Illegal Alien Shrubs

The Big Three: Invasive Shrub Species of the Eastern U.S.

It's National Invasive Species Awareness Week! Houston, we have a problem. In fact, much of North America has a problem. Today I want to talk about shrubs, specifically what I consider to be the big three of the eastern US: bush honeysuckle (Lonicera maackii), Chinese privet (Ligustrum sinense), and autumn olive (Elaeaganus umbrellata).  An honorable mention to multiflora rose (Rosa multiflora) and others I have not included here.

Before we get too deep, let me explain. I am not anti-shrub. I am definitely pro-shrub. Shrubs are an important part of many ecosystems and provide habitat and food for some of our favorite wildlife species including nesting habitat for neotropical migrants, woody browse for rabbits and deer, and good escape/roosting/hiding cover for northern bobwhites, wood ducks, etc. Shrubs often mark transitions between habitat types. In a healthy natural system, grassland does not meet deciduous forest abruptly with hard, linear edges. Nor does that same forest become an emergent wetland without some gradual indication from the local vegetation. Shrubs often occupy these transition zones.

My beef is with non-native, invasive shrub species. "They're not sending us their best. They're sending honeysuckle. They're sending privet... and some I assume are good shrubs." Shrubs that know their place. The big three outcompete native shrubs, grasses, and forbs, change forest structure, and spread like wildfire. Sometimes these "bad arbustos" can create structural homogeneity, resulting in a decrease in biodiversity.

Why are these non-natives so successful?

Perhaps you have seen that leaf out of invasive shrubs like Chinese Privet and bush honeysuckle occurs 1-3 weeks earlier than most native trees and shrubs, giving them a competitive advantage for sunlight. By the time any neighboring native shrub's leaves develop, the poor little plant is partially shaded out by the dense foliage of the invader. Likewise, these shrubs can often tolerate low light conditions. Additionally non-natives don't have the same tight evolutionary history with our native herbivores as do our native taxa. Therefore, deer, rabbits, etc. generally prefer to browse on native species, further giving the non-natives an advantage. All three of these species spread rapidly across the landscape with help from the birds that consume the berries and deposit seeds in their poop. Those same seeds have high germination rates, readily establishing new seedlings.

Let's take a closer look at these problem species individually.

1. Lonicera maackii; bush honeysuckle

Lonicera maackii or bush honeysuckle in a dense hedgerow along a forest edge at Parkersburg Wildlife Area in Ohio. Bush honeysuckle can tolerate relatively low sunlight and can become established on the forest floor where canopy coverage is not 100%. In turn these shrubs shade out beneficial native grasses, wildflowers, and shrubs.

These are large, upright shrubs native to Asia that can grow to a height of 20'. The simple leaves are arranged on the stems in pairs (opposite) and are slightly fuzzy underneath. The 1" white/pink, tubular flowers bloom in May and June and emit a beautifully fragrant aroma that is hard to dislike. Growing up I, like many, used pick the flowers and suck the nectar out like some oafish hummingbird. Once pollinated the ovaries develop into bright red berries that mature in September-October. The bark is grayish brown with visible grooves and ridges. Once you learn to recognize this species, you will see it everywhere!

Bush honeysuckle is spread mainly by frugivorous birds (American robins, catbirds, etc.) that eat the berries and deposit the seeds far and wide. So not all bad, right? At least they feed our native birds. Wrong. Turns out while high in sugars, the fruit of honeysuckle is low in both lipids and protein (Ingold & Craycraft 1983). So while tempting to some species, it is the bird equivalent of eating candy. Additionally, researchers have found that robins and wood thrushes that nest in bush honeysuckle, where nests are made at a relatively low height and are unprotected by thorns, experience higher rates of nest predation than those individuals nesting in native shrub species (Schmidt & Whelan 2001).

Perhaps more important is how bush honeysuckle affects the local plant community and forest structure. The vigorous growth of these shrubs can quickly shade out native grasses, wildflowers, and woody vegetation. One study showed a 53% reduction in species richness and 63% lower ground cover beneath the canopy of bush honeysuckle (Collier & Hughes 2002). If allowed to become abundant, bush honeysuckle will undoubtably change the structure and composition of our forests for decades and perhaps generations.

2. Ligustrum sinense; Chinese privet

Ligustrum sinense at a public park in northwest Arkansas. This shrub has become naturalized in many of our southeastern riparian forests, especially in urban/suburban areas. 

Like many of our now invasive flora, Chinese privet was introduced to the U.S. from China for ornamental plantings and is now common throughout the Southeast especially in riparian forests. Semi-evergreen, an individual plant will grow multiple stems from the ground up to 20' high. Branches are very long with many small (1-2"), simple, oblong leaves that are oppositely arranged. The creamy white flowers bloom from April-June and develop in terminal axillary clusters. The very small fruits develop from green to a dark blue/purple.

Like honeysuckle, Chinese privet is spread mainly by birds but can also colonize new areas by root sprouts. The presence of privet has been shown to have a negative impact on herbaceous vegetation by reducing the survival,  growth, and species richness of native vegetation (Greene & Blossey 2011). Privet's negative effect on regeneration threatens future forest structure and biodiversity. One study found that a forest with 40% privet coverage resulted in a >50% drop of regeneration rates (Brantley 2008). Some bird lovers will surely say, "but it feeds my feathered friends! don't kill it." However the results of one study suggest that privet removal would not substantially affect songbird populations (Wilcox & Beck 2007). Additionally, some will claim that the fight against privet is a loosing battle so why spend money and time trying to control it. However, one study found that five years after removal only 7% of sites had become re-infested with Chinese privet while herbaceous cover occupied 60-70% of the removal plots (Hudson et al. 2014). So the war can be won!

Interestingly, Stromayer et al. 1998 found Chinese privet to be an important fall and winter dietary component of a white-tailed deer population in Georgia and suggest that the shrub's benefit to deer should be considered against the damage done to biodiversity. However deer populations are high across most of the east and would probably do just as well browsing on the native woody species that the privet and other invaders are crowding out.  Regardless, certainly we should not consider deer populations to be more important than entire forest ecosystems.

3. Elaeagnus umbellata; Autumn olive

The silvery undersides of Elaeaganus umbrellata leaves help with identification. This specimen photographed in the Ozarks of Arkansas had its red berries which are just visible in the background. 

This large shrub was first introduced from East Asia on strip mine reclamation sites to control erosion and benefit wildlife. Some gardeners continue to plant this species despite the ecological damage. Autumn olive prefers full sun but can tolerate some shading and generally does well in poor soils. As a consequence of its sun and soil preferences, autumn olive generally invades grasslands and open woodlands. The alternately arranged leaves are elliptical with wavy margins and undersides that are covered in silvery scales that give them a dusty silver look. Twigs are generally orangish in appearance, but upon close inspection spots of orange dot a brownish gray twig.

As primarily a weed in grasslands and other open areas, autumn olive is the most common undesirable shrub that I see when assessing land enrolled in the Cropland Reserve Program (CRP). This shrub, like the other two species mentioned above, have the ability to transform the habitat which they invade, resulting in habitat degradation. As a nitrogen fixer, autumn olive increases local soil nutrient levels which can be problematic for communities that have evolved to grow in nutrient poor conditions. Additionally, autumn olive density is negatively correlated with grassland bird species abundances (Graves et al. 2010). One study found that large herbivores didn't eat as much autumn olive when compared with native shrub species (Knapp et al. 2008), once again giving this non-native a competitive advantage.

The Take Aways
1. Invasive species can completely change the physical structure and community composition of an ecosystem resulting in negative effects for the native flora and fauna.
2. Don't plant these species of shrub! Just say no. You may love the look, smell, etc. but I promise you there is a native that you will love even more which will also not take over the neighbors yard.
3. If you have or think you have bush honeysuckle, Chinese privet, or Autumn olive on your property, contact your county extension office for detailed  information on control strategies.
4. Replace invasives with native species of shrub like, dogwoods, serviceberry, American cranberry, winterberry, plums, hawthorns, hazelnuts, ninebark, etc.
5. Know how to identify invasive species.
6. Let government officials know where you find invasives on public property.