Without any bad intent many ornamental plants worldwide have been released into gardens and landscapes of their new homes, only to become serious invasive pests. Some were simple stowaways, others were brought as food, a few to purposely modify landscapes, and yet others simply because they were beautiful – it is this last group, the ornamental plants, I wish to address in this article.
With the advent of international exploration and trade, there also began the movement of exotic plants and animals across the planet. Early colonizers and traders at first brought useful plants, such as grains, vegetables, fruits and fiber crops to new lands, but in time ever more unusual and exotic species made their way as well. While both Asian and early American cultures had been trading in various plants for over a thousand years, movements of these was relatively slow and limited. It wasn’t until the Europeans with their ocean going ships did the flow of exotic species go from a trickle to a torrent across the world. And so began the age of invasive exotic species.
This flood of invasive species is well known these days and even makes the news frequently. In the USA we all have heard about Japanese knotweed overtaking disturbed land throughout the northern states, kuzu vines strangling acres of southern forests, cheat grass crowding out native grassland species in the western mountains, and hydrilla hopelessly clogging innumerable waterways.
But there remains a real danger with regard to ornamental invasive plants – the desire to grow them regardless of the threat to natural systems and the species that these support. The truth is that many ornamental plants have been in cultivation for just a relatively short time, so well meaning gardeners could have ticking time-bombs in their midst and not even realize it. Moreover, once an invasive has entered a natural system it is hard to know the effect it is having on other species, or indeed the very physical elements such as water and soil.
The following list is just a sample of beautiful exotic pests that have already run rampant over native ecosystems or are threatening expand their range. Some may indeed to be found to be mere localized pests, or so limited by the type of habitat they can invade that their effect is not pervasive. In any event, it is my opinion that the following plants be avoided when considering what to grow in your garden. The numbering is not intended as a scale of invasiveness, but rather arbitrarily assigned.
1.) European Wand Loosestrife, Lythrum virgatum – this a close relative to purple loosestrife, Lythrum salicaria, the famous invader of wetlands. Nowadays it is illegal to grow L. salicaria throughout much of the USA and Canada, and to date it can be found in all but six states in the USA, and is still spreading in the west. This plant is so invasive that it literally transforms wetlands into near loosestrife monocultures.
The rub is that the very similar European wand loosestrife, Lythrum virgatum, is still sold by many nurseries. Cultivars sold as sterile, safe garden plants have been found to be capable of setting seed if pollinated by other wild growing Lythrum species, including L. salicaria. Germination rates of Lythrum seed is very high, and rampant colonization of wetlands is a constant threat when seed is in the vicinity. Though L. virgatum is reported only from a handful of states in the USA, the truth is that its actual distribution is not well known since it can be easily confused with L. salicaria, and hybrids as well may be much more prevalent that previously thought. To make matters worse, both species are adapted to a wide range of temperatures, growing from USDA cold hardiness zones 3-9 (in other words, capable of living from Florida to southern Canada).
Not surprisingly a number of states have already banned the sale of this species within their borders, including Arizona, Iowa, Montana, Minnesota, North Carolina, North Dakota, Nevada, New Hampshire, Tennessee, Wisconsin as well as Pennsylvania and Washington, where it is already considered a noxious weed. Remarkably, even to this day there are people who grow and advocate growing both species. In my opinion, growing and distributing either is unethical, and should be stopped wherever they are not native. Remaining wetlands have undergone extensive modification or outright destruction in past decades, and combined with increasing human need for water resources, are under tremendous pressure already. Adding to that by growing these highly invasive pests is the coup de grace, and thoroughly inadmissible.
2. Yellow Flag Iris, Iris pseudacorus – another ornamental plant that has literally gone amuck, invading wetlands throughout the USA and adjoining areas of Canada. Currently is it found in all but four states within the US, one being Hawaii. Amazingly even Alaska has populations. Brought in as a garden plant in the 1700s, it has escaped and colonized freshwater as well as brackish water wetlands from central Alaska, thru much of southern Canada, all the US, parts of Mexico and even South America. It rapidly colonizes wetlands forming huge monospecific rafts, out-competing native plants, and can even flourish in shaded swamp forests.
If that weren’t bad enough, it is also toxic, so is a danger to both domestic stock as well as wild animals. Care should be taken when removing it since its sap can cause skin irritation. The seeds have air pockets that allow them to float for months and thus are distributed to new areas. The rhizomes as well break off with age and similarly can be transported by flowing water to colonize other habitats. It is tolerant of a wide range of temperatures, growing from USDA cold hardiness zones 2 thru 10, which is in itself almost unbelievable.
Remarkably, Iris pseudacorus is still sold online, and otherwise traded. Regardless, simply put, this species has no redeeming features beyond its pleasant yellow flowers. If you want to grow an iris, there are many other species and hybrids within the genus that are harmless to the environment and you. Here is another species that should never be grown outside its native range.
3. Periwinkles, Vinca minor and Vinca major – these lovely little evergreen vining plants complete with pretty lilac blossoms have one major flaw – they are terribly invasive. Brought in as ornamental ground covers as early as the 1700s, both have spread over much of the eastern USA, with V. minor favoring the northern forested eastern and western states, and V. major more frequent in the southeast and Pacific coast states. Though lovely, both are serious invasive pests wherever they are found. Plants that escape from gardens and old homesteads rapidly colonize woodlands, forming dense mats that exclude native species from growing.
This is another one of those, “oh isn’t it lovely and cute” plants (even the name is non-threatening) that is literally more sinister than it seems on the surface. Though the plants are rather short and seem innocuous, their root systems are extremely vigorous and penetrate several feet into the ground, out-competing other plants and thus creating sterile monocultures. In southern Indiana for example there are many hundreds of acres of these periwinkle monocultures dominating forest floors there.
Natives of Europe and the Mediterranean region, they need to remain where they are found naturally. Even here in southern Japan I see them escaped into the local forests. Remarkably, to this day both are commonly traded and sold, and as far as I know, not legally controlled by any state in the USA. Another plant to avoid regardless of legality in my book.
4. Japanese climbing fern, Lygodium japonicum – this odd little Asian fern is a true vine, growing high into the forest canopy when happy, and is cousin to Lygodium microphyllum, which has become a menace to the wetlands of central and south Florida. Though relatively not well known, L. japonicum is established in much of the southeastern USA, particularly in Alabama, Florida, Georgia, Louisiana, Mississippi, South Carolina and eastern Texas where it rapidly colonizes disturbed sites and plantation forests.
It frequently grows from the ground up into the canopy (to 30 meters, or 90 feet) forming a “petticoat” up the tree, and in fire events this can aid in setting crown fires which destroy trees outright, but have little effect on the fern since it rapidly regrows. In Florida this fern is establishing in native habitats as well, particularly wetlands, and seems to be increasing in distribution, apparently from north to south. It has also been found in remote regions of the Everglades and Big Cypress Swamp, so it is effectively moving on its own.
Brought in sometime around the turn of the last century, this plant is now firmly established in many southeastern states. While its relative L. microphyllum is now illegal to import or distribute in the USA without a permit, this species is not regulated as yet. Given it is more capable of handling cold, surviving up to at least USDA cold hardiness zone 7, it seems it could become an even greater threat than its cousin to southeastern wetlands and forests. For this reason I highly recommend avoid growing this species either in the open garden or under glass.
5. Coral Ardisia, Ardisia crenata – here’s another lovely plant that at face value looks like a winner for southern gardens, and so it seemed for many years. This plant was first introduced into Florida about a hundred years ago and seemed to be just another beautiful shrub with its broad evergreen leaves and copious umbels of brilliant red berries. Then in the 1980s folks started to notice it was finding its way into nearby woodlands. At first it seemed to be restricted to Florida, but then infestations were reported in eastern Texas, and more recently in Louisiana, Alabama, and even Hawaii.
The problem with this plant stems from its red berries, that while lovely, are produced in vast quantities and have very high germination rates. A single adult plant that has escaped into a native woodland can have a literal carpet of seedlings around its base. In time these grow into adults and before you know it, you have a vast grove of A. crenata dominating the forest floor. This species is a native of dark forest floors, needing little light to thrive, and easily crowds out native herbs, grasses and ferns. Populations are now well established in both urban areas as well as native forests, and it is probably still expanding its range outside of Florida.
Currently coral ardisia is illegal to possess, sell or transport within the state of Florida where it is now widespread. In other southern states it remains a growing threat. Though its advance northward will be thwarted by cold winter temperatures, it is likely it will be able to spread into much of the warmer parts of the southeast, certainly to USDA cold hardiness zone 8, and even perhaps the warmer parts of zone 7. Given its high invasiveness, here’s another one to avoid in the garden even outside of Florida.
6. Mimosa tree, Albizia julibrissin – this lovely tree with its pink pom-pom shaped flowers and soft, fern like foliage has earned it many colorful names such as silk tree, silky acacia and powderpuff tree. Originally a native of southern Asia, it was introduced into the USA in the mid 1700’s and has firmly established itself throughout the southeastern states, mid-Atlantic region, Ohio River valley, and parts of the west as well.
It’s elegant tiered branching and soft pink flowers beguile, but beware, this tree is highly invasive. An opportunist, it quickly dominates disturbed land such as fallow fields and roadsides in particular, but is highly adaptable to different conditions, often favoring natural waterways. It is fast growing (up to a meter a year), quickly resprouts if mechanically damaged (including severe pruning, frost damage and fire), and produces abundant seed that can remain viable for decades. It can quickly form dense thickets that blocks out sunlight for other plants. Trees growing along rivers and streams pose a greater threat since the seeds are easy spread downstream.
To this day the mimosa tree is grown as an ornamental tree throughout the world. There are even deep purple-red leaf cultivars that are nearly irresistible to exotic garden hobbyists. In the southeastern US it is considered a weed species and is being closely monitored and controlled. In Tennessee it is considered a severe threat, while in Georgia it is on the top ten most invasive plants, and a weed capable of altering plant communities in Florida. In other areas is listed as moderately invasive, but this could change with time. Though largely unregulated, this remains a tree that should not be grown outside of its native lands in my opinion.
7. Chinese Tallow Tree, Triadica sebifera – another angel in disguise, this native of China and Japan is now a rampant weed in subtropical regions around the world. It was brought into the Charleston, South Carolina area in the 1700s since the coating of its seeds can be made into wax, soap, and even fuel. For years it was allowed to grow unchecked, and current now populations exist throughout the southeastern USA from coastal North Carolina to central Texas. More worrisome are reports of disjunct populations in northern Tennessee, Kentucky, California, and even southern Wisconsin, indicating that its spread may not yet be full realized. In California it is a relative newcomer, with populations along river systems still on the increase. In Florida it is now almost widespread, with just a handful of counties still free of the pest.
This is one adaptive plant, capable of producing flowers in the first year of growth, and adult trees are estimated to produce 100,000 seeds each year. While the plant seems to thrive in riparian zones (wetlands along rivers and streams), it also happily invades marshlands. Here it isn’t fussy either, growing in fresh, brackish, or salt water marsh. If that weren’t enough it can grow on fallow land, in both full sun or deep shade. Very quickly infested lands can become vast monocultures of this tree, replacing other trees, grassland species, and even effecting reproduction of frogs. Moreover, while the seeds are dispersed by hungry birds, both leaves and seeds are poisonous to livestock and humans, causing skin irritation and if eaten, vomiting and diarrhea.
Amazingly, this tree was sold until very recently as an ornamental since it can grow in almost any soil, is drought tolerant, fast growing and has lovely red to orange fall foliage. It now is classified as a noxious weed through the gulf states, and efforts are under way to control it wherever it is found in abundance. This one is another unsuitable plant for any garden, and existing trees should be removed ASAP.
8. Asian Privet, Ligustrum species – this group of Asian evergreens, capable of growing to small tree proportions, are represented by the following species in the USA: L. lucidum, L. japonicum (pictured above), L. ovalifolium and L. sinense. Of these, L. sinense is the most widely distributed, occurring even into the lower Midwest and southern New England states, while L. lucidum and L. japonicum are found largely in the southeast, particularly Florida and the gulf states, with outlying populations in California. L. ovalifolium is far more widespread, but with much less impact (so far) than its relatives.
Privets were brought into the USA around the 1800s as ornamental shrubs and for making hedgerows. In early summer they produce large heads of white flowers, and in the fall bunches of black-purple berries which are readily eaten by wildlife and dispersed. Plants can spread from root sprouts as well, thus forming dense thickets that crowd out native species, particularly low growing herbs and grasses, and to a lesser extent sprouting tree seedlings. They can invade all manner of habitats, from open fields to closed canopy forests, and even wetlands.
Although Florida lists L. sinense as a noxious weed, these bushes remain largely unregulated in the USA, and though they are being closely watched, there are no federal controls on growing and distributing them. Here’s another group of plants that are just too invasive to be good candidates for the garden.
9. Mexican Petunia, Ruellia simplex – here’s another would be innocuous herbaceous perennial for southern gardens that has turned into a nightmare plant. A native of Mexico and South America, it was introduced into Florida sometime in the 1940’s. This vigorous weed now is found in throughout peninsular Florida as well as parts of South Carolina, Georgia, Alabama, Mississippi, Louisiana and Texas, as well as Hawaii, Puerto Rico, and the US Virgin Islands. It has also become a pest in eastern Australia, and to some extent, Japan as well.
Mexican petunia readily invades wetland communities in particular, including marsh/prairie habitats as well as more shaded forested swamp lands. It has been found that even after being killed back by herbicidal treatments, this plant quickly recolonizes, crowding out native species. Not surprisingly, it can withstand a wide range of environments, the seed has high germination rates and plants quickly recover if cut down, burned, etc. Simply put, it has all the characteristics of an invasive weed.
Ruellia simplex is now considered a noxious weed in Florida, capable of altering native plant associations. Though its range is spotty north of peninsular Florida, it is cold hardy to at least USDA cold hardiness zone 8, making its expansion northward just a matter of time. Efforts have been made to breed sterile forms for the outdoor garden, but honestly I personally do not believe they should be trusted. There are just too many safer choices for the garden, both exotic or native, so why take a risk with this invasive time-bomb?
10. Spanish Bluebells, Hyacinthoides hispanica – what could be more wonderful than a vast field full of bluebells in the spring? Answer – some other plant that doesn’t take over everything. Although this species is not yet considered a serious problem in the USA or Canada, it is a problem in western Europe where it is hybridizing with native bluebell species (H. non-scripta) to produce the invasive, fertile hybrid H. x massartiana. In Vancouver, B.C., Canada, all three of these bluebells exist, and like in the UK and Ireland, they are causing concern, but as yet are not considered seriously invasive. Although a beautiful plant, all parts are toxic to humans and animals.
That said, this bulb forming plant originally native the Iberian Peninsula and north Africa has been found to be very unruly in the garden. Its bulbs easily multiply and are deeply rooted, making them difficult to fully dig up and remove. Anyone who has seen them take over, often in fairly shaded woodlands, can attest to their ability to dominate. I have seen them growing on Salt Spring Island, B.C., Canada in a native woodland (admittedly along a road), thus proving their ability to expand into native plant communities. Add to that, this species can thrive in a wide range of temperatures, from USDA cold hardiness zones 3-8, making its potential spread truly frightening.
As yet completely unregulated in the USA or Canada, H. hispanica, H. non-scripta, and their hybrid, H. x massartiana, are in fact still openly sold and traded as safe, reliable garden plants. Again, given the danger to native ecosystems, and the potential for becoming yet another prolific garden weed, why consider growing this plant?
So there they are, ten species of exotic ornamental plant you’d never find in my garden. Lovely, perhaps, but at what cost? Ethical concerns aside, we are unfortunately already experiencing the effects of their presence, and the real question now is how can we stem the tide in the decades to come? One answer that is simple is to just not grow any of them. That’s my ten cents.