Trees and Shrubs That Self-sow Excessively

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The Norway maple (Acer platanoides) produces so many seedlings it can easily become a garden pest. Source: www.massaudubon.org.

All plants reproduce. If they didn’t, they’d go extinct! But most do so modestly, producing a plant here and there, just enough to maintain their population. Others, though, do so profusely, becoming an annoyance to gardeners and spreading into the wild far from where they are native. Most such plants are simply called weeds and they tend to be annuals, perennials or biennials. However, there are also trees and shrubs that overdo it and can become invasive due to aggressive self-sowing as well.

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False spirea (Sorbaria sorbifolia) can be highly invasive locally, but tends to spread via suckers rather than seed, so doesn’t get far. Source: Fanghong, Wikimedia Commons

Of course, there are different ways in which a plant can become invasive. Through suckers, for example. Think of false spirea (Sorbaria sorbifolia) or staghorn sumac (Rhus typhina). They certainly get around, but since they sprout from wandering roots, they only tend to be invasive on a very local scale. Woody plants that spread by seed can get much farther. What with birds, squirrels and wind to carry them greater distances, they can really get around.

I, for example, have no Norway maple on my property, nor do any of my immediate neighbors, but there are several further down the street and as a result, I find hundreds of Norway maple seedlings in my gardens every year.

The Ones That Overdo It

What follows is a list of trees and shrubs that have the reputation of being invasive through their seeds, but…

Not all plants on the list will be invasive under all conditions. They’ll only cause problems when the local environment is appropriate to their needs. For example, tamarisk or saltcedar (Tamarix ramosissima) is very invasive in drier climates, but not a problem at all in more humid ones, while Japanese barberry (Berberis thunbergii), which can be terribly invasive in moderate climates, doesn’t produce fertile seed in colder ones and thus is not a problem there.

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Frangula alnus ‘Ron Williams’ Fine Line is a sterile columnar form of the otherwise invasive alder buckthorn (F. alnus). Source: springmeadownursery.com

Also, there are sterile forms of many of the trees and shrubs listed here, such as Frangula alnus ‘Ron Williams’ Fine Line, a sterile form of the otherwise invasive alder buckthorn (F. alnus), while new sterile varieties of Japanese barberry (Berberis thunbergii) and winged euonymus (Euonymus alatus) are either under development or being introduced. If there’s a tree or shrub you like, but it has invasive tendencies, you can often find a sterile or nearly sterile form you can use with impunity.

Mulching Can Help

Fortunately, using a good mulch will prevent most tree and shrub seeds from germinating. They simply can’t germinate through a thick mulch. That said, some trees—especially nut trees—are among the few plants whose extra robust seeds really can germinate through a mulch. Those that are invasive even under 2 inches (5 cm) of mulch are marked with an asterisk (*).

You Choose

I’m not saying do not plant the plants listed here—some are great garden plants!—but forewarned is forearmed!

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Himalayan blackberry (Rubus armeniacus, syn. R. discolor, zone 7) is extremely invasive in some climates, but not hardy enough to be a problem in colder areas. Source: www.nwcb.wa.gov

  1. Alder buckthorn (Frangula alnus, formerly Rhamnus frangula) zone 3
  2. American elm (Ulmus americana) zone 3
  3. Amur honeysuckle (Lonicera maackii) zone 2b
  4. Ash (Fraxinus spp.) zone 2 to 7, according to species
  5. Autumn olive (Elaeagnus umbellata) zone 4
  6. Bird cherry (Prunus padus) zone 2
  7. Black locust (Robinia pseudoacacia) zone 4b
  8. Box elder* (Acer negundo) zone 2
  9. Blackberry (Rubus spp.) zone 2 to 8, according to species
  10. Bramble (Rubus spp.) zone 2 to 8, according to species

    20180927C www.plantes-shopping.fr.jpg

    Renowned for its stunning fall colours, burning bush (Euonymus alatus) can nevertheless be invasive in some areas. Source: www.plantes-shopping.fr

  11. Burning bush (Euonymus alatus) zone 5
  12. Butterfly bush (Buddleia davidii) zone 6b
  13. Chinese elm (Ulmus pumila) zone 2
  14. Chokecherry (Prunus virginiana) zone 2b
  15. Common buckthorn (Rhamnus cathartica) zone 2
  16. Dog rose (Rosa canina) zone 4
  17. European birch (Betula pendula) zone 3
  18. European privet (Ligustrum vulgare) zone 4
  19. Guelder-rose (Viburnum opulus) zone 3
  20. Horse chestnut* (Aesculus hippocastanum) zone 4b
  21. Japanese barberry (Berberis thunbergii) zone 4
  22. Morrow’s honeysuckle (Lonicera morrowii) zone 4
  23. Mountain ash (Sorbus spp.) Zone 3
  24. Multiflora rose (Rosa multiflora) zone 5b
  25. Norway maple* (Acer platanoides) zone 4b
  26. Norway spruce (Picea abies) zone 2b
  27. Pin cherry (Prunus pensylvanica) zone 2

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    Common buckthorn (Rhamnus cathartica) is very invasive in most temperate climates. Source: nyc.books.plantsofsuburbia.com

  28. Plane (Platanus spp.) zone 5 to 9, according to species
  29. Raspberry (Rubus idaeus) zone 3
  30. Redleaf rose (Rosa glauca, formerly R. rubrifolia) zone 2
  31. Rose of Sharon (Hibiscus syriacus) zone 6b
  32. Rowan tree (Sorbus spp.) Zone 3
  33. Rugosa rose (Rosa rugosa) zone 3
  34. Russian olive (Elaeagnus angustifolia) zone 2b
  35. Saltcedar (Tamarix spp.) zone 5 to 9, according to species
  36. Scots pine, (Pinus sylvestris) zone 2b
  37. Sea buckthorn (Hippophae rhamnoides) zone 2b
  38. Siberian elm (Ulmus pumila) zone 2
  39. Siberian peashrub (Caragana arborescens) zone 2
  40. Silver maple* (Acer saccharinum) zone 2
  41. Small-leaved lime* (Tilia cordata) zone 3
  42. Sugar maple* (Acer saccharum) zone 4
  43. Tamarisk (Tamarix spp.) Zone 5 to 9, according to species

    20180927B www.florafinder.com.jpg

    Tatarian honeysuckle (Lonicera tatarica) can be quite invasive in temperate climates. Source: www.florafinder.com

  44. Tatarian honeysuckle (Lonicera tatarica) zone 4
  45. Tree of heaven (Ailanthus altissima) zone 6b
  46. Walnut* (Juglans spp.) Zones 4b to 8, according to species
  47. Wayfaring tree (Viburnum lantana) zone 2b
  48. Winged euonymus (Euonymus alata) zone 5
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Norway Maple: the Great Invader!

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Norway maple: a nice, symmetrical shade tree… but with a hidden vice!

Sometimes when you try to solve one problem, you create another. That’s what happened in the 1950s and 1960s with the massive introduction of the Norway maple (Acer platanoides) in Eastern North America. At that time, North America’s most popular street tree, the American elm (Ulmus americana), was being decimated by Dutch elm disease and people were looking for a substitute. The Norway maple, although largely untested, quickly became the street tree of choice. It was planted by the hundreds of thousands across North America and remains, in most areas, the tree most commonly sold in garden centers.

Many varieties of Norway maple have decorative leaves, like this purple-leave Acer plantanoides 'Schwedleri'.

Many varieties of Norway maple have decorative foliage, like this purple-leaved cultivar, Acer plantanoides ‘Schwedleri’.

It’s not an unattractive tree: relatively fast-growing, with dense, dark green foliage (or purple or variegated foliage in the case of some of its many cultivars) and a thick trunk. Also, it’s very tolerant of street conditions: air pollution, compacted soil, road salt, etc. It certainly looked like a good choice… at first. Over time, it showed more flaws: its dense root system uplifts and destroys sidewalks, it kills lawns and it suffers severe breakage during storms, causing untold millions of dollars of damage. Also, it turns out, for reasons unknown, to be relatively short-lived in North America: only about 50 to 60 years compared to up to 250 years in Europe. But who thinks that far ahead when planting a tree?

But the main problem turns out to be its invasive nature.

Who knew? In Central and Eastern Europe, where it is native, Norway maple is not an invasive species. It simply melds into the local forest, one tree among many. In Eastern North America, though, its seeds fall everywhere and germinate under a wide variety of conditions. And, unlike most other invasive introduced species, which tend mostly to stick to disturbed habitats, Norway maple quickly moves into and starts to dominate local forests, outcompeting native trees and especially its North American counterpart, the sugar maple (A. saccharum).

Sugar maple in its fall glory.

Sugar maple in its fall glory.

The sugar maple is the dominant tree in much of eastern North America’s climax hardwood forest, from Lake Superior to Pennsylvania (even further south in mountainous regions) and from the Eastern seaboard to the Prairies. It is well known as the source of maple sugar… and of the brilliant fall coloration that attracts so many tourists to the area in October.

 War of the Species

Norway maples produce huge seed crops, much greater than sugar maples, and their seeds, carried by the wind, tend move into maple forests where they grow readily. Their seeds are capable of germinating in deep shade, even more so than those of the sugar maple, although it too is highly shade tolerant. Norway maple seedlings grow quickly and densely and generally shade out any sugar maple seedlings that do sprout. They are more heat-tolerant that young sugar maples and thus better able to survive a climate influenced by global warming. As they grow, Norway maples create greater shade and drier soil conditions than native woodland species can tolerate, creating woodlands barren of undergrowth… other than more Norway maple seedlings! It is feared that, if the spread of Norway maples into the wild continues to be tolerated, they will create a dense monoculture, eliminating entire North American ecosystems.

You can see the signs already. In many neighborhoods, Norway maples sprout abundantly from hedges, along property lines, and pop up in shade borders. When the care in local parks is neglected, they move in and take over any areas that are not regularly clipped by lawn movers. And where city and suburb meet native woodland, the Norway maple is usually already established, slowly working on becoming the dominant tree.

In natural forests in Montreal’s Mount Royal Park, for example, Norway maple seedlings were found to outnumber sugar maple seedlings 4 to 1 in 2003 and it was estimated the species would dominate the park’s forest “within a generation”.

Some authorities have “seen the light” and banned Norway maples. It is illegal to plant them in Massachusetts and New Hampshire, for example, and many municipalities have similar laws. One major chain store, Meijer Garden Centers, has voluntarily removed the plant from its stores. Unfortunately, I know of no other nurseries that have followed suit and Norway maples remain widely available in most regions where they continue to be planted on a large scale.

It would be nice if more governments banned the tree… but they seem to be turning away from ecological issues these days. And I must admit I’m amazed and puzzled that garden centers continue to produce and promote Norway maples in areas where it could easily escape into native forests. The whole problem would be reduced considerably if shoppers were simply offered other trees instead. Most probably wouldn’t even notice the difference! I don’t think home owners set off to a nursery looking for a Norway maple; they’re looking for a “shade tree”, period, and would happily choose among whatever species are offered. No government ban would be needed if garden centers showed more concern about the environment!

Help from the Strangest Place

20150408HOddly, a plant disease may become a major player in the fight against the invasive Norway maple.

Tar spot disease (Rhytisma acerinum), a disease specific to Norway maples and certain other maples (but not the sugar maple*), has been in North America since at least the year 2000 and has spread throughout most of eastern part of the continent, especially over the last 5 years. It causes disfiguring dark black spots on Norway maple leaves, making the entire tree unattractive from midsummer until fall. No effective treatment for this disease is known other than removing the affected tree. Municipalities have been spouting nonsense about raking up and destroying leaves to reduce the effect of tar spot, but there is not one iota of proof that striving to pick up and destroy every single diseased leaf in the fall (it takes only one to cause the following year’s outbreak!) does anything other than annoy gardeners. For more information, see Yes, You Can Compost Diseased Leaves.

*There are other tar spot diseases that target the sugar maple, but they are not very common and tend to appear only sporadically and on a much more limited basis.

The more gardeners see this striking disease, which has turned entire city streets into something like a scene from a horror movie, the more likely they are to think twice about buying a Norway maple. Unfortunately, new leaves appear healthy, so during the prime season for selling trees, spring, even seriously contaminated trees show no symptoms. Still, as tar spot disease gets more and more press and the information circulates that the only real solution is avoid planting Norway maples, this can’t but help reduce Norway maple sales.

So, spread the word. Tell friends and neighbors not to plant Norway maples and, if you have that kind of influence, try to get your local government to ban planting them. Also, express your disapproval to the manager each time you see them in a local garden center. These measures may be little and late, but at least they’re a step in the right direction.

How to Distinguish Between Sugar Maples and Norway Maples