The Best Invasive Plants
Across the globe many introduced plants have led to widespread ecological problems including loss of diversity. Exotic plants that naturalize readily into the environment are labeled as invasive. Typically they are demonized for having adverse effects on plant communities and wildlife. Billions of dollars and countless gallons of herbicides are used to combat these ‘evil’ plants. Communities will hold gatherings to eradicate invasive plants to ‘fix the problem’.
I believe the story is much more complex than good natives and bad exotics. Invasive plants can choke out native vegetation and be far less palatable to herbivorous insects (which are a huge part of the ecosystem). However, there is a good and bad side to everything. Often times invasive plants are extremely useful to people and wildlife. Originally, many of these plants were introduced on purpose for their beneficial qualities.
So much negative press and talk has been made of these aggressive exotic plants that I am excited to shed some light on their positive effects. It will not be hard for hard for anyone to find information on the harm caused by invasives. I am writing this, to add another element to this fascinating discussion of our changing world.
Before I get started on the positive attributes of plants like European buckthorn and multiflora rose, it is important to look at why they are invasive at all.
Invasive plants can only invade and take over an area when conditions are ripe. They can not just appear and thrive anywhere, especially into a healthy ecosystem. When plants demonstrate invasive qualities it is a symptom of a bigger problem. For example, nature abhors bare soil, and will cover the ground as fast as possible. If a garden plot is rototilled and then abandoned annual and biennial weeds will sprout very fast. A sun loving plant of disturbed soil like lamb’s quarters can only become invasive in that situation.
In the Northeastern U.S., when left alone, land turns to either forest or wetland. In a forest or wetland, there is no place for autumn olives or japanese honeysuckle. They simply have no habitat. When the land is cleared and the soil exposed to light is when these exotic shrubs can become invasive. They are a response to an extremely violent and traumatic action. They are the indicators of disturbance rather than the cause of environmental degradation.
With that said here are some of the most common and useful invasive plants in the Northeastern U.S. (many of them are widespread across the continent).
Black Locust (Robinia pseudoacacia):
Black locust is a tall tree with a narrow crown growing to about 60 feet tall and 1-2 feet in diameter. It creates a light shade conducive to other plants growing beneath them. Black locust groves are often hard to walk through because of all the undergrowth.
Depending on who you talk to black locust is native to different parts of the U.S. ranging from the Appalachian mountains from Pennsylvania to Alabama and parts of the Ozark mountains. It has spreadly widely from it’s native range to now inhabit every continental state.
Black locust grows fast, it demands sun and grows extremely well in disturbed sites, road sides, and old fields. It spreads by seed and by underground root runners. The root system of a single tree can form large colonies as big as an acre. When cut down black locust can sprout from the stump and put on 9 plus feet in a single year.
Many people think trees that grow fast will have soft wood. Black locust proves that theory wrong. It is one of the hardest of all woods in the diverse temperate forests of the U.S. As hard as hickory, black locust is extremely rot resistant. It is 2.5 times more durable than white oak, the traditional ship building wood.
Black locust can be grown and managed to produce high quality firewood and fence posts. A grove can be cut down again and again. It is being bred by a handful of people to exhibit straighter growth for timber. Locust can be a viable alternative to pressure treated lumber, it is certainly more rot resistant.
The benefits of locust do not end with the wood. It is a member of the pea family. Black locust is a giant nitrogen fixer with edible flowers and seeds. The flowers really do taste like peas when picked fresh. They look like pea flowers and hang down in beautiful white clusters. When black locusts are blooming, a diverse range of pollinators are all over them. Just ask any beekeeper about black locust honey.
Because of black locusts ability to grow fast and spread by horizontal roots, it is an excellent soil stabilizer and builder. Add that to the light shade it creates, it’s ability to grow in poor or no soil and fix nitrogen, and its relatively short life span, make black locust an ideal tree for reforesting abused land. Species such as oaks, chestnuts, walnuts, and hemlocks can be established under the light canopy of a locust grove, as it gives some shade without smothering them.
Multiflora Rose (Rosa Multiflora):
Multiflora rose looks like a billowing mass of red canes, hooked thorns, and bright red berries in the winter. In the summer it looks like a green monster taking over everything next to it. Multiflora is a vigorous shrub that can grow anywhere there is some light and moisture. In partial shade it will grow almost as a vine and climb up and over small trees. Twelve feet of growth in a single year is not uncommon.
It is native to East Asia and was brought to the U.S. for two reasons. It was originally brought here from Japan in the early 20th century to be used as a rootstock for grafting cultivated roses onto. Multiflora was again introduced in the 1930s and 40s to be used for living fences on cattle farms. State nurseries and soil and water conservation districts encouraged farmers to plant hedges of multiflora rose and sold them a great number of plants.
A mature multiflora rose shrub is capable of producing half a million seeds in a single year, all of them viable. The tiny bright red rose hips are a popular food among song birds in winter, so it is no surprise that multiflora has been able to spread rapidly. It is can be found growing in open fields, fence lines, and woods with light shade. Though it prefers moist or even wet soils, multiflora can tolerate virtually all soil conditions.
The thorns on this plant are particularly fierce. They are hooked and very good at tearing clothing and skin. It almost seems like they are reaching out to grab you. Sometimes multiflora will colonize a field forming such dense thickets that even a deer can not pass through.
With such an aggressive thorny plant, it may seem hard to imagine the positive attributes. In order to do so, we must see things from the perspective of wildlife. Wherever there is thickets of multiflora, there is a usually an abundance of songbirds and often rabbits and small rodents (a necessary ingredient for more popular fauna like hawks, owls, and foxes).
Songbirds are able to nest in the dense thorny branches without any fear of predator raids. In the winter, rose hips, are an excellent food for birds (and people) that is packed with vitamin C.
The life of a rabbit or a small rodent is a precarious one, but in a rose thicket, they can find cover and food all winter. Not only can they eat fallen berries, but they will also chew the bark off of the rose stems throughout the winter.
When spring comes and the roses bloom, it is a beautiful sight and smell. They are white, five petaled roses that are pollinated by a great number of insects.
There are virtually no plants in the Northeastern U.S. that provide as good cover for birds and small mammals as multiflora. Combine this with the fact that it also provides food at a critical time in winter, and you have a tremendous wildlife plant. Wherever I see multiflora rose, I can usually count on finding rabbits, foxes, and lots of songbirds.
Autumn Olive (Eleagnus umbellata):
Autumn olive or autumn berry is one of the most useful plants to people and wildlife. It grows as a large shrub, to about 15 feet and wide. They can best be identified by their silver speckling. When looked at closely, you can see shiny metallic silver speckles covering the leaves, young twigs, flowers, and berries. This silver color can be seen from roadsides when they are in leaf, especially on windy days.
Autumn olives can form dense thickets and are capable of colonizing whole acres. They will do very poorly in the shade, but thrive in sunny fields, roadsides, and abused areas.
Autumn berries were brought here from Eurasia for planting wildlife shelter belts, and perhaps occasionally for their edible fruit. They have had no problem escaping cultivation because birds readily consume the fruit, and because they are extremely tough plants. Autumn olives can tolerate extreme cold and heat, excessive moisture and drought, and a pH anywhere from 3 to 8. They also can fix nitrogen out of the atmosphere and grow in the sterile conditions of abandoned coal fields.
Rabbits find good cover in autumn olive thickets, and will chew the bark off stems in the winter. However it is the birds who really benefit from this plant. The branches are often so heavy with fruit, it is almost impossible to describe how generous these plants are, as their limbs bend to the ground with endless berries.
The tiny flowers of autumn berry are yellowish (with silver speckles), 4 petaled blossoms that smell amazingly unique and strong. Autumn olive thickets can be smelled from a speeding car with the windows shut. In ancient Persia, women were locked up during autumn olive’s bloom time to prevent any excitement.
There is no shortage of insects that will pollinate the endless number of tiny autumn olive blossoms. By late summer the berries will begin to ripen. Bushes will ripen fruit as early as August or as late as November. Occasionally berries will hang on into winter in a particularly good year, but most of the time they either fall off or stripped by birds and children.
The taste of autumn berries varies a great deal from plant to plant. Some individuals will have delicious sweet berries as good as any raspberry, while others will have fruit so astringent that it’s barely edible. People have stepped in and selected superior plants for home gardens, but here in the U.S. there is no commercial cultivation.
Autumn berries are excellent for fresh eating, jams, juice, and fruit leather. The fruit leather made from them is so good that it rivals strawberry. The berries have about 17 times as much lycopene as tomatoes, making them an excellent source of anti-oxidants. In parts of central Asia, whole branches laden with ripe fruit are sold in markets for a snack to eat while walking around. The branches snap off easy and are a great gift to a child stuck in a car seat during road trips.
European Buckthorn (Rhamnus Cathartica):
Buckthorn is a very common shrub or small tree along fence rows, hedges, young woods, and abandoned fields. It can be 20 feet high, often with multiple trunks. Short thorns that resemble pointed sticks more than thorns are found sporadically on the trunks and branches. Dark purplish black berries abundantly cover female plants during fall and winter. Most people have noticed this plant because it is so common, and the berries make it conspicuous.
One interesting thing about identifying buckthorn is that it has both opposite and alternate branching patterns, often on the same twig. I know of no other tree or shrub that does this.
Buckthorn was introduced here from Eurasia in the 1700s, and quickly became naturalized. It was brought here for it’s medicinal properties, and to be used in creating hedges.
Buckthorn can easily become a dominant understory species in woods with ample light. Though, buckthorn really thrives and shows it true potential in full sun, wet or dry. It seems to have no problem with heavy clay soils that are often mud. Buckthorn will grow fast in all conditions, and responds extremely vigorously to being cut. The roots of buckthorn are incredibly dense and fibrous, forming black hairy mats underground that I am convinced improve soil.
Seed is how buckthorn propagates itself. It is not uncommon to find carpets of buckthorn seedlings near mature trees that have sprouted from fallen fruit. However, buckthorn really gets around with the help of birds and deer.
I consider European buckthorn to be one of the most important winter deer foods. Underneath mature specimens in the wintertime, the snow will be trampled by hoof marks and stained purple from the berries. A group of deer will often visit the same productive shrub, every day for most of the winter, as the berries slowly fall off.
While buckthorn seedlings may crowd out other tree seedlings like oak and maple, it may be the fruit that feeds the deer and lessens the browse pressure. The world is never as simple as it seems.
The wood of buckthorn is very hard. It produces a dark orange pink heart wood at an early age that resembles the color of fire. Because it is a small tree, it is unknown to most woodworkers.
I hope the next time you pass by a hedge of buckthorn that you do not view it with disdain, but can see it for what it is, a valuable winter food for an abundance of wildlife.
Wild Apple (Malus spp):
Many folks might be surprised to see wild apples listed here as invasive. The reason is because they are so loved by people that we have chosen to ignore the fact that apples are primarily exotic and have become widely naturalized. Through our own prejudice we have decided which exotic plants are invasive and which are not. Wild apples can form thickets and choke out native vegetation. Is this bad?, are invasive plants bad?
Wild apples are much more common than many people realize. They really are all over the place when you have an eye for the wild forms. The apple tree has so many forms and fruits that they can be unrecognizable to someone used to seeing apples in orchards. Wild apple trees can be gnarled shrubs with spikes, they can be tall slender trees, or massive spreading specimens 50 feet wide. The fruit can be as big as a softball or as small as a pea. The fruit’s skin can be red, orange, green, yellow, and on rare occasionsblue. The flesh can be white, red, or pink.
Wild apple trees that were not planted by any human can be found in nearly every hedgerow, abandoned field, and woods edge in upstate N.Y. For the most part these are not native crabapples, they are escapes of the widely cultivated apple. The seeds are spread by an abundance of mammals that flock to these trees every fall. Many wild apples appear in old pastures where cows or horses pooped out seeds. The seeds germinate easily, and the trees are tolerant of both drought, and heavy water logged soils. They can compete with weeds and withstand considerable browsing.
People who have planted apple trees might disagree that they are this easy to grow. Cultivated apples are not nearly as tough as wild ones. They are typically grafted onto wimpy dwarfing rootstocks. Also wild trees produce so many seedlings that only a small percentage need to survive to spread the species.
It is hard to overstate the significance to wildlife. Fruits can drop from trees starting in August all the way through winter and early spring depending on individuals. When trees are fruiting, they will be visited by virtually every mammal that is not a pure carnivore, and even the carnivores will benefit from the improved habitat as every hunter can tell you. A long list of birds will also feed on the fruit.
In winter the buds are eaten by deer, and the bark by rodents and rabbits. The beautiful fragrant blossoms are significant sources of nectar for a wide variety of pollinators. Apple wood is hard and heavy, it has a dark color and a wonderful smell when cut or burned.
The endless gifts of the apple tree are enjoyed around the world by countless species including our own. Numerous books have been written about the lore and value of this tree, it would be a crime not to list it here in this list of the best invasive plants. If we are to condemn exotic species that readily naturalize, then we would need to include the honey bee, the earhtworm, the apple tree, and all of us who are not Native Americans.
Which plants we view as invasive or beneficial or native is really just that, our view. The world has always been in a constant state of change, and will continue to be so. Plants and diseases will continue to move around the planet and shift things around. Nature’s balance looks more like upheaval in reality. The only thing sustainable is change.
As gardeners, farmers, or land stewards, when we look at plants, let’s see them for what they really are. All prejudices aside, what is their value to wildlife?, their uses to humans?, their role in the surrounding community of plants? I hope that when you see exotic plants, you can see them beyond the literature. Most of the time you will likely have no desire to plant or encourage wild exotics in any way, and you might want to work on limiting their expanse or even eradication. It is also possible that you might see the nearby multiflora rose thicket is home to most of the song birds on your property. The point is not to judge a species based on its origins, but on its merits and drawbacks in your particular setting. Let’s observe plants and see what they really are with our own eyes without judging them first based on their origin.