How to Plant & Grow Winterberry (Ilex verticillata)


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Winterberry fruits in winter
Winterberry loses its deciduous foliage in the winter, making room for its vivid red fruits to take center stage. Bonnie Semmling / CC BY 4.0

An important resource for wild animals all throughout the year, Ilex verticillata is a holly shrub with many uses. Its appearance can markedly change through the seasons, making it a fascinating ornamental plant to have in any garden. Its many common names, including black alder, winterberry, Canada holly, fever bush, Michigan holly, and coralberry, hint at its natural distribution and popular features.

Native to a wide variety of biomes in eastern North America, this showy plant is classified under the Aquifoliaceae family. Unlike evergreen trees, it loses its deciduous foliage toward winter. The loss only makes room for a spectacular show of red fruits, which can appear to glow dramatically against a backdrop of stark, white snow.

Come spring, alternately arranged, glossy green leaves begin to appear once more. These are slightly serrated and oblong-shaped. The leaves eventually turn purple to black shortly before they are dropped towards the end of fall. The tiny flowers may be light green, white, or yellow and usually begin to appear in spring. A fully grown shrub can be as tall as 10 – 15 feet (3 – 4.5 m), though there are smaller cultivars as well.

Facts, Benefits & Uses of Winterberry

Winterberry flowers
The winterberry plant has small flowers that attract specialized pollinators, such as honeybees. SB Johnny, CC BY-SA 3.0, via Wikimedia Commons

The winterberry favors wetland conditions and is frequently found in riparian forests. Its tolerance for poorly draining soil makes it an ideal shrub for rain gardens and the edges of water features. Its shoots and branches provide protection for many wild animals, while its berries are a vital food source for birds, such as the American robin and cedar waxwing, each winter. Small mammals may occasionally feast on the berries too.

The leaves, though mildly toxic, are grazed in moderate amounts by deer and rabbits. The tiny flowers attract highly specialized pollinators, including honeybees. Winterberry is also a larval host for many of the insects found in its natural environment. It is fairly resistant to the grazing activities of most animals. Despite its preference for moist soil, it may withstand physical extremes, including brief droughts, intense summer heat, and mild forest fires.

I. verticillata is an ornamental plant that can be grown in hedges, as a border shrub, or in mass plantings. Apart from having its own mild toxins, it has the capacity to absorb toxic substances and stormwater pollutants. Its high adaptability and resistance to various pests make it easy to cultivate into maturity.

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Winterberry Fact Sheet:
PLANT TYPE
Deciduous shrub
HARDINESS ZONES
USDA 3 – 9
LIGHT REQUIREMENTS
Full sun, partial shade, deep shade
BLOOM COLOR
Yellow, green, white
BLOOM PERIOD
Spring
MAXIMUM GROWTH
15 feet (4.5 meters)
PLANTING DEPTH
Length of root ball
WATER QUALITY
pH 4.5 – 6.5 (slightly acidic)

Winterberry Growth, Hardiness & Climate

Female winterberry plant
You will need to look closely to identify the winterberry plant’s sex – female plants have a green nub that protrudes at the center of each flower, as seen here. Fritzflohrreynolds, CC BY-SA 3.0, via Wikimedia Commons

I. verticillata is a dioecious species, which means that male and female reproductive organs are found on separate plants. As a result, both sexes need to be within close range of one another for fruit production. Unfortunately, many lone winterberry trees that are grown outside of their native range may never produce fruits due to the absence of conspecifics. Moreover, some cultivars are not compatible with one another. Cultivation in mass plantings increases the chances of fruit production, particularly if the site is frequently visited by bees.

To distinguish between male and female plants, closely inspect the blooms. Female plants have a green nub that protrudes at the center of each flower, whereas males have anthers that bear pollen. Even when these trees are planted closely together, it may take up to 2 years for them to become well-established enough for fruit and flower production.

Winterberry thrives best in moist areas that receive around 6 – 8 hours of sunlight per day. It can tolerate a wide range of humidity and temperature levels, with a minimum of around -28˚C (-20˚F). It also favors slightly acidic soil.


How to Plant Winterberry

Winterberry plant
You can use softwood cuttings to propagate winterberry – this will generate established plants much quicker than using seeds. Michael Ellis / CC BY 4.0

Plant nurseries usually sell I. verticillata cultivars as rooted, young shrubs, or as seeds. If you’ve managed to acquire a rooted shrub, you can transplant it directly into your garden’s substrate. All you have to do is uproot the entire plant, keeping the full root ball intact. Select a sunny location for the shrub. Dig a hole with a depth that is equal to that of the shrub’s root ball. If there are lengthy roots or if you intend to transplant the root along with the entire volume of substrate in the pot, the hole can be made deeper. Just keep in mind that the base of the shoot should be level with the ground.

Softwood cuttings, which will generate established plants much quicker than seeds, can be used for propagation. In spring or summer, use garden shears to collect a few stem tips from mature plants. These should be around 3 inches (8 cm) long. Dip the base of the cutting in rooting hormone and remove most of the stem’s leaves apart from the ones at the tip. Plant the cuttings in a propagation setup with moistened potting soil. The setup should ideally have a high rim so that you can cover the surface without dislodging the cuttings. This will help keep the soil moist.

Seeds will need to be cold-stratified prior to being sown. Distribute them over a pre-moistened germination mix. Both cuttings and seedlings should remain in moist soil until they have produced enough roots to be transplanted. Use a similar setup for transplantation, but place the young plants further apart to leave room for their roots to expand. Keep them in a protected area until they are large enough to withstand exposed conditions.


How to Care for Winterberry

Winterberry plant with fruits
If winterberry fruits aren’t appearing, it’s likely that the male or female plant is missing or too far away. Tom Scavo / CC BY 4.0

Apart from occasional pruning, winterberry will require minimum care through the years. The best time to prune holly shrubs is in early spring, just before the new growths appear. Pruning at another time of year, particularly once the plant has flowered, may prevent its fruits from developing. If flowers or fruits fail to appear, especially on well-established plants, it is likely that the male or female pair is absent or planted too far away. To encourage flower production and pollination, the paired plants need to be less than 50 feet away from each other. A single male shrub can produce enough pollen for up to 6 female trees.

Winterberry is fairly pest- and disease-resistant when grown in its native range. Regardless, regularly monitor the foliage for powdery mildew and leaf spots. Monitor the soil pH and ensure that it remains slightly acidic. Alkaline conditions may cause leaf chlorosis, which may lead to the death of the plant.


How to Winter Winterberry

As suggested by its name, winterberry is known for being remarkably winter-hardy. It will not require special care even if temperatures drop to below zero. However, if the winters in your area are particularly harsh, with temperatures dropping to below -30˚C (-22˚F), it may be prudent to mulch the base of the plant so that the roots are given ample protection. The branches are likely to lose all of their foliage towards the end of fall. Leaves will sprout back once temperatures begin to rise the following year.


Is Winterberry Invasive or Toxic?

Winterberry is not considered an invasive species due to its relatively slow growth rate. Its dioecious nature also prevents it from forming colonies when it is grown as a solitary plant or when it is situated too far away from compatible cultivars.

I. verticillata contains low-severity toxins. All plant parts may cause undesirable symptoms, such as vomiting and diarrhea, when ingested. Alkaloids make up the bulk of its mildly toxic compounds, which are largely concentrated in the fruits and leaves. While many wild animals can safely consume small amounts of the raw fruits and foliage, domesticated pets and humans may be adversely affected. Always monitor young children and pets if they are permitted to explore gardens with winterberry shrubs.


Is Winterberry Edible? Do Animals Eat it?

Despite harboring mildly toxic compounds, winterberry fruits are often used as a main ingredient in cooked or processed treats. These include pies, syrups, jellies, and even wine! Its toxic components are likely denatured by high heat.

Although they may have some medicinal uses, the berries should not be eaten raw. Herbalists, particularly those of Native American heritage, may occasionally use the winterberry shrub to produce a medicinal tea or poultice. Decoctions of the plant can supposedly reduce blood pressure, treat malaria, or hasten the healing process of skin diseases.

Dozens of animals benefit from consuming nutritious winterberries each winter. In natural forests, red fruits usually disappear before the end of winter. In areas where winterberry shrubs are not accessible to birds, the increasingly bitter berries may last until summer.


Where to Buy Winterberry & Seeds? (UK & US)

Ilex verticillata comes in a wide variety of cultivars that can be purchased in plant nurseries and garden centers all throughout its native range. Young shrubs may also be available as potted individuals in North American online plant portals. To ensure that your shrub will produce flowers and fruits, you need to purchase both sexes of identical or compatible cultivars. Ideally, reputable plant nurseries should be able to accurately identify and sex each of their winterberry plants after the first bloom period.

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