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

What Was I Thinking When I Planted That!

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20161119iWhen it comes to gardening, my own insatiable curiosity is often my worst enemy. I keep trying new plants, either new hybrids or simply plants I’d never seen before. Of course, most of the time the worst that happens is I spend money for nothing on a dud plant, but still, I often do discover some really great varieties. But sometimes the result is much worse.

You see, most of the worst weeds in my yard are ornamentals I planted without having checked them out adequately. Had I looked into them a bit more, I never would have planted them.

Here are a few examples:

The Euphorbia that Ate my Flowerbed

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‘Fen’s Ruby’ cypress spurge (Euphorbia cyparissias ‘Fen’s Ruby’)

I planted ‘Fen’s Ruby’ cypress spurge (Euphorbia cyparissias ‘Fen’s Ruby’) because it just looked so cute in the nursery, almost like a little purple conifer with contrasting chartreuse flowers. But what was I thinking when I planted it? I knew that several (but not all) spurges (euphorbias) were invasive, but this one was so tiny and dense, how could it possibly do any damage?

12 years later, I’m still fighting it. The plant spreads thanks to underground stolons that head off in all directions. And to be honest, it’s not even all that attractive, because its pretty spring coloration doesn’t last, the leaves turning a more boring glaucous green for the rest of the summer. Worse, when I yank it, it releases a sticky, poisonous latex you must not get into your eyes. In spite of my efforts, every year its spreads further and further. I may have lost the war against this one!

Self-Sowing by the Thousands

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‘Ravenswing’ cow parsley (Anthriscus sylvestris ‘Ravenswing’).

‘Ravenswing’ cow parsley (Anthriscus sylvestris ‘Ravenswing’) is in itself totally charming. And I see it mentioned recommended on lots of Websites as a great garden plant. With its glossy, deeply cut foliage in a superb dark purple, almost black, it looks like a black fern… when it’s not in bloom, at least. Of course, the umbels of tiny white flowers, reminiscent of those of its cousin, the wild carrot (Daucus carota), prove it isn’t a fern (ferns don’t bloom), but they do make a nice contrast with the foliage.

It didn’t take me long to realize I’d made a mistake in planting this one. The following spring, hundreds of seedlings popped up in my garden, about half with the same shiny dark purple leaves as the cultivar, the other half with plain green foliage. (Obviously it does not come totally true to type from seed).

Subsequently I learned that the secret to keeping ‘Ravenswing’ under control is to remove the flowerheads before they go to seed, because the plant itself never suckers: it only spreads by seeds. Besides, removing the flower stalks before they go to seed helps extend the life of this plant which is essentially a biennial. If it doesn’t produce seed, it will tend to sprout anew the following season.

But it’s too late for that now. Pretty clearly, the seeds can live for many years, because even if I pull out or cut back the seedlings as soon as I see them and never let them bloom, new seedlings grow back every year.

I intend to keep fighting this one: I don’t want to be accused of having introduced this invader to my region (the green form is known to be a noxious weed!)… and self-sowing plants get around much faster than stoloniferous ones!

An Aggressive Invader

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Plume poppy (Macleaya cordata).

I must admit that I was still a young and naive gardener when I planted my first plume poppy (Macleaya cordata). I don’t think I was yet 25, so that would have been nearly 40 years ago. I simply assumed that garden centers would never sell weedy plants, that if it was being sold, it had to be something desirable. I’ve since learned otherwise.

Some people actually like plume poppy. Its proponents claim it’s not all that invasive and that it’s easy to control. If so, how come every time I see it in a garden, it seems to have pretty much taken over? And it’s a big bruiser of a plant, too, about 8 feet (2.5 m) high, big enough to crowd out the competition.

Part of the difference in attitude might be that it is less invasive in heavy soils. Well, I planted mine in practically pure sand. The plant spread from one end of the flower bed to the other, about 20 feet (6 m) in just two years. The whole bed turned into a macleaya jungle, and it would have taken over the lawn too if I didn’t mow it regularly, chopping off the sprouts as I went. When you try to pull it up, it covers you in sticky orange sap that stains everything. Worse yet, all that effort is for a plant I honestly don’t even find all that attractive.

I solved this problem quite promptly: I moved and left it to the new owner to handle.

Mislabeled

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Common false spirea (Sorbaria sorbifolia): beautiful, but what a garden thug!

I knew that common false spirea (Sorbaria sorbifolia) was invasive and I would never have planted it on purpose, but I was done in by a lying label.

You see, I was looking for Kashmir false spirea (S. tomentosa angustifolia, syn. S. aitchisonii), common false spirea’s more restrained cousin. This species doesn’t sucker and is not invasive. In fact, that’s its main selling point: it’s just as pretty as common false spirea, but it stays put! The two species pretty much resemble each other (Kasmir false spirea has somewhat finer leaves, though), and before I realized the error, the damage was done. I’d let the devil loose in my backyard!

The problem wasn’t obvious at first: like many woody plants, the mislabeled common false spirea took a few years to settle in. Then suckers began popping up… everywhere. This shrub made a beeline for the fence and moved into my neighbor’s yard from which it now  regularly makes forays into my garden. It follows the fence for a good 50 feet (15 m) and every year I cut back and pull out hundreds of suckers that dare cross the fence back into my yard.

The phenomenon of mislabeled plants is unfortunately very common in the horticultural world. Some nurseries just don’t seem to give a damn about what the label says as long as the plant sells. You can see where you could make a lot more money selling the prolific common false spirea labeled as the slow-to-multiple Kashmir false spirea. But I don’t want to make it sound like all nurseries do this: many take the correct identification of their plants very seriously.

I really don’t have a solution for this problem except to always complain when you find a mistake. It’s not your local garden center that is to blame (usually), but the wholesale nursery that shipped out the plants with the wrong labels. I like to think that if enough people complain about getting the wrong plant, it will make a difference.

Sold the Wrong Knotweed

The false spirea incident was not the only time I got into a pickle because of a mislabeled plant. There was also the case of the mislabeled Japanese knotweed and that could have been much, much worse!

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The wild, unselected Japanese knotweed (Fallopia japonica): exactly what I didn’t want in my garden.

Japanese knotweed (Fallopia japonica) has such a bad reputation as an invasive plant that the species itself is rarely seen in plant nurseries. It’s considered one of the worst weeds in the world! But if the species itself is highly invasive, there are a few ornamental cultivars that don’t sucker or only do so to a very limited degree. The variegated cultivar, F. japonica compacta ‘Milk Boy’ (apparently the correct name for ‘Variegata’), for example, is easy to control and fairly widely available.

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‘Crimson Beauty’ Japanese knotweed (Fallopia japonica ‘Crimson Beauty’) is a beauty and totally non-invasive… if you can find it!

But I wasn’t looking for ‘Milk Boy’. I wanted ‘Crimson Beauty’, an extraordinary giant perennial with bright mossy red flowers I first saw at Longwood Garden decades ago. It is non-suckering and produces a huge shrublike plant with flowers that last 3 months. Really, I have rarely seen such a beautiful perennial! But I never seemed to be able to find it in nurseries, not even in catalogs, at least, not in Canada (for me, ordering from the US is a major complication). Then, in 2003, I finally saw one in the mail order catalog of an Ontario nursery and of course of course sent for it.

It didn’t bloom the first year, but when the second year came around, the flowers were not red, but white. I thought maybe they’d turn red over time, but no such luck. Then I figured (wishful thinking!) that maybe the red flowers only appeared when the plant was mature. But by the third summer, I realized that I had made a huge mistake, that my plant was not the non-suckering ‘Crimson Beauty’, but the species. I knew this for sure not just because of the still-white flowers, but because suckers started popping up everywhere, even 3 or 4 feet away from the mother plant.

I’ve been known to put things off, but my entire yard was at stake here, and I didn’t hesitate. I dug up the mother plant (quite a struggle, as the roots were incredibly deep), then hacked out all the thick rhizomes I could see as well, pretty much trashing a huge section of garden. Soon new shoots appeared: strong, dominant shoots as thick as my thumb that seemed to be saying “you’ll never be able to stop us”. But I cut them to the ground. And cut back the new sprouts that followed as well. And their replacements. Each time I cut them back, the next generation had thinner, shorter stems. By year 3, the invasion was nearly over. Still, the occasional stray sucker came up in year 4 and even a few in year 5. None appeared in year 6: I had succeeded in eliminating Japanese knotweed from my yard: not many gardeners can make that claim.

I did contact the nursery that sold me the plant and they apologized. It wasn’t one of their plants, but a plant they’d bought in under the name ‘Crimson Beauty’. I know that sort of thing happens, but it’s annoying to have had to put 5 years of efforts into correcting someone else’s mistake.

Caveat Emptor

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Check before you plant!

Anyone can make the mistake of buying an invasive plant by accident. After all, the fact that the plant is a weed is not the kind of detail that the seller ever mentions on the label! But don’t do as I did and plant an unknown plant without checking into it. The Internet can be a great tool for that sort of thing and it takes only minutes to do a reasonable search. You won’t want to be struggling today to control a plant you put in years before.

Trees and Shrubs that Get Around

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A typical suckering shrub: multiple stems, each with its own roots.

Most trees produce only a single trunk over their entire life. The average shrub, too, produces all its branches from a single base. But there are exceptions to this rule, woody plants that send out suckers (basal shoots) from creeping roots, stolons, or rhizomes. Sometimes these sprout near the mother plant, but other times they can be a good distance away. And suckering trees and shrubs do tend to get out of hand over time. Sometimes these plants sucker only under specific conditions, such as when their roots are damaged by hoeing or when the mother plant is under stress. Others, on the other hand, sucker happily under just about any condition!

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The false spirea (Sorbaria sorbifolia) is an example of a widely grown and attractive shrub… whose suckers do tend to get out of hand!

You can turn this disadvantage into an advantage in areas prone to erosion because suckering trees and shrubs create a complex mass of rhizomes, stolons and roots that holds the soil in place. Laidback gardeners can also benefit from their tendency to wander by letting them fill in empty spaces in the garden. A single shrub that covers a large area means a lot less planting… and a lot less expense! Also, most suckers are not just shoots, they are rooted plants: you can easily dig them up and replant them elsewhere, which gives you plenty of free green material for future plantatings! But no matter where you plant suckering plants, and even if you appreciate their ability to proliferate, you still need to know how to stop them. They won’t be able to escape, for example, if they are surrounded by a terrace or trapped between the wall of the house and a walkway. And cities often use them as low-cost fillers for traffic medians: no way they’ll find their way of that kind of barrier.

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A bucket or pot whose bottom has been removed will control almost any suckering shrub… but won’t be big enough for many trees.

You can also plant them inside a barrier, such as a big pot or plastic bucket whose bottom has been removed (necessary for proper drainage).

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Anti-rhizome barrier.

Or install an anti-rhizome barrier, also called a bamboo barrier, a sort of semi-rigid plastic film of about 2 feet (60 cm) in height that can be inserted into ground around the plantation. (This product is widely available in Europe and in parts of the United States, but I know only of one source in Canada: Canada’s Bamboo World).

List of Suckering Trees and Shrubs

Here are some temperate climate shrubs and trees that tend to sucker, at least under certain conditions.

  1. American Beech (Fagus grandifolia) Zone 3
  2. Balsam poplar (Populus balsamifera) Zone 2
  3. Blackberry (Rubus) Zone 5
  4. Black locust (Robinia pseudoacacia) zone 4b
  5. Bramble (Rubus) Zones 3 to 7
  6. Buffaloberry (Shepherdia) Zone 2
  7. Bush honeysuckle (Diervillea) Zones 3-5
  8. Chokeberry (Aronia) Zone 4
  9. Chokecherry (Prunus virginaniana) zone 2b
  10. Cliff Green (Paxistima canbyi) zone 4b
  11. Common buckthorn (Rhamnus cathartica) Zone 2
  12. Common dogwood (Cornus sanguinea) Zone 3
  13. Common lilac (Syringa vulgaris) zone 2b
  14. Devil’s walkingstick (Aralia spinosa) Zone 6
  15. European Aspen (Populus tremula) Zone 2
  16. False spirea (Sorbaria sorbifolia) Zone 2
  17. Fragrant currant (Ribes odoratum) Zone 2
  18. Fragrant sumac (Rhus aromatica) Zone 3
  19. Gray dogwood (Cornus racemosa) Zone 2
  20. Hazel (Corylus) Zones 3-6
  21. Japanese angelica tree (Aralia elata) zone 5b
  22. Lombardy poplar (Populus nigra ‘Italica’) Zone 4
  23. Oregon Grape (Mahonia aquifolia) Zone 5
  24. Pin cherry (Prunus pennsylvanica) Zone 2
  25. Raspberry (Rubus idaeus) Zone 3
  26. Red osier dogwood (Cornus sericea, syn. stolonifera) Zone 2
  27. Rugosa rose (Rosa rugosa, some cultivars)  Zone 3
  28. Running serviceberry (Amelanchier stolonifera) Zone 3
  29. Russian olive (Elaeagnus angustifolia) zone 2b
  30. Sea buckthorn (Hippophae rhamnoides) zone 2b
  31. Shadblow serviceberry (Amelanchier canadensis) Zone 4
  32. Siberian dogwood (Cornus alba) Zone 2
  33. Silverberry (Elaeagnus) Zones 1b to 7
  34. Smooth hydrangea (Hydrangea arborescens) Zone 3
  35. Snowberry (Symphoricarpos) zone 3
  36. Staghorn sumac (Rhus typhina and glabra) Zone 3
  37. Sweet fern (Comptonia peregrina) Zone 2
  38. Sweet Gale (Myrica gale) Zone 2
  39. Tree of heaven (Ailanthus altissima) area 6b
  40. Trembling aspen (Populus tremuloides) Zone 2
  41. White poplar (Populus alba) Zone 4
  42. Wild Rose (Rosa, many species) zones 2 to 5
  43. Yellowroot (Xanthorhiza simplicissima) Zone 4