Ilex opaca
American Holly
Type Tree, woody plant
Hardy range 5B to 9A
Height 25' to 50' / 7.60m to 15.20m
Spread 15' to 25' / 4.60m to 7.60m
Growth rate Slow
Form Pyramidal
Exposure Full shade to full sun
Persistence Evergreen
Hardy range 5B to 9A
Height 25' to 50' / 7.60m to 15.20m
Spread 15' to 25' / 4.60m to 7.60m
Growth rate Slow
Form Pyramidal
Exposure Full shade to full sun
Persistence Evergreen
Bloom Color Green and white
Bloom Time Spring
The flowers are fragrant.
Bloom Time Spring
The flowers are fragrant.
Environment
This plant tolerates drought, occasional wetness and salt well.
This plant will grow in very dry to occasionally wet soil.
Suitable soil is well-drained/loamy, sandy or clay.
The pH preference is an acidic to slightly alkaline (less than 6.8 to 7.7) soil.
This plant tolerates drought, occasional wetness and salt well.
This plant will grow in very dry to occasionally wet soil.
Suitable soil is well-drained/loamy, sandy or clay.
The pH preference is an acidic to slightly alkaline (less than 6.8 to 7.7) soil.
Leaf Color Green
Fall Color No change in fall color
This plant has attractive foliage.
Fall Color No change in fall color
This plant has attractive foliage.
Culture Notes
The Pennsylvania Horticultural Society selected this plant as one of the Gold Medal Plants for 2001. This award recognizes exceptional woody plants hardy from New York to Washington, D.C. Growing well in full sun to partial shade, American Holly should be located on fertile, well-drained but moist, slightly acid soils below 6.5 pH. Berry production is highest in full sun and only on female trees if pollinated by a male plant. American Holly foliage thins during drought but insect and disease infestations are usually minimal. Foliage can burn in winter if exposed to direct wind. It tolerates salt air very well, growing along the secondary dunes in its natural habitat. Trees attract cedar waxwings, mockingbirds, robbins and many other birds.
Trees maintain a central leader and a conical shape even in the shade. The central leader form allows them to resist ice damage. Trees with double leaders will split in ice storms. American holly resists deer browsing which can be a problem on other hollys such as Nellie R. Stevens. Foliage can brown in strong winter winds. This plant is considered mostly allergy free and causes little or no allergy problems in most people. Trees are tolerant of urban conditions and have performed well as street trees. Plants serve as hosts for butterfly larvae.
Plants in the wild tolerate wet soil but those planted in yards appear to be intolerant of poorly drained soil. I (Dr. Gilman) think that if you plant trees when they are young they will be more successful in wet soils than if you plant a tree with a 2 inch trunk diameter or larger. Killing point for mature roots 9 degrees F.
This is the state tree of Delaware.
Wood is considered diffuse porous meaning that there is little difference in size of pores between spring and summer wood.
Tree establishment specifications
Choose good quality trees for planting. The most common cause of young tree failure is planting too deep. In most instances, the point where the top-most root in the root ball originates from the trunk (referred to as the root flare zone or root collar) should be located just above the soil surface. You may have to dig into the root ball to find the root flare. If there is nursery soil over this area, scrape it off. Never place ANY soil over the root ball. The planting hole should be at least twice the width of the root ball, preferably wider because roots grow best in loose soil. In all but exceptional circumstances where the soil is very poor, extensive research clearly shows that there is no need to incorporate any amendments into the backfill soil. Simply use the loosened soil that came out of the planting hole. Simply planting with the topmost portion of the root ball slightly higher than the surrounding soil might still install the tree too deep - be sure to locate the root flare.
Weed suppression during establishment is essential. Apply a 3-inch thick layer of mulch to at least a six-foot diameter circle around the tree. This area should be at least two feet in diameter for each inch of tree trunk diameter and maintained during the establishment period. Apply a thinner layer of mulch directly over the root ball but keep it at least 10 inches from the trunk. This allows rainwater, irrigation and air to easily enter the root ball and keeps the trunk dry. Placing mulch against the trunk and applying too thick a layer above the root ball can kill the plant by oxygen starvation, death of bark, stem and root diseases, prevention of hardening off for winter, vole and other rodent damage to the trunk, keeping soil too wet, or repelling water.
Regular irrigation after planting encourages rapid root growth that is essential for tree establishment. Trees provided with regular irrigation through the first growing season after transplanting require about 3 months (hardiness zones 9-11), 6 months (hardiness zones 7-8), or one year or more (hardiness zones 2-6) per inch of trunk diameter to fully establish roots in the landscape soil. Trees in desert climates may take longer to establish. Trees that are under-irrigated during this establishment period (and most trees are) often require additional time to establish because roots grow more slowly. Be prepared to irrigate through the entire establishment period, especially during periods of drought.
Irrigation also helps maintain and encourage the desirable dominant leader in the tree canopy on large-maturing trees. Instead of a dominant leader, trees that are under-irrigated during the establishment period often develop undesirable, low, co-dominant stems and double leaders that can split from the tree later.
Unlike established plants, which do best with deep, infrequent irrigation, research clearly shows that recently transplanted trees and shrubs establish quickest with light, frequent irrigation. For trees planted in spring or summer, provide one (cooler hardiness zones) to three irrigations (warmer hardiness zones) each week during the first few months after planting. Daily irrigation in the warmest hardiness zones provides the quickest establishment. Following the initial few months of frequent irrigation, provide weekly irrigation until plants are fully established. With every irrigation, apply one (cool climates) to two (warm climates) gallons of water per inch trunk diameter (e.g. 2 to 4 gallons for a 2-inch tree) over the root ball only. In most landscapes that receive more than 30 inches of rain or irrigation annually, if the mulch area is maintained weed-free, irrigation does not need to be applied outside of the root ball. Never add water if the root ball is saturated.
In cooler hardiness zones, in all but the driest years, irrigation of spring- and summer-planted trees usually can be discontinued once fall color has begun. Irrigation of fall planted trees, however, should be continued until foliage has dropped from the deciduous trees in the region. In warmer climates, irrigate fall-and winter-planted trees as described for the spring- and summer-planted trees.
In drier, desert climates there is benefit to be gained from applying additional irrigation outside of the root ball area. This is best done by making a large diameter berm four to six inches high, then filling it with water so it percolates into the soil. For the first two years, irrigate twice each week through the spring, once per week in summer provided monsoons arrive, and twice each week again in fall if it remains warm. Taper off watering to once or twice each month in winter and resume twice weekly next spring. For years three to five, water twice per month in spring, summer, and fall and once or twice per month in winter. During years five through seven, water once every three weeks in warm weather and once every six weeks in winter. After this, the drought-tolerant desert trees should be able to survive on natural rainfall.
Trees with good, strong structure need no pruning at planting, except to remove broken twigs. Do not remove branches to compensate for root loss - research has shown that this can be detrimental to establishment.
The Pennsylvania Horticultural Society selected this plant as one of the Gold Medal Plants for 2001. This award recognizes exceptional woody plants hardy from New York to Washington, D.C. Growing well in full sun to partial shade, American Holly should be located on fertile, well-drained but moist, slightly acid soils below 6.5 pH. Berry production is highest in full sun and only on female trees if pollinated by a male plant. American Holly foliage thins during drought but insect and disease infestations are usually minimal. Foliage can burn in winter if exposed to direct wind. It tolerates salt air very well, growing along the secondary dunes in its natural habitat. Trees attract cedar waxwings, mockingbirds, robbins and many other birds.
Trees maintain a central leader and a conical shape even in the shade. The central leader form allows them to resist ice damage. Trees with double leaders will split in ice storms. American holly resists deer browsing which can be a problem on other hollys such as Nellie R. Stevens. Foliage can brown in strong winter winds. This plant is considered mostly allergy free and causes little or no allergy problems in most people. Trees are tolerant of urban conditions and have performed well as street trees. Plants serve as hosts for butterfly larvae.
Plants in the wild tolerate wet soil but those planted in yards appear to be intolerant of poorly drained soil. I (Dr. Gilman) think that if you plant trees when they are young they will be more successful in wet soils than if you plant a tree with a 2 inch trunk diameter or larger. Killing point for mature roots 9 degrees F.
This is the state tree of Delaware.
Wood is considered diffuse porous meaning that there is little difference in size of pores between spring and summer wood.
Tree establishment specifications
Choose good quality trees for planting. The most common cause of young tree failure is planting too deep. In most instances, the point where the top-most root in the root ball originates from the trunk (referred to as the root flare zone or root collar) should be located just above the soil surface. You may have to dig into the root ball to find the root flare. If there is nursery soil over this area, scrape it off. Never place ANY soil over the root ball. The planting hole should be at least twice the width of the root ball, preferably wider because roots grow best in loose soil. In all but exceptional circumstances where the soil is very poor, extensive research clearly shows that there is no need to incorporate any amendments into the backfill soil. Simply use the loosened soil that came out of the planting hole. Simply planting with the topmost portion of the root ball slightly higher than the surrounding soil might still install the tree too deep - be sure to locate the root flare.
Weed suppression during establishment is essential. Apply a 3-inch thick layer of mulch to at least a six-foot diameter circle around the tree. This area should be at least two feet in diameter for each inch of tree trunk diameter and maintained during the establishment period. Apply a thinner layer of mulch directly over the root ball but keep it at least 10 inches from the trunk. This allows rainwater, irrigation and air to easily enter the root ball and keeps the trunk dry. Placing mulch against the trunk and applying too thick a layer above the root ball can kill the plant by oxygen starvation, death of bark, stem and root diseases, prevention of hardening off for winter, vole and other rodent damage to the trunk, keeping soil too wet, or repelling water.
Regular irrigation after planting encourages rapid root growth that is essential for tree establishment. Trees provided with regular irrigation through the first growing season after transplanting require about 3 months (hardiness zones 9-11), 6 months (hardiness zones 7-8), or one year or more (hardiness zones 2-6) per inch of trunk diameter to fully establish roots in the landscape soil. Trees in desert climates may take longer to establish. Trees that are under-irrigated during this establishment period (and most trees are) often require additional time to establish because roots grow more slowly. Be prepared to irrigate through the entire establishment period, especially during periods of drought.
Irrigation also helps maintain and encourage the desirable dominant leader in the tree canopy on large-maturing trees. Instead of a dominant leader, trees that are under-irrigated during the establishment period often develop undesirable, low, co-dominant stems and double leaders that can split from the tree later.
Unlike established plants, which do best with deep, infrequent irrigation, research clearly shows that recently transplanted trees and shrubs establish quickest with light, frequent irrigation. For trees planted in spring or summer, provide one (cooler hardiness zones) to three irrigations (warmer hardiness zones) each week during the first few months after planting. Daily irrigation in the warmest hardiness zones provides the quickest establishment. Following the initial few months of frequent irrigation, provide weekly irrigation until plants are fully established. With every irrigation, apply one (cool climates) to two (warm climates) gallons of water per inch trunk diameter (e.g. 2 to 4 gallons for a 2-inch tree) over the root ball only. In most landscapes that receive more than 30 inches of rain or irrigation annually, if the mulch area is maintained weed-free, irrigation does not need to be applied outside of the root ball. Never add water if the root ball is saturated.
In cooler hardiness zones, in all but the driest years, irrigation of spring- and summer-planted trees usually can be discontinued once fall color has begun. Irrigation of fall planted trees, however, should be continued until foliage has dropped from the deciduous trees in the region. In warmer climates, irrigate fall-and winter-planted trees as described for the spring- and summer-planted trees.
In drier, desert climates there is benefit to be gained from applying additional irrigation outside of the root ball area. This is best done by making a large diameter berm four to six inches high, then filling it with water so it percolates into the soil. For the first two years, irrigate twice each week through the spring, once per week in summer provided monsoons arrive, and twice each week again in fall if it remains warm. Taper off watering to once or twice each month in winter and resume twice weekly next spring. For years three to five, water twice per month in spring, summer, and fall and once or twice per month in winter. During years five through seven, water once every three weeks in warm weather and once every six weeks in winter. After this, the drought-tolerant desert trees should be able to survive on natural rainfall.
Trees with good, strong structure need no pruning at planting, except to remove broken twigs. Do not remove branches to compensate for root loss - research has shown that this can be detrimental to establishment.
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