temperature is higher than 30 ℃ in summer to prevent root damage. Insufficient fertilization can cause thin and weak plants, sparse fruits, and thin and dull leaves. Large but dull leaves may be caused by over-fertilization and signal that fertilization should be stopped immediately.

Planting Instructions

Though its red fruits are a Christmas symbol, it is generally best to plant american holly in spring. Large seedlings over 2 years old are preferred for planting. plants should be spaced about 1 m apart and there should be about 1 to 1.2 m between rows. Compacting the soil and watering after planting will allow the roots to be in closer contact with the soil. It should be noted that american holly is dioecious, so it requires both female and male plants present to produce fruits. Successful pollination only happens when plants are spaced no more than 9 m apart. One male plant can pollinate 6-10 female plants.

Pruning

American holly flowers and fruits only grow on new branches, so the plant needs to be pruned to promote the germination of new branches. It’s best to prune before early spring, ideally during winter. In addition to promptly cutting off old, yellow, dried, and diseased leaves, one-third of the old branches should be cut off from the base every year. This promotes vertical growth, flowering, and fruiting. After winter pruning in cold areas where it snows, the remaining branches can be bundled with ropes to avoid damage caused by snow or ice on the branches.

Propagation

Creating new holly plants by rooting some stem cuttings is fairly easy to do, but it takes quite a lot of time. In late summer, cut 6-inch pieces of stem from the tips of new growth branches. Strip away all but the top three or four leaves. Plant the cuttings in a container filled with a mixture of sand and peat moss. Thoroughly moisten the potting mix, then place it in a sealed plastic bag. Set the pot in a warm location (60 to 70 degrees Fahrenheit) and check it weekly, watering when necessary.

In four to six weeks you should see some new growth beginning, and at this time you can remove the plastic bag and set the pot in a sunny window. After a few months, you can transplant each growing holly sapling into a large container filled with ordinary peat-based potting mix. Continue to grow the plant in a sunny window until spring, when you can move the potted seedlings outdoors to continue growing—or plant them in the landscape if they are big enough.

Pests and Diseases

Holly plants can be susceptible to many insect problems, such as leaf miners, spider mites, whitefly, and scale. Horticultural oils will generally help with most pests, but systemic insecticide may be necessary if leaf miners become too disfiguring.

These plants are also prone to fungal rots, which will usually require that the plant be removed. These are more likely to occur in dense, boggy planting locations. The fungus can persist in the soil for many years, so it’s usually best to change to another plant altogether when fungal disease appears.

Holly plants can also be victims of leaf drop and sun scorch in very hot conditions. If the leaves turn yellow, it may be due to soil that is too alkaline.

III. Uses and Benefits 

  • Ornamental uses

In landscaping, American holly can be used as a specimen plant, in groups, or in foundation plantings, where they add good winter color and attract birds with their colorful berries. Holly is perhaps most commonly used to add visual interest to color-starved landscapes in winter. And, of course, holly is prized for Christmas decorations, both indoors and outdoors.

Several bird species are attracted to holly shrubs, including thrushes and blackbirds. According to the USDA Forest Service, holly berries are also eaten in winter by wild turkeys, cedar waxwings, mourning doves, goldfinches, bobwhites, and cardinals.

  • Other uses

The wood is very pale, tough, close-grained, takes a good polish, and is used for whip-handles, engraving blocks and also cabinet work. It can also be dyed and used as a substitute for ebony. It has a density of 0.58 to 0.64. The sap is watery, and contains a bitter substance used as an herbal tonic.

Leaves from the American holly can be used to make a tea-like beverage. American holly tea does not contain caffeine.

American Holly (Ilex opaca) Details

Common name American Holly, Holly, Winterberry
Botanical name Ilex opaca
Plant type Native Plant
Hardiness zone 5a, 5b, 6a, 6b, 7a, 7b, 8a, 8b, 9a, 9b
Growth rate Slow
Harvest time Fall
Height 40 ft. 0 in. - 60 ft. 0 in.
Width 40 ft. 0 in. - 60 ft. 0 in.
Sunlight Full sun (6 or more hours of direct sunlight a day)
Soil condition Clay
Flower color Gold/Yellow
Leaf color Green