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

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    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

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    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

When Tumbleweeds Take Over

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Call in the troops! Airmen battle a tumbleweed invasion in Clovis, Colorado. Source: www.cannon.af.mil

For me, tumbleweeds blowing lazily down the street of a ghost town are the very epitome of the American West, but while tumbleweeds may seem bucolic to gardeners in greener climes, real Westerners are getting fed up with tumbleweeds. Iconic or not, they’re a major hassle.

Road overwhelmed by tumbleweeds. The road had just been cleared by a snowplow! CLICK ON THIS: IT’S AMAZING! Source: Tim Tower, http://www.youtube.com.

When the big, round shrub-like masses of dry vegetation break loose from their roots and start to roll, they have to stop somewhere … and that somewhere can be drainage ditches, fences, firebreaks, gardens and homes. You certainly wouldn’t let a child play outdoors when they’re around in any number: that could be fatal! Sometimes so many tumbleweeds build up against homes that people have had to climb out through second-story windows or call neighbors for help. Some homes become entirely covered and can’t be recognized as buildings.

And Western tumbleweeds can be toxic to wildlife (they’re edible when young, then more and more toxic as they mature), hinder traffic and, being highly flammable, are a major fire hazard. Imagine a rolling, burning bush heading towards your home! Their thorny stems make handling them painful (hint: wear thick gloves!) and scratches can cause rashes, itching and inflamed skin in sensitive individuals. Going barefoot in tumbleweed territory is unthinkable and in many areas, you have to protect the legs of pets and livestock to keep them from going lame. And each plant produces up to 200,000 seeds a year. Yikes!

What Is Tumbleweed?

A tumbleweed can be any plant that breaks off and blows around. This one is a giant tumbleweed (Salsola x ryanii), only recently discovered in California. Source: giphy.com

Tough question! The term tumbleweed can refer to many different plants in 10 different plant families. It actually refers any plant sharing the same seed distribution technique. Tumbleweeds can be flower heads or entire plants that spontaneously break free and roll in the wind, spreading their seeds or propagules as they go.

Tumbleweeds tend to found in arid climates with open environments where there are few obstacles to hinder their roll-about distribution method: steppes, savannas, plains, deserts, beaches, etc. They don’t get very far, although may well be present, in dense, green, shrubby environments. Moist soils are an anathema to them: they much prefer drought!

Tumbleweeds in the Garden

You may be growing tumbleweeds without knowing it.

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The flower head of the tumbleweed onion (Allium schubertii) is a typical tumbleweed. Source: www.johnscheepers.com

The huge spherical umbels of rose-purple flowers of the ornamental onion Allium schubertii break off and roll about in its native Middle East environment. It is sometimes called—and why not?—tumbleweed onion. Boophane and Brunsvigia, two extraordinary African bulbs likewise bearing balls of flowers, are also tumbleweeds.

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The so-called Resurrection Fern (Selaginella lepidophylla) is one of the rare fern relatives that is a tumbleweed. Source: Nicole-Koehler, Wikimedia Commons

Or you may have tried (mostly likely in vain) to grow a “Resurrection Fern” or “Rose of Jericho” from a dried-up ball of leaves. Such dried plants commonly are sold as a novelty plant in horticultural fairs and will green up (although rarely come back to life) when soaked in water. The Resurrection Fern is not actually a fern, but a spike moss (Selaginella lepidophylla) and in the wild in its native Chihuahuan Desert, it breaks free and rolls about, spreading its tiny spores.

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Planting of kochia just staring to turn its bright fall color. Source: lookfordiagnosis.com

Another homegrown tumbleweed is kochia, also called burning bush and summer cypress (Bassia scoparia, syn. Kochia scoparia), a garden annual that forms a dense ball of apple green leaves that turn brilliant red in the fall and is sometimes used as a temporary hedge. It too breaks off and rolls away at the end of the season, sowing its seeds far and wide. Although widely available in seed catalogs, growing kochia is illegal in many areas.

The Not-So-Friendly Tumbleweeds

The tumbleweeds devastating California, Colorado, Arizona, New Mexico, Texas and Mexico, just to mention a few hot spots, are quite different from the garden plants described above and indeed, are not even native to the New World, but were imported accidentally from the steppes of Russia in field grains starting in the late 1800s. Others possibly arrived from elsewhere in Asia and one species seems to have come from Australia or Africa.

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When in growth, so-called Russian thistles (Salsola spp.) look nothing like thistles, but rather like shrubs. Source: Pablo Alberto Salguero Quiles, Wikimedia Commons

Because of the Russian origin of the main species and the plant’s thorny leaves, these tumbleweeds are often called Russian thistles. Of course, they are clearly not true thistles (that title rightly belongs to the thistle subfamily of the Asteraceae or sunflower family). Instead, they are placed are in the genus Salsola, now in the Amaranthaceae or amaranth family. Actually, the genus name has been changed to Kali … but I’ve heard rumors it will be moved back to Salsola, so I’ll be using that name in this text.

Confusing Nomenclature

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Most botanists would call these plants Salsola tragus, a sort of “name by consensus” for any plant in the hopelessly muddled Salsola genus. Source: wnmu.edu

Whether you call them Salsola or Kali, the plants’ identification is hopelessly muddled. Traditionally, the North American plants have long been called Salsola tragus, but apparently several very similar species were actually introduced, including S. kali, S. pestifer, S. australis and S. iberica, so it’s rarely clear which species you’re actually looking at. In fact, just to make things more confusing, some of these species have hybridized, creating new species, such as S. x gobicola and “giant tumbleweed” (S. x ryanii).

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The tiny flowers of Russian thistle are pretty enough … when seen close-up. Source: Forest & Kim Starr, Wikimedia Commons

Salsola species are shub-like annuals of varying heights, from 4 inches to 4 feet (10 to 120 cm tall) according to growing conditions, with small, sparse, linear, spine-tipped leaves and numerous narrow, branched, tangled stems. The stems are green, red or striped when young, beige at maturity. The tiny sessile mallow-like flowers have no petals, only sepals. They are whitish to pink, depending on the species.

The plant dries up when the seeds mature, then breaks off at its base and starts its travels, dropping its seeds as it rolls. Plants have been clocked at 30 miles (48 kilometers) per hour on windy days and will often take to the air!

Going Places

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Field filling in with young Russian thistles. Source: Stefan.lefnaer, Wikimedia Commons

Starting theoretically from an accidental introduction in South Dakota in 1873, Russian sage has pretty much conquered the North American continent, being found in 48 US states except Alaska and Florida, northern Mexico and all Canadian provinces, although none of its territories. In more humid climates, it’s mostly found on railrway embankments (plants are carried far and wide by trains) where they thrive in gravel and near the ocean, as it will grow in saline soils, including sand. It has likewise naturalized pretty much all over the world: Australia, South Africa, Asia, Europe and South America.

The extreme drought in the US Southwest has caused a massive explosion of tumbleweed in many areas where it was once fairly rare, since it is extremely drought tolerant and easily fills in where other plants fail.

Stopping a Pest Plant

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Mass burning of tumbleweed in a secure setting. Source: CBS

Control measures so far have been mostly limited to cutting the plant back, spraying it with herbicide and mass burnings of mobile plants, but the USDA is testing specific insects and diseases, including a virus, imported from the plant’s main homeland, Eurasian Russia. Some look quite promising, but the concern is that they may attack other plants and that requires further study.

In the meantime, S. tragus can still be useful under certain circumstances, offering browse and shelter for wild animals, a source of seed for birds, soil rehabilitation and phytoremediation. It also acts as a pioneer plant in disturbed environments, helping to launch ecological successions. It is not very competitive, disappearing quickly when taller vegetation creates shade, and thus will only thrive under near barren conditions. Also, the seeds are short-lived (about two years).

However, the prolonged drought in the US Southwest means the situation has gotten so out of hand in recent years that there is little sympathy for tumbleweed there. Most people want to see it gone … forever!20180425A www.cannon.af.milJPG