Introduction

Ilex aquifolium goes by several common names, including English Holly, European Holly, Common Holly, and Christmas Holly. It is a highly invasive plant in the moist climates of North America, particularly on the west coast, where it is beginning to dominate the understory of some forest ecosystems. Despite this, it is still widely sold in stores throughout this region.

I grew up on the coast of BC, Canada, and remember finding it all the time under the canopy of the forest. I recently returned to the coast and spent some time there and was horrified by the amount it appears to be spreading and replacing our native understory species. In a single forest walk, I managed to pull out about 40 immature plants from the forest canopy. There were several larger ones I could not pull out by hand due to their deep and aggressive root system.

Next time I return to the coast and go for a forest walk, I will be sure to bring a trowel or a small shovel. Do your part on your forest walks and pull out any small holly shrubs you find growing before they get large and too difficult to remove. Just make sure you are not pulling Oregon Grape or native hollies if you live on the east coast; see Similar Species below for more information.

Description of Ilex aquifolium

Leaves & Stems of Ilex aquifolium

Common Holly is a slow-growing evergreen perennial shrub or small tree that grows from 5-25 m tall. However, it usually only grows to the shorter 5-10 m shrub height. It may grow from a single trunk, or it may be multi-stemmed. Their trunks are usually less than 40 cm in diameter. Larger trunks can be from 40-80 cm in diameter, but they rarely exceed 100 cm. The bark is green when young but turns grey when it matures. While specimens as old as 500 years have been reported, it seldom lives longer than 100 years.

The leaves of English Holly are arranged alternately on the branches. They are tough, thick, and glossy green and measure 3-12 cm long and 2-6 cm wide. The margins are often characteristically undulate (wavy) and toothed with spiny tipped teeth. Sometimes the leaf margins are smooth, particularly on older specimens.

Flowers & Fruits of Ilex aquifolium

English Holly is a dioecious plant meaning that there are separate male and female plants. The flowers are whitish and have 4 petals and 4 sepals that are both fused near their base. In males, the flowers are usually more yellowish and appear in axillary groups (in clusters in the leaf axils). Female specimens have single flowers or in groups of three, and the flowers or white or somewhat pink. The male and female shrubs are impossible to tell apart until they begin to flower from 4 – 12 years of age.

Fruits are small red or yellow drupes, berry-like in appearance, and only grow on female plants. They are 6-10 mm in diameter. Each drupe contains 3-4 small stony seeds.

Toxicity of Ilex aquifolium

The drupes are toxic to humans and pets, causing vomiting and diarrhea, but they are generally not lethal. Birds typically eat them later in the winter after the frosts have softened them and made them less bitter; they are immune to their toxic effects.

Similar Species Ilex aquifolium is Frequently Confused With

There are a few other plant genera that have similar leaves to Ilex aquifolium. The Oregon-Grape genus Berberis of which we have a few native species growing in North America have evergreen spiny-toothed leaves, but their leaves are opposite rather than alternate, and they have blue-colored berries rather than red or yellow drupes. Osmanthus heterophyllus or Holly Olive is a member of the olive family native to Europe that also has similar leaves and has been introduced in the USA. Holly Olive differs from true hollies by its opposite rather than alternate leaves and bark that turns grayish to blackish with age and has a characteristic cracking pattern to it.

The other Ilex species can be distinguished as follows:

• Ilex opaca American Holly – native to the eastern and south-central USA. It has matte yellowish-green leaves that are seldom glossy, as in Ilex aquifolium. Its trunk is grey with wart-like bumps, and its mature branches turn brown rather than grey. It also grows more of a tree than a shrub, with trunk diameters commonly of 50 cm and up to 120 cm in diameter.
• Ilex cornuta Chinese Holly – native to China but widely introduced throughout the eastern and south-central USA. It has more rectangular-shaped leaves with only 5 (4) spines, but they are glossy green like Ilex aquifolium. Chinese Holly never grows to tree size, it rarely exceeds 3 m in height, and its drupes are larger than European Holly. The bark is smooth and grey but becomes flaky with age.
• Ilex vomitoria Yaupon Holly – native to southeastern North America. It is a small tree or shrub with glossy green leaves, but the leaves are much smaller and are not spiny. Instead, the leaves have crenate or coarsely serrated margins.

Native Distribution of Ilex aquifolium

European or Common Holly is native to western and southern Europe, northwest Africa, and southwest Asia.

Habitat Types Where European Holly is Found

European or Common Holly is typically found growing under forest canopies, oak, and beech, especially in its native range. It typically requires moist and shady environments, making it particularly suitable as an undergrowth species in any moist forest. Also, it is common on moist and shady slopes, cliffs, and mountain gorges. Sometimes, however, it can also form dense thickets in more open fields and meadows. It is a rugged pioneer species that can tolerate both frosts and droughts.

Human Uses of Common Holly

European or Common Holly is widely used as an ornamental shrub or small tree in gardens. It is also extremely popular in Christmas decorations, where it is put in wreaths, garlands, and various other displays.

In traditional European herbal medicine, the leaves of Ilex aquifolium were used as a diuretic, to treat fevers, and as a laxative. It is seldom used in modern herbal medicine, however, and its berries are considered toxic, so the plant should be used with caution.

Distribution of Ilex aquifolium in North America

In Canada, Ilex aquifolium has been recorded as introduced in BC and possibly in Ontario. It is becoming particularly invasive in coastal BC.

In the USA, English Holly is found in Washington, Oregon, and California in the west and Virginia and New Jersey in the east. It has also been introduced in Hawaii. According to iNaturalist, it is located in many more states, but many may still be in cultivation, and some could be native Ilex spp that have been misidentified.

In Mexico, Ilex aquifolium has not yet been reported so far.

English Holly has been introduced on every continent. It seems particularly bad in New Zealand, Tasmania, and North America.

How English Holly Spreads

It is primarily spread long-distance by deliberate human introductions as an ornamental species in gardens.

Short-distance dispersal occurs through birds that eat the abundant drupes later in winter after they have softened and their taste improves. They spread the seeds through their feces, where the seeds then germinate in any suitable habitat in both shade and sun.

English Holly also spreads readily vegetatively through suckering. This is how most of the thickets of Common Holly form and how it becomes a dominant understory species of the forests.

Habitats at Risk of Invasion in North America

The way that English Holly spreads through the feces of birds puts all habitats at risk providing there is enough moisture. They will not grow in permanently wet wetlands, and they also will not grow in arid or semi-arid areas. Any moderately moist to the wet climate, however, is at risk of invasion.

Their ability to grow in both sun and shade makes them a common invader of forests in particular but also roadsides, fields, meadows, cliffs, gorges, and anywhere else as long as there is ample moisture.

European Holly has proven to be particularly invasive in the temperate rainforests of northwestern North America, where it easily invades the mixed forest canopies and thrives in the moist environment there.

Impacts of Invasion

Ilex aquifolium has deep and aggressive roots that tend to out-compete the native species for nutrients and water. There is also evidence that English Holly alters the soil conditions by adding sulfur and large amounts of organic matter to the soil, which makes the conditions less hospitable for native species.

English Holly forms dense thickets that can dominate the understory of the forest and out-shade native shrubs and other native plants, reducing the biodiversity of the forests it invades.

Potential Benefits of Invasion

Thickets of Ilex aquifolium are known to be used as a refuge by small birds and deer. The flowers are attractive to bees, small butterflies, wasps, and flies, and the berries provide an important late-winter food source. However, as with all invasive plants, the native wildlife would have depended on the native plants that grew there before the Common Holly replaced them.

Methods to Remove English Holly

As always, prevention is the preferred method of control. It, like most invasive species, is still widely sold online and in most local garden stores. Do not buy or transport any European Holly, and never plant it in your yard.

If you see them being sold online or in your local garden stores, please inform them of their invasive status and ask them to do their part and stop selling them. Ask them to instead sell more native species as ecologically friendly garden alternatives to invasive species.

If you live in the eastern part of North America, why not grow one of the two native hollies instead, Ilex opaca or Ilex vomitoria? If you live in western North America and want some spiny glossy green foliage, try growing native Berberis species instead. They have lovely foliage and beautiful blue-colored berries. The native Ilex and Berberis species can even be substituted for Common Holly in Christmas decorations.

Physical Control of European Holly

Once already established, however, physical control is always the most effective means. Physical control is labor-intensive and time-consuming but causes the least amount of environmental damage.

Physical methods to remove European Holly generally involve uprooting the plants. This is best done when the plants are young before their aggressive roots have had a chance to establish themselves. Fortunately, they are a slow-growing species, so on a single walk through the forest, you can single-handedly destroy dozens of young plants with very little effort. Larger plants will need to be dug out with a shovel to get their roots. If the roots are not dug out, they can re-grow from the stump.

For larger shrubby plants, use pruners to remove the upper growth before using a shovel to dig out the root mass. For tree forms of Ilex aquifolium you can try girdling it. This involves removing a strip of bark from all the way around the circumference of the tree. As with all trees and woody shrubs, the only part of the tree’s trunk that is alive and transporting nutrients is the layer directly under the bark. If you cut down to the heartwood and remove the bark in a strip all around the tree, it can no longer transport water and nutrients to the leaves. The tree will then die and can be left to rot or be removed.

The best time to remove European Holly is before it is in berry, in the spring or early summer. If you need to remove them when in berry, be sure to clip off the berries before removing the shrub or tree and place them in the bag for proper disposal.

Disposal of the Shrubs Once Removed

Trees or shrubs without berries on them can be burned, cut into firewood, or left to rot. If you have plants that have mature berries on them, they must either be burned or solarized. If you are unable to burn in your area, then solarizing is a viable option. To solarize, put the shrubs under a thick black tarp or into thick black garbage bags. Then leave them in the full sun for a good 8 weeks at least to be sure that all seeds are no longer viable. The plants can then be disposed of. They can be brought to a garbage dump where you can inform them they are an invasive species so they can deal with them accordingly.

Chemical Control of European Holly

Chemical applications are almost never an ideal method of control for any invasive species. That is because chemical alteration of the environment often makes the environment more suitable for invasive species than native species. Furthermore, it is often difficult to keep the chemical control method contained so that it does not directly affect any native species that are there during the application process itself. As a result, plots where chemical control is used usually show a decrease in species richness. On the other hand, in plots where only physical control is used, species riches significantly increase.

Furthermore, there are no chemical control methods that effectively target only European Holly. If you do use chemical control, it must be stem injected. The tough evergreen leaves readily resist the absorption of pesticides making foliar applications ineffective.

Chemical control is generally not recommended.

Biological Control of Ilex aquifolium

Biological control involves the use of a predator, herbivore, disease, or some other biological agent to control an invasive species once it is fully established in the environment. The problem with biological control is that the agent used must be entirely specific to only the target organism before releasing it into the environment. This is often difficult to determine since the agent of control is also not native to the environment and could behave differently when released there.

Biological control methods are extremely risky and should only be carried out by professionals after years of rigorous study. The use of biological control methods can never be used alone. They must be part of an integrated pest management approach. However, using biological control in conjunction with physical control and ongoing monitoring can be very effective for some invasive species.

Unfortunately, there are no current methods of biological control for English Holly in North America other than goats. Domestic goats are aggressive and non-selective browsers. They will literally eat anything. This will work best on shrubs and thickets. Often goats will eat the bark off of trees so that they can even kill off the trees if they manage to eat all the way around the tree.

To use goats, the patch should be penned in, and then the goats left there to graze for at least 2 years. They will eventually destroy all the above-ground growth and then eat the new sprouts as they come up. This can then be followed with rootstock removal for any stubborn plants that may remain. Alternatively, simply leave the goats in the area for another year until you see no more sprouts. At this time, they can be moved to another patch, and that patch can be replanted with native vegetation. An ongoing monitoring program should then be implemented to be sure they do not sprout again.

Integrated Pest Management & Ongoing Monitoring

Integrated management is always the best approach. In its simplest form, this involves physical removal methods, possibly biological control methods, replanting, and ongoing monitoring. Integrated management is required because the area needs to be monitored for returning sprouts or seedlings. Otherwise, all the hard work done in removal could be wasted if the invasive species is allowed to regrow.

Replanting With Native Species is Sometimes Crucial

Removal of single plants from the understory of the forest will not require replanting with native species. Simply remove and monitor, and other plants will grow in their place. However, in cases of the removal of thickets that dominate the area, the site should be replanted with native species. A replanting program should already be planned and ready to implement once the English Holly has been removed. You will need to research species that are native to your area. Ideally, use ones from your local environment to ensure local ecotypes are represented. Alternatively, you can purchase suitable plants from a nursery in your area that specializes in native species.

Ongoing Monitoring is Essential

In all cases of invasive European Holly removal, ongoing monitoring is absolutely essential. Yearly monitoring programs should be put in place to ensure that any surviving individuals are removed so that the population is not able to recover. This is required whether the area is replanted or not. Sometimes roots left behind will re-sprout into new plants. Furthermore, birds will keep bringing in new seedlings if there are still plants in the wider surrounding environment. For these reasons, yearly monitoring should be put in place to remove young plants before they have a chance to become established, especially since that is the easiest time to remove them. With ongoing monitoring, we can keep invasive species in check and promote local biodiversity in the process.

References and Resources

Best Management Practices for English Holly in the Metro Vancouver Region http://www.metrovancouver.org/services/regional-planning/PlanningPublications/EnglishHollyBMP.pdf

Canadensys Plant Search https://data.canadensys.net/vascan/search

Dictionary of Botanical Terms – Lyrae’s Nature Blog Dictionary of Botanical Terms

iNaturalist Plant Search https://www.inaturalist.org/home

USDA Plants Database https://plants.sc.egov.usda.gov/home

Wikipedia on Ilex aquifolium https://en.wikipedia.org/wiki/Ilex_aquifolium

Willis, Lyrae (Unpublished).  Plant Families of North America.

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